Natural Ecosystem and Agroecosystem Comparison

Pest species can be present in agroecosystems, but not cause significant crop yield loss or livestock productivity reductions. Why? What factors prevent pest populations from reducing yield? One explanation may be that the crop or livestock is resistant to the pest. For instance, a crop plant may produce compounds that fend off pathogen infection or deter insect feeding. And if environmental conditions and resources are ideal, the plant may be able to grow and recover from pest infestation. What other ecological processes and factors might contribute to agricultural resilience to pests or other stresses such as climate change?

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In natural ecosystems there tend to be more niches and a higher diversity of species compared to most managed agroecosystems that are simpler, have fewer predatory and parasitic species, and less genetic diversity within a species. As the table below indicates with fewer trophic interactions, there are fewer species to reduce pest populations and prevent them from reducing agricultural yield and quality. Further, with low genetic diversity within agricultural species and across the landscape, the agricultural system is more vulnerable to pest outbreaks than natural ecosystems.

Insects

Insects are the most diverse group of animals that are found in most environments. In the Animal kingdom, Insects are in the Phylum Arthropoda; Arthropods have an exoskeleton of chitin that they shed as they grow; they also have segmented bodies and jointed appendages. In addition to the Class Insecta, the Arthropoda also includes the arachnids (spiders and mites), myriapods (ex. centipedes), and crustaceans (crabs, lobsters, etc.). Insects are distinguished from the other Arthropod classes by the following features:

1. As adults and in some species in the juvenile stages, insects have three body parts: the head, thorax, and abdomen. Although in some insect species, some of the three body parts are fused together and may be difficult to distinguish. See this website for images and more discussion of insect anatomy: Purdue University, College of Agriculture, Department of Entomology, 4-H and Youth: Insect Anatomy [8]

2. The adults have antennae on their heads that they use to sense their environment, and they have three pairs of legs or six legs.
   

   Figure 8.1.1: Honeybees are important pollinator insects. Note the three body parts (head, thorax, and abdomen) two antennae, and three pairs of legs. 

   Credit: MaryAnn Frazier, Department of Entomology, Penn State University [9]
3. Most insects undergo a morphological change that occurs between the time they hatch from eggs and develop into adults. The morphological change is called either complete metamorphosis or incomplete metamorphosis referring to how significantly the insect's appearance changes from the early stage of development to the adult stage. Go to these links to see images of the types of metamorphosis and read more about insect metamorphosis: ASU School of Life Sciences: Metamorphosis [10]

Feeding Types

Insects may be herbivores or omnivores. Herbivorous insects may eat plants by directly feeding on plant tissues such as leaves or roots. Herbivorous insects include caterpillars, beetles, grasshoppers, and ants. Some insects pierce plants and suck plant nutrients from the plant vascular system, typically the phloem, (the cells that transport plant carbohydrates and amino acids); although some insects feed on the xylem, the vascular cells that transport water and nutrients. Examples of piercing-sucking insects include aphids and mosquitoes. By contrast, butterflies and moths have siphoning mouthparts for drinking nectar. Omnivore insects consume multiple kinds of food including other insect prey and plant tissues such as leaves and/or nectar and pollen.

Beneficial insects

Although insect pests are major agronomic pests, only about 1% of insect species are agricultural pests. Insects also contribute to important ecosystem processes, including: i. pollination, ii. predation and parasitism (ex. lady beetles, lacewings, praying mantis, parasitic wasps); iii. decomposition of organic materials such as crop residues and manure (Ex. dung beetles) iv. providing food for other organisms, such as fish and birds. Review the photos below for some categories of beneficial insects, and some of their characteristics here: National Pesticide Information Center [13]

Figure 8.1.2: Dung Beetles contribute to decomposing dung or manure. This photo shows a Large Copper Dung Beetle (Kheper nigroaeneus) on top of its dung ball. 

Credit: Bernard DUPONT from FRANCE (CC BY-SA 2.0 [14]), via Wikimedia Commons

Figure 8.1.3: A beetle (Chlaneius sp.), predating on insect larvae

Credit: Heidi Myer

Figure 8.1.4: Parasitic wasp laying eggs on an alfalfa weevil

Credit: Arthur Hower, PSU

Figure 8.1.5: Caddisflies: An important food source for fish.

Credit: Jason Neuswanger, Troutnut [15]

Pest Management

Humans have developed methods of insect and pest control for centuries.

Pesticide Resistance

Soon after the development of DDT in 1939 and the dawn of the modern insecticide era in the 1940s, scientists began to understand that pesticides were not the silver bullet of pest control. Particularly when a pesticide or one effective pest control strategy is relied on, the control tactic acts as a strong selective force for the development of resistance to the tactic in the target pest population. With the continuous application of the same pesticide, individuals that are susceptible to the pesticide are killed, leaving the few resistant individuals that survive to reproduce a offspring that are resistant to the pesticide. See the figure below for an illustration of how frequent reliance on one insecticide can select for a resistant insect population. Further, since many early pesticides were broad spectrum pesticides, the natural enemies of agricultural pest populations were also destroyed, contributing to pest population outbreaks.

