Ecosystems have many trophic levels of organisms including primary producers, herbivores, omnivores, carnivores; parasites, and decomposers. Agroecosystems are ecosystems managed for food and fiber production that have less diversity and typically fewer trophic interactions than natural ecosystems. But diverse organisms and their trophic interactions provide important functions in agroecosystems including, for instance, decomposition and nutrient cycling; plant pollination, and pest suppression. Organisms that reduce agricultural productivity and quality and are referred to as agricultural pests; these include weed pathogens, insects, and other herbivorous organisms. Mammals that graze or browse crops (ex. deer and rodents), and other arthropod species such as mites and slugs (mollusks), can also reduce crop yields through grazing and seed predation.
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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Draw a food web pyramid and label the trophic levels as categories of organisms with i. primary producers at the bottom, ii. herbivores next, ii. omnivores and carnivores at the top of the pyramid. Chose a natural ecosystem and list all of the species you can think of that are found at each trophic level in the natural ecosystem. Then draw a second food web pyramid for a type of farm that you are familiar with, and list all of the species you might find at each trophic level. Describe how the natural ecosystem and the agroecosystem compare. How do they differ?
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Odum (1997), an Ecologist summarized some of the major functional differences between natural and agroecosystems that are shown in the table below. Consider how your natural and agroecosystem food pyramids offer examples of the below ecosystem differences. How many predatory and parasitic species are there in the natural ecosystem and agroecosystem? How might the presence of predatory and parasitic organisms impact agricultural pests? How might genetic diversity contribute to pest management and ecosystem stability?
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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.
Property | Natural Ecosystem | Agroecosystems |
---|---|---|
Human Control | Low | High |
Net Productivity | Medium | High |
Species and Genetic Diversity | High | Low |
Trophic Interactions | Complex | Simple, Linear |
Habitat Heterogeneity | Complex | Simple |
Nutrient Cycles | Closed | Open |
Stability (resilience) | High | Low |
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:
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 [1]
Browse the following websites for two major agricultural crop pests. What kind of organisms are they? In what stage of their lifecycle do they cause the most damage to the crop plants?
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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.
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 [6]
Read the following website: Omnivorous Insects: Evolution and Ecology in Natural Agriculture Ecosystems [9].
Then answer the following questions:
What did scientists observe happened to cotton plants and insect herbivores after cotton plants were injured by herbivorous insects?
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To conserve or maintain predatory insects, what is required? What can farmers do to attract and conserve predatory insects?
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Humans have developed methods of insect and pest control for centuries.
Read the following brief history of pesticides and then answer the questions that follow:
Pesticide Development: A Brief Look at the History [10]. Taylor, R. L., A. G. Holley and M. Kirk. March 2007. Southern Regional Extension Forestry. A Regional Peer-Reviewed Publication SREF-FM-010 (Also published as Texas A & M Publication 805-124)
What chemicals were used to control pests from 1700 to the early 1900s?
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When was DDT invented and what was it first used for?
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When and why was DDT banned?
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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.
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 [12] and figure 8 [13]), as well as how difficult and costly it was becoming to develop cost-effective pesticides (see figures 12 and 13 [14] 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.
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:
The Integrated Pest Management (IPM) Concept [15]. D. G. Alston. July 2011. IPM 014-11. Utah State University Extension and Utah Plant Pest Diagnostic Laboratory
Describe three things that are integrated into IPM.
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On the IPM figure below, which IPM pest population terms from the article could describe the lines labeled A, B, and C?
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How would you describe the damage that the pest had caused to the crop at each of these pest population densities?
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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.
What are the potential benefits of scouting for the European red mites and predatory mites in Pennsylvania orchards?
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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.
Narrator: Now another aspect of the overall push for improved farming practices, is how we control pests; and Jane Drinkwater reports on the latest approach to pest control while looking after the environment.
Jane Drinkwater: Australia's crop production systems are forever improving. A prime example is how we manage insect pests. Where once broad-spectrum, often highly toxic, insecticides were used to blanket eradicate insects, there's a move towards a more holistic approach, and with good reason. Integrated pest management, or IPM, presents a win-win, less damage to the environment and to your hip pocket.
Rowan Peel (Mount Pollock VIC: I love the environment and I want to look after the environment, but I have to make a living. IPM has given us the opportunity to do all of these things, both look after the environment and to make more money.
Jane: IPM uses multiple strategies to manage insect pests. One of the tactics is to let an army of the insects’ natural predators, or beneficials, fight the battle for you, and that means holding off on the use of broad-spectrum chemicals.
Rowan: I've probably learned that nature has its way of handling things its own way. You just have to give those beneficials that time. And when you understand that when you are using a broad-spectrum insecticide that you might control it straight away, but you'll get another flight straight in. But you've killed all your beneficials, and you've killed beneficials for other pests later on. And some of these beneficials don't have the lifecycle of an aphid. You know, their lifecycle might be only once or twice a year. And so you know, economically, if you look at the long-term, you're a long way worse off.
Jane: For insects without natural predators, or where the ratio of pests to beneficials is high enough to affect yield, strategies include the application of pesticides to problem areas only and the use of chemicals which target the problem pests, without damaging the beneficial insects. Rowan: We actually treat the seed for earwig infestation to give ita protection. But if there is a further problem, and that may well only be in certain areas of the paddock, which we tend to know where they will be, we will make up a brew of wheat, a little lawsben, and a little bit of vegetable oil. And we'll go out and spread just on that area. So as the earwigs are attracted to that bait, rather than all the other insects.
Jane: Peter Enkelmann’s been using IBM for more than a decade. While his beneficials successfully control silver leaf whitefly, there are still a few pests without natural predators.
Peter Enkelmann (“Riverview” Byee QLD): The chemistry that we use here, it takes out the beneficial insects. So the attitude is to delay spraying any product at all basically, apart from very few natural viruses, right through until the very last.
Jane: And using IPM means, when you do need to pull out the big guns, they're more likely to work.
Peter: One of the big advantages is that resistance to our traditional chemistry is just dropped dramatically.
Jane: But how do growers know when to take action? Well thanks to research funded by GRDC, entomologists have data on the density of pests in each crop that will lead to economic damage. Growers measure the number of pests in their fields and only take action once they've reached this threshold.
Hugh Brier (Senior Entomologist, Primary Industries and Fisheries, DEEDI QLD): So the short-term gain is you might avoid unnecessary sprays. Another short-term gain is by not spraying when you didn't need to, you might avoid flaring another pest which is more expensive to control, so that's another benefit. Longer term, if you avoid spraying unnecessarily, you build up beneficials in the whole system and the system is much more stable.
Jane: Fundamental to successful integrated pest management is the ability to correctly identify pests and beneficial