Figure 8.1.6: How the repeated use of a pesticide results in pest resistance

Source: How pesticide resistance develops [18]. Michigan State University. Excerpt from Fruit Crop Ecology and Management, Chapter 2: Managing the Community of Pests and Beneficials by Larry Gut, Annemiek Schilder, Rufus Isaacs and Patricia McManus

In 1984, the US Board of Agriculture of the National Academy of Sciences organized a committee to explore the science of pest resistance and strategies to address the challenge. A report called "Pesticide Resistance: Strategies and Tactics for Management" was co-authored by the Committee on Strategies for the Management of Pesticide Resistant Pest Populations and published in 1986 by the National Academies Press, Washington D.C. In Chapter 1, G. P. Georghiou (1986) documented the development of pest resistance across multiple pest organisms (see pages 17 and 28 for figure 2 [19] and figure 8 [20]), as well as how difficult and costly it was becoming to develop cost-effective pesticides (see figures 12 and 13 [21] on page 36).

In the report, the Committee recommended using Integrated Pest Management or IPM to reduce the evolution of pesticide resistance and provide more long-term, effective pest control. As early as 1959, a team of scientists (Stern et al.) in California had also proposed that pest control that integrated both biological and chemical control approaches, was needed to prevent pest resistance to pesticides and pest control. Stern et al. (1959) defined terms and concepts that are fundamental to IPM today.

Understanding Economic Thresholds

Read the following two fact sheets for a description of Integrated Pest Management and the terms that Stern and his colleagues defined in 1959, which are still used today (economic injury level, economic threshold, and general equilibrium position).  Then watch the following short video and answer the questions below:

1. The Integrated Pest Management (IPM) Concept [3]. D. G. Alston. July 2011. IPM 014-11. Utah State University Extension and Utah Plant Pest Diagnostic Laboratory

2. IPM Pest Management Decision-Making: The Economic-Injury Level Concept [4]. D. G. Alston. July 2011. IPM 016-11. Utah State University Extension and Utah Plant Pest Diagnostic Laboratory

Activate Your Learning: IPM Concept and Decision-Making

Describe three things that are integrated into IPM.

Click for the answer.

ANSWER: Pest control strategies are integrated: such as cultural, mechanical, biological, and chemical. The FAO website provides multiple examples of these practices.

On the IPM figure below, which IPM pest population terms from the article could describe the lines labeled A, B, and C?

Figure 8.1.7: Hypothetical pest density over time graph. What IPM pest population term could apply to each line in the above figure?

Credit: Heather Karsten

Click for the answer.

ANSWER:

Figure 8.1.8: Hypothetical pest density over time graph with IPM pest population terms included

Credit: Heather Karsten

How would you describe the damage that the pest had caused to the crop at each of these pest population densities?

Click for the answer.

ANSWER: A: Economic Injury level- the pest damage has caused a significant economic cost or loss in the value and/or quality of the crop; B: Economic threshold: the pest population has reached a density that is cost-effective to control the pest population and prevent economic losses to the crop; it has not yet caused irreversible economic damage; C: The pest population is in equilibrium with natural enemies: pest damage is minor and natural enemies are preventing the pest population from increasing to the density of being an economic threat to the crop.

Watch the first 4.11 minutes of the below video: Integrated Pest Management (IPM) in Apple Orchards, which describes European Red Mite pests and predatory mites in Pennsylvania apple orchards.

Video: Integrated Pest Management (IPM) in Apple Orchards [22] (8.34)

Click for a transcript of the Integrated Pest Management video.

Fruit growers do their best to assure consumers their food is grown in ways that are environmentally, socially, and economically sustainable. Regular field scouting and weather monitoring are key to achieving the production goals of conserving soil and water, reducing pesticide use, and being good responsible employers. In this short video, you will learn some basic orchard scouting principles for common disease, apple scab, and also mite pests and beneficials. Weather stations provide site-specific data on temperature, rainfall, relative humidity, leaf wetness, and degree days, to alert you when conditions are favorable for diseases and insect pests. Routine inspection of trees and the use of pheromone traps to determine thresholds, will help you minimize and better time sprays. Penn State is known for its early work on IPM for biological control of European red mites. European red mite is a major pest of apples, if controlled only with mitacides, With eight to ten generations per year, this pest can build in numbers very quickly and has historically been able to develop resistance to many new mitacides, in only three to five years, if biological control by predators is conserved. European red mite is a sporadic minor pest that is relatively easy to control with only an occasional selective miticide application. And miticide resistance is not an issue. A quick way of determining light levels in your orchards is to use a magnifying hand lens or a headpiece magnifier to determine the percentage of might infested leaves. Select ten trees in the orchard, on the most susceptible variety, and count ten spur leaves from each tree for the presence or absence of mites. Then use this graph to determine the mite threshold level. As a general rule in apples, a spray threshold of only two point five mites per leaf exists early season, before June. The threshold increases to 5 mites per leaf from June through mid-July. Use a threshold level of 7.5 mites per leaf through the rest of the season. If the mites per leaf do not reach these levels, no control action needs to be taken. Orchards with stable populations of T. pyri never reach these thresholds, as long as there's at least one predatory mite for every 10 pest mites per leaf. Our current population of T. pyri probably came to Pennsylvania on Apple bins moved between states, or on nursery stock. A program developed by Penn State, and funded by the USDA conservation programs, move T. pyri from known seed orchards to many new grower orchards, and over eighty percent of Pennsylvania apple orchards have this predator present at some level. Where conserved, T. pyri has reduced the use of miticides by over ninety percent, and some growers have not sprayed mite-susceptible varieties in more than ten years. Establishment of T. pyri into orchards where it is absent is relatively simple and can be accomplished in one to two seasons, once donor orchards with abundant T. pyri populations have been identified as a source. Transfers of T. pyri from these orchards can be successful by physically moving spur leaves in May and June. Transfers after July appear to be less likely to establish populations. If not controlled, apple scab can cause losses of seventy percent or greater where humid, cool weather occurs during the spring months. Losses result directly from fruit infections or indirectly from repeated defoliation, which can reduce tree growth and yield. The pathogen generally overwinters in fallen leaves and fruit on the orchard floor. As a result, orchards are self-infecting. Primary spores develop during the winter and begin to mature early spring. Around bud break, the first mature spores will be released from the infected leaves and or fruit. The length of time required for infection to occur depends on the number of hours of continuous wetness and the temperature during the wetting period. Leaf wetness hours can be calculated by either beginning the count at the time leaves become wet and ending the count when the relative humidity drops below ninety percent, or by adding consecutive wetting periods (hours), if the leaves are again wetted within eight hours from the time relative humidity dropped below ninety percent. For example, if the average temperature is between 61 to 75 degrees Fahrenheit, a minimum of six hours of leaf wetness is required for spores to be dispersed. Once the primary spores have established infection on the plant tissue, and approximately nine to ten days, symptoms can be observed. At that time, secondary spores called conidia, are being produced and will do so the remainder of the season, being dispersed by rain or wind on susceptible tissue. Monitor rainfall and duration of wetness closely, beginning at green tip, since mature spores begin to be released around this time. Peak mature spore release is around bloom time through petal fall. Continue to monitor rainfall and duration of wetness through mid-June, as the final mature spores are released during this time. Start monitoring for lesions (spots) around 10 to 14 days after bud break, which is when the first symptoms can occur, if disease conditions are favorable. For each orchard block, follow a “w” shape pattern within the block when scouting. Evaluate ten trees by examining 20 leaves on each of the five limbs per tree, and record the number of leaves showing any scab lesions. Number one: begin with the flower bud (spur) leaves where early infections are most likely to be noticed. Number two: start with observing the undersurface of leaves, since the undersurface of leaves may become spotted before the top surface. Take notice of early lesions which may be small, light brown, black spots. Number three: as scouting continues during early spring, be sure to observe both the top side and underside of the leaf. Apple scab infection appears as brown velvety lesions, which will become darker as they age. After fruit have set, in addition to leaf observations, examine 20 fruit on each tree and record the number showing any scab lesions. Use this information to better manage scab in the future. It is important to scout and control apple scab early in the season to prevent secondary infections from becoming established. Even if you have a professional consultant who monitors your orchard, it is important to become knowledgeable about basic principles of integrated fruit production. Penn State Extension offers educational programs on current best management practices in nutrition, pruning, tree training, crop load management, farm employee health and welfare regulations, food safety practices, and IPM. For a list of courses, visit the Penn State Extension Tree Fruit Production website. And for timely recommendations, sign up for Penn State Extension, Fruit Times.

What are the potential benefits of scouting for the European red mites and predatory mites in Pennsylvania orchards?

Click for the answer.

Answer: If the European red mites have not reached the economic threshold, a farmer can avoid spraying an insecticide and protect beneficial insects that can reduce the pest population and/or provide ecosystem benefits (ex. pollination, nutrient cycling). Avoiding spraying can also save money and time for the farmer and reduce the farmers’ exposure to pesticides.

Formative Assessment: Australian Grain Crop IPM and Determining the Economic Threshold of Potato Leafhoppers in Alfalfa

Part 1: Australian grain crop IPM

Watch the following video that explains IPM adoption in grain crops in Australia; then answer this question:

1. Identify and explain three benefits of utilizing IPM discussed in the Australian video from the GRDC.

Video 1: GCTV2: Integrated Pest Management (5:46)

If the video does not load for you, go to GCTV2: Integrated Pest Management [23]

Part 2: Determining the Economic Threshold of Potato Leafhoppers in Alfalfa

Read the Penn State University Potato Leafhopper on Alfalfa Fact Sheet [24].

Figure 8.1.9: Economic Threshold for Potato Leafhoppers

Credit: Penn State University Potato Leafhopper Alfalfa Fact Sheet [24]

Scenario

Assume that you followed the procedure described in the Penn State fact sheet to scout for Potato Leafhoppers in an alfalfa field by sweeping 20 times with your sweepnet in each of 5 different locations in the alfalfa field. The number of leafhoppers that you found in the 5 different locations was: 15, 12, 16, 7, 13 when the alfalfa crop was about 11 inches tall. You would like the alfalfa to grow about 25-30 inches height before harvesting it for hay, this could require 2 to 3 more weeks of growth, depending on rainfall. Based on current alfalfa hay prices in your region, you estimate your alfalfa hay is worth about $250/Ton, and the insecticide you would spray to control the leafhoppers would cost about $16/A. If you spray the alfalfa field, it cannot be harvest until 7 days after spraying the insecticide; and due to toxicity to bees, the alfalfa should not be sprayed if it is flowering.

Answer the following questions:

1. Calculate the average number of leafhoppers per sweep. Add the number of potato leafhoppers from the 20 sweeps in each of the 5 locations (20 X 5= 100 sweeps). Divide by 100 to calculate the number of leafhoppers per sweep. Use the Economic Threshold Table from the Fact Sheet for Potato Leafhoppers, shown here. Has the insect population reached an economic threshold for your crop at this height?
2. Based on the average number of leafhoppers per sweep, what should you do? Why?
3. If your crop height was 7 inches tall and you had the same number of leafhoppers per sweep that you calculated here, would your pest management decision change and how?
4. Discuss at least two potential benefits of using the economic threshold decision tool rather than spraying as soon as potato leafhoppers were first visible.

Files to Download

Module 8 Formative Assessment Worksheet [25]

Submitting Your Assignment

Please complete the Module 8 Formative Assessment in Canvas.

Weeds

A weed is a plant that is not wanted or a plant growing in the wrong place. In agricultural systems, weeds tend to be unwanted because they compete with crops for light, water, and/or nutrients, and can reduce crop yield and/or quality, particularly if weeds are permitted to grow and reproduce. Weeds may reduce crop quality through contamination with seeds or plant parts that may be toxic, or of poor nutritional or culinary quality (produce off-flavor compounds). Some weeds may harbor crop insect pests or pathogens; and when weeds have a significant negative impact, they can reduce the economic value of agricultural land. On the other hand, if weeds are not numerous enough to reduce crop yield and quality, weeds can provide some agroecosystem benefits. For instance, weeds can provide:

1. protection from soil erosion

2. pollen, nectar, and habitat for beneficial organisms and wildlife
3. forage for grazing animals

Competitive Characteristics of Weeds

Weeds tend to be plants that are adaptive and competitive in a range of environmental conditions. They typically have seeds or perennial storage organs that enable them to grow rapidly and produce aboveground canopies that compete with crop plants for light, and root systems that compete for nutrients and water. Annual weed species often grow and mature relatively quickly, producing seeds earlier than crops. To increase survival of their offspring, annual weeds often produce many seeds, and some species produce large seeds. Strategies to control annual weed species target terminating them early, to prevent them from competing with crops and producing seeds.

Figure 8.2.1: Common ragweed (Ambrosia artemisiifolia L.) may produce between 3000-4000 seeds per plant. See Invasive Species Compendium [26]for more information.

Credit: Heather Karsten

Figure 8.2.2: Annual Lambsquarter (Chenopodium album L.) weed in corn. Lambsquarter can produce up to 72,000 seeds per plant. For more information about this weed species see Extension Utah State University [27].

Credit: Heather Karsten

If perennial weeds are growing from the small seeds they produce, they establish more slowly than annual crop seeds. But seeds are not their primary form of reproduction, recall that perennial plants often spread and reproduce via established storage organs such as taproots, tubers, bulbs, and rhizomes (belowground modified stems that store reserves and enable a plant to spread horizontally), or aboveground stolons or storage stem bases. Perennial weeds growing from storage organs can be very competitive with crop plants. If perennial weed storage organs are cut and distributed over a larger area and reburied or partially covered, they can also establish and spread across a larger area. If weed storage organs are left on the soil surface to freeze and thaw over winter or desiccate in mid-summer, then tillage can terminate perennial storage organs. Repeated mowing of plant regrowth may weaken or deplete plant reserves, particularly if it is prior to the end of the growing season when perennials tend to translocate plant reserves to storage organs. Chemical control of perennials is also often most successful at this time when herbicides can be translocated to storage organs.

Figure 8.2.3: This Phragmites weed spreads by stolons and produces roots and shoots every few inches, that once established could survive as separate plants.

Photo Credit: Rod Stolcpart (Rock County, Nebraska Weed Superintendent)

Figure 8.2.4: Rhizomes on Johnsongrass (Sorghum halepense L.)

Credit: Jack Kelly Clark, courtesy University of California Statewide IPM Program

Figure 8.2.5: Tubers on yellow nutsedge (Cyperus esculentus) 

Credit: Jack Kelly Clark, courtesy University of California Statewide IPM Program

Weed Survival Characteristics

In addition, many weeds have traits that enhance their survival and reproductive success such as: i. hard-seeds or seeds that can remain dormant for long time periods until environmental conditions for germination are good, enhancing weed seed success, ii. plant protective characteristics such as thorns, toxic tissues, protected growing buds, iii. adaptive growth to a wide range of environmental conditions also referred to as plasticity. For example, in a field or lawn that is grazed or mowed to a short height to control weeds, adaptive weeds can produce leaves very close to the soil surface and flowers on short stems below the mowing height.

Figure 8.2.6: Buttercups (Ranunculus L. ) contain protective compounds that are toxic to most grazing livestock. For more information about buttercups see Creeping Buttercup, Pacific Northwest Extension Publications [28].

Photo: Heather Karsten

Figure 8.2.7: Thistles with protective thorns.

Photo: Heather Karsten

Activate Your Learning: Weed Control Practices

Recall what you have learned about crop plant lifecycle classification and characteristics in Module 6.

Read the Australian Department of the Environment website that describes Integrated Weed Management [30]. Click on and read the links that describe each type of weed management technique. After you have read both of the above readings, answer the questions below.

Herbicide Resistance

Although integrated pest management was introduced in the 1980s, the number of weeds that have evolved resistance to new herbicides continues to grow (See Figure 8.2.8 below).

Figure 8.2.8: Global Increases in Unique Resistant Cases

Credit: Dr. Ian Heap, International Survey of Herbicide Resistant Weeds [31]

Similar to other pests, weeds evolve resistance when exposed to the same strong selective force, such as an application of the same herbicide over consecutive years. When the same herbicide is applied numerous times to a field, susceptible weeds are killed, leaving resistant individuals to reproduce and dominate the population, as illustrated in figure 8.2.9 below.

Figure 8.2.9: Selection for herbicide resistance begins when an herbicide resistant biotype survives a particular herbicide application. The resistant biotype survives, matures, and sets seed. If the same herbicide continues to be applied and the resistant weeds reproduce, eventually the majority of the weeks will be resistant to the herbicide.

Credit: J. L. Gunsolus. Weed Science, Department of Agronomy and Plant Genetics. Herbicide-resistant weeds [32].

Integrated Weed Management

Integrated weed management (IWM) is an IPM approach for weeds that can provide long-term weed control of weeds by integrating multiple control strategies. Some weed scientists have described IWM as utilizing “many little hammers” as opposed to continuously employing one “big hammer” such as an herbicide (Liebman & Gallandt, 1997). Weed control tactics fall into the IPM control categories that you learned about for insect control in Module 8.1. Examples of weed control practices include the following:

1. Cultural control practices are management practices humans can employ to prevent weed establishment and maintain vigorous crop growth. Examples include: crop rotation with crops of different life cycles and seeding densities, planting certified seed that is managed to have minimal weed contamination, planting adapted crop varieties, adjusting row spacing, population density, and timing for a competitive crop and successful crop establishment, maintaining soil health and fertility, and using practices that prevent weed establishment such as cover crops and mulching.

   

   Figure 8.2.10: Straw mulch to suppress weeds in garlic.

   Credit: Heather Karsten
   

   Figure 8.2.11: Buckwheat (Fagopyrum esculentum L. ) cover crop planted mid-season to smother weeds, produce organic matter and provide habitat for beneficial insects.

   Credit: Anna Santini
   

   Figure 8.2.12: Crop rotation of summer annual row crops such as corn (first photo) with densely seeded perennial alfalfa (second photo), and/or densely planted winter annual wheat and spring oats (third photo, wheat is on the right, oats are on the left).

   Credit: Heather Karsten
2. Mechanical or physical weed control includes practices such as plowing, cultivation, hoeing, targeted hand-weeding, and flaming.
   

   Figure 8.2.13: This flex tine cultivator is used to control weeds when they are very small.

   Credit: Heather Karsten
   

   Figure 8.2.14: Rotary harrow for weed cultivation.

   Credit: Heather Karsten
3. Biological control: conserving or introducing herbivorous insects, grazing or browsing animals, or plant pathogens to reduce weed populations. Biological control of weeds is typically used on extensive rangeland where other more labor intensive or expensive methods are not cost effective.
4. Chemical control: the application of herbicides, or the use of herbicide-resistant crops with herbicide applications to control weeds
5. Genetic resistance: selecting crop varieties that are well adapted to an environment and competitive with weeds. Herbicide-resistant crops may also be considered a form of genetic resistance.

Transgenic Crops for Pest Control 

Transgenic crops or animals are often referred to as GMO’s or genetically modified organisms. This is misleading because all cultivated crop plants and livestock have been genetically modified through centuries of human selection and traditional breeding. A more accurate name for the genetically engineered organisms that are referred to as GMOs, is transgenic organisms. Transgenic crops or livestock contain genetic material that was transferred from a different species through biotechnology techniques or genetic engineering.

In the 1980s, agricultural input companies began developing and using transgenic techniques to develop new crop varieties. The first traits that were inserted into major crop plants and commercialized on a large scale were genes from two different species of bacteria. The transgenic traits were for insect resistance (Bt) and resistance to the herbicide glyphosate (commercially marketed as Round-up). Since the first commercial release in 1996, these technologies have been widely adopted in the US and other parts of the world (See Figure below).

Figure 8.2.15: Adoption of genetically engineered crops in the United States from 1996 - 2014. Data for each crop category include varieties with both HT (herbicide-resistant) and Bt (stacked) traits. Credit: USDA Economic Research Service [33], using data from Fernandez-Cornejo and McBride (2002) for the years 1996-99 and USDA, National Agricultural Statistics Service [34], June Agricultural Survey for the years 2000-14.

Insect Resistant Bt Crops 

Bt is an abbreviation for Bacillus thuringiensis a bacteria that produces an enzyme that is toxic to the digestive system of insects in the Beetle; and Moth and Butterfly families. These two insect families include some major crop pests. Scientists have transferred the genes that code for the production of the toxins into crop plants. Because the Bt trait confers insect pest resistance, the adoption of Bt corn and Bt cotton has contributed to a significant reduction of insecticide use in these crops (See Figures 8.2.16- 8.2.18 below).

Figure 8.2.16: (Figure 12) Insecticide use in corn and cotton production, 1995-2010

Credit: Jorge Fernandez-Cornejo, Seth Wechsler, Mike Livingston, and Lorraine Mitchell. Feb. 2014. Genetically Engineered Crops in the United States [35]. USDA Economic Research Report Number 162.

Figure 8.2.17: (Figure 18) Pounds of insecticide active ingredients (a.i.) per planted acre and percent acres of Bt corn, 1996 - 2008.
Figure 8.2.18: (Figure 19) Pounds of insecticide active ingredient (a.i.) per planted acre and percent acres of Bt cotton, 1996 - 2008.

Click for a text description of Figures 8.2.18 and 8.2.19.

Figure 18 is showing that as the pounds of insecticide/acre is decreasing ( except for an increase from 1998-1999), the percent acres of Bt corn is increasing from 1996 until 2008. Figure 19 is showing that as the pounds of cotton insecticides are generally decreasing, the percent of Bt cotton is increasing from 1996-2008.

Credit: Fernandez-Cornejo, Jorge, Richard Nehring, Craig Osteen, Seth Wechsler, Andrew Martin, and Alex Vialou. Pesticide Use in U.S. Agriculture: 21 Selected Crops, 1960-2008, EIB-124, U.S. Department of Agriculture, Economic Research Service, May 2014.

Prior to the development of Bt crops, spores of the bacteria Bacillus thuringiensis were sometimes used as biological control for insect pests in forestry and agriculture, often on organic farms. Initially, commercial Bt crops were released in the US without any regulations to prevent resistance. But science had shown that pest populations could quickly evolve resistance to Bt, and planting Bt crops on a large scale across the agricultural landscape would create a strong selective force for pests to evolve resistance to Bt. Therefore, in response to public concern about the high risk of pests evolving resistance to Bt, the EPA convened a committee that developed a resistance management plan for Bt crops.

In addition to the crop expressing a high dose of the Bt toxin, to prevent or delay pest resistance to Bt farmers who plant transgenic Bt corn and cotton, are required to plant a refuge, a percentage of the crop field or a field close by that does not express the Bt trait. The refuge area conserves a population of insects that are susceptible to Bt, so that the susceptible insects can reproduce with insects that might have resistance to Bt, sustaining some Bt-susceptible individuals in the population. Depending on the presence of Bt crops in a region, the EPA regulation requires that farmers plant between 5% and 20% of their crop field without the Bt trait. For stacked Bt corn hybrids (with 2 or more Bt traits) farmers must plant the required refuge for each Bt trait that their crop expresses. For more information on refuge requirements, read Insect Resistance Management and Refuge Requirements for Bt Corn [36], from the University of Wisconsin.

Pest Resistance to Transgenic Bt Crops

Despite the refuge requirement in the US, western corn rootworm resistance to Bt corn was reported in multiple Midwestern states (Jakka et al., 2016). The first reported Bt-resistant corn rootworm populations were found in cornfields in Iowa that had been planted to Bt corn consecutively for at least three years, and the authors suggested that the fields likely did not include refuge corn (Gassman et al., 2011). Additional studies also found that the Bt toxin dose was not sufficiently high to delay the evolution of insect resistance and that corn rootworm could evolve resistance to additional Bt toxins in three to seven generations (Gassman, 2016). Further, in 2013 pest resistance to Bt crops was reported in 5 of 13 major pest species in a survey of 77 studies from eight countries across five continents, where resistance management requirements and enforcement varied. Practices that delayed resistance to Bt included the Bt crop expressing a high dose of the Bt toxin and an abundance of refuge crop planting (Tabashnik, et al., 2013).  In accordance with integrated pest management principles, entomologists also recommend that other control tactics be utilized to control pests targeted by Bt crops.

In a number of countries (ex. most countries in the European Union), Bt crops and transgenic crops were not approved for commercial production.  Concerns about the potential human health and ecological risks of transgenic crops limited acceptance of Bt crops and other transgenic crops. Applying the precautionary principle, some policy-makers and the public require more research and long-term assessment of transgenic traits on human health and ecosystems.  An interest in protecting domestic seed markets and companies may also contribute to policy decisions to prohibit the adoption of transgenic seeds produced by multi-national seed companies.

Herbicide Resistant Crops

Herbicide-resistant (or tolerant) crops, such as glyphosate-resistant crops are transgenic crops that are resistant to the herbicide glyphosate. Glyphosate is a broad-spectrum herbicide that controls a wide range of plants and breaks down relatively quickly in the environment; it was first marketed under the trade name: Round-up. Round-up Ready soybeans were released in the US in 1996, and since then, additional glyphosate-resistant crops (corn, cotton, canola, sugarbeet, and alfalfa) have been developed and widely adopted in the US and other countries (Fernandez-Cornejo J. and S. J. Wechsler, 2015; Benbrook, 2014; Duke and Powles, 2009). See Figure 8.2.15 on the Transgenic Crops for Pest Control page: Adoption of genetically engineered crops.

Figure 8.2.19: Roundup Ready Soybeans

Credit: Heather Karsten

Herbicide-resistant (HR) crops such as glyphosate-resistant crops have facilitated the increased adoption of no-till or direct seeding of some HR crops because tillage is not needed for weed control. Once a crop has emerged, the risk of glyphosate herbicide damage to the HR crop is eliminated, making it easier for farmers to plant crops and control weeds without tillage. However, although Bt crops reduced insecticide use, the glyphosate herbicide must be applied to glyphosate-resistant crops to control weeds. Since they were first introduced in 1996, glyphosate use has increased. See the Figure 8.2.20 from the USGS Pesticide National Synthesis project below.

In addition, in contrast to Bt crops, the EPA did not require farmers to employ a glyphosate resistance management plan or refuge, and the number of weeds that are resistant to glyphosate has increased. Weeds have evolved resistance to glyphosate particularly in cases where farmers consistently applied glyphosate to manage weeds in HR crops and terminated cover crops and/or perennials with glyphosate prior to planting an HR crop. See the Figure 8.2.21 from the International Survey of Herbicide Resistant Weeds illustrating the increase in glyphosate-resistant weeds below.

Figure 8.2.20: Glyphosate Use in the United States by Year and Crop

Credit: USGS, Nat’l Water Quality Assessment Program, Pesticide National Synthesis project [37]

Figure 8.2.21: Increase in the Number of Glyphosate-Resistant Weeds Worldwide

Credit: Ian Heap. International Survey of Herbicide Resistant Weeds [38].

Stacked Herbicide-Resistant Crops

When the number of glyphosate-resistant weeds increased and became difficult to control, the agricultural-input industry developed transgenic herbicide-resistance crops that are resistant to additional herbicides. Dow AgroSciences developed a transgenic trait for resistance to 2,4-D, an herbicide that controls broadleaf weeds (dicot plants) and the company stacked or added the trait to soybean and cotton crops that also have resistance to glyphosate. And Monsanto produced a transgenic trait for resistance to an herbicide called dicamba that they stacked (or added to) soybeans that have glyphosate resistance. Dicamba and 2,4-D herbicides are volatile, and there is a risk that when the herbicides are sprayed, they will drift into neighboring fields and field edges, potentially damaging other crops and other plants. Wild plants in field edges and natural ecosystems often provide habitat for beneficial organisms, such as pollinators, pest predators, and wildlife. In 2017, Monsanto's crops with stacked dicamba and glyphosate resistance were available for use in some midwestern and southern states, where glyphosate-resistant weeds were particularly problematic. In 2017, there were so many reports and complaints from farmers about crop damage due to dicamba drift, that the states of Arkansas and Missouri banned dicamba spraying for some of the growing season. The EPA also investigated the complaints, and in autumn 2017, the EPA announced that the companies had agreed to new steps to reduce the risk dicamba drift with dicamba-resistant crops. For more information, see the EPA Registration of Dicamba for Used on Genetically Engineered Crops. [39]  

We will explore concerns about the stacked, herbicide-resistant technologies and tactics to manage glyphosate-resistant weeds more in the Summative Assessment.

Plant Pathogens

Pathogens include fungi, bacteria, nematodes, and viruses, all biological organisms that can cause disease symptoms and significantly reduce the productivity, quality, and even cause the death of plants. Pathogens can also infect agricultural animals, but for this module, we will focus on plant pathogens. Read the following brief overview of plant pathogens, Plant Disease: Pathogens and Cycles [40].

Pathogens can be introduced and spread to host plants in many ways. Bacteria and fungal spores can be transferred by wind, in rain, and from the soil via rain splashing onto plant tissues. Insects can vector or infect a plant with a pathogen when they feed on an infected host plant, and then move and feed on an uninfected plant. Pathogens can also spread through infected seeds, transplants, or contaminated equipment, irrigation water, and humans.

Figure 8.2.22: This pumpkin plant is infected with Bacterial wilt (Ralstonia Solanacerarum) that was vectored (introduced) by a cucumber beetle insect.

Photo Credit: Beth Gugino, Penn State University, Associate Professor of Vegetable Pathology

Figure 8.2.23: Center pivot irrigation of crops, such as this canola, could facilitate pathogen infection and disease development via soil-splashing and by creating high humidity in the plant canopy that could favor some pathogens.

Photo Credit: Chad Swank, courtesy of USDA Natural Resources Conservation Services, via Wikimedia Commons [41].

Plant Disease Triangle: Plant pathologists have identified three factors that are needed for a plant disease to develop:

i. a susceptible host Some pathogens have a narrow host plant range, meaning they can infect just a few host species. For instance, the primary host crops of Late blight (Phytophthora infestans) are tomato and potato. For more information see Tomato-Potato Late Blight in the Home Garden [42].  By contrast, pathogens with a wide host plant range can infect many different host species. There are almost 200 plant species that can be infected by Bacterial wilt (Ralstonia solanacearum). For more information, see Bacterial Wilt - Ralstonia solanacearum [43].

ii. a disease-causing organism (pathogen). Plant pathogens include fungi, bacteria, viruses, and nematodes. For examples, again, see the reading: Introduction to Plant Diseases [5], A. D. Timmerman, K.A. Korus. 2014. University of Nebraska-Lincoln. Extension. EC 1273.

iii. a favorable environment for the pathogen. Pathogens usually require specific humidity and temperature conditions for pathogen infection and disease symptoms to manifest. For instance, Late Blight disease symptoms are most likely to occur when the weather is cool and wet.

Figure 8.2.24: Disease Triangle

Credit Heather Karsten

Disease Diagnosis

The three disease triangle factors are important for diagnosing the cause of disease symptoms. Pathologists consider the weather, environmental conditions and the host species to diagnosis what pathogen is causing disease symptoms. Pathologists also consider other factors that could favor and help diagnose a disease, such as i. the field history, particularly what crops and pathogens were present in the past, ii. current crop management practices, iii. when disease symptoms were visible, and on what other species. To assist farmers and others with disease diagnosis, many land-grant universities in the US have crop and animal diagnostic disease clinics where one can submit diseased tissue samples with detailed information that can aid in the diagnosis, such as the host species, environmental conditions, the site history, and management.

Pathogen Management

Although disease control practices could be categorized into the pest control approaches that were discussed earlier for managing insects and weeds (genetic, cultural, chemical, etc.), plant pathologists typically describe pathogen control tactics with more specific language. For instance, Exclusion tactics involve rejecting infected transplants from being introduced to a farm.

Prevention or Avoidance of pathogen introduction and spread tactics include:

• crop rotation, particularly for pathogens with narrow host ranges
• sanitizing equipment for planting, trellising, pruning, and harvesting
• managing for healthy vigorous crops with optimal soil and water nutrient management
• managing to avoid environmental conditions that promote pathogens, such as avoiding very humid conditions due to over-watering, or promoting drying of plant surfaces with wide row spacing to facilitate air flow, or using drip-tape irrigation that waters plants at or below the soil surface versus over the canopy
• using physical barriers such as row covers, mulch to reduce water splashing; and high tunnel/hoop houses or greenhouses to prevent the introduction of rain and wind-borne pathogens

Figure 8.2.25: Multiple pest control practices on this organic farm help prevent pathogen infection and spread, while also helping to control insect pests and weeds. Wide row spacing allows for inter-row weed cultivation as well as airflow to reduce crop canopy humidity; crop rotation and intercropping plants from different plant families interrupts pathogen and insect spread; and straw mulch that prevents soil-borne pathogens from splashing onto plants also suppresses weeds.

Photo Credit: Heather Karsten

Figure 8.2.26: Drip tape irrigation under plastic mulch avoids splashing soil-borne pathogens onto crops and is also a more efficient use of irrigation water.

Plastic mulch also raises soil temperatures, promoting crop growth and helps to suppress weeds.

Photo Credit: Elsa Sanchez, Penn State, Professor of Horticultural Systems Management, Dept. of Plant Science

Figure 8.2.27: High tunnels, plastic mulch, and sanitized stakes all avoid introducing pathogens to these tomato plants, while also promoting crop growth.

Photo Credit: Elsa Sanchez, Penn State, Professor of Horticultural Systems Management, Dept. of Plant Science

Genetic resistance to pathogens is a very valuable and important pathogen control tool. Many plant breeding programs select for genetic resistance to pathogens. When available, pathogen resistance traits are included in most crop variety descriptions to help growers select appropriate crop varieties for their farm.

Figure 8.2.28: This tomato variety trial included tomato varieties that were resistant or susceptible to late blight.

Photo Credit: Beth Gugino, Penn State, Associate Professor of Vegetable Pathology.

If disease symptoms develop, infected plants may be Eradicated or destroyed. And materials that may have been contaminated with pathogens, such as the soil and planting containers, can be heated to very high temperatures with pasteurization equipment or through solarization. For instance, soil may be solarized by placing black plastic over the crop bed (planting zone) during the warm season to increase the soil temperature and destroy pathogens prior to planting the crop.

Therapy or Fungicides (chemical control) may be applied to infected plants to terminate pathogens. Particularly when plant pathogen symptoms are identified early and favorable weather conditions for the pathogen are projected to continue, fungicides can prevent disease spread and significant economic losses. In some high-value crop systems, the soil may be fumigated prior to planting crops.

Similarly, in agricultural livestock systems, animals with disease symptoms can be treated with antibiotics. And in some livestock production systems, antibiotics and vaccinations are administered to animals to prevent diseases and pathogen infection.

Summative Assessment: Herbicide-Resistant Weed Interpretation and Management of Multiple Pest Types

Note

Explore the International Survey of Herbicide Resistant Weeds [45] and answer the questions below. On the International Survey of Herbicide Resistant Weeds Homepage, examine the trends of herbicide-resistant weeds. In the left column chose "Summaries" by "US State Map" and by "Country". By moving your cursor over the states and scrolling down the list of countries, compare the number of herbicide-resistant weed species across a range of geographical regions. You can also view the data as a “Global Map.”

You will also use the following information in the proposed scenario:

Dow AgroSciences developed a transgenic trait for resistance to 2,4-D, an herbicide that controls broadleaf weeds (dicot plants), that has been transferred to soybean, corn, and cotton crops. The trait is stacked or added to soybeans that also have resistance to glyphosate, and another herbicide called glufosinate.

Monsanto has produced a transgenic trait for resistance to an herbicide called dicamba, that they are stacking (or adding to) soybeans that have glyphosate resistance. Some formulations of the dicamba herbicide are volatile, and there is a risk that when farmers spray dicamba it will drift into neighboring fields and field edges, potentially damaging other crops and wild plants in field edges and natural ecosystems. These field edges and other plants often provide habitat for beneficial organisms, such as pollinators, pest predators, and wildlife.

Please also read these short NPR articles on dicamba:

With ok from EPA use of controversial weedkiller is expected to double [46]

A wayward weed killer divides farm communities harms wildlife [47]

If you are interested in more information about the use of dicamba and Arkansas' recent restrictions on the herbicide you may read or listen to the following brief (3-minute) story: Arkansas defies Monsanto moves to ban rogue weed killer [48]

Directions

Read the following scenario and in approximately 450-500 words answer the questions below. I suggest you write your response in a separate document and then copy and paste it into Canvas. Once you have posted your own answers, you need to respond to ONE classmate. Your response should be approximately 150 words.

Assume you manage a 200-acre corn and soybean farm in Southern Pennsylvania. You keep up with the latest technological advances in farming and use seeds from either Dow or Monsanto depending on what your seed salesperson recommends. You are proud of your farm and strive to keep your crops free from both weeds and harmful insects that could damage your crop and cut into your profits. Your immediate neighbors on the East side have a large organic vegetable production farm. Based on what you have observed on the above website, the information mentioned above, and from what you have learned through the readings in this module, answer the following questions:

• What are some potential problems you might encounter if you adopt seeds with the new herbicide traits listed above? How do these problems differ between now and in the future?
• What other weed-control strategies could you use to control glyphosate-resistant weeds?
• How might your pest management system affect your neighbors?
• Are there strategies you can use to mitigate potential harm to your neighbors’ fields?
• If you were farming in Thailand or Egypt instead of in Pennsylvania, what might explain differences in the number of herbicide-resistant species you encounter there compared to your farm in the United States?
• If you farmed in Canada, Australia, or Western Europe what might explain the herbicide-resistant species you encounter there?

Consider the following possible questions when responding to a classmate:

• How do their answers differ from yours?
• Are there suggestions you can make to help them improve their IPM practices-for weeds as well as insects?
• Do they have possible solutions you could use on your farm?
• Is there a question you have about why or how they answered the way they did?

Files to Download

Module 8 Summative Assessment Worksheet [49]

Submitting Your Assignment

Submit your response in Module 8 Summative Assessment Discussion in Canvas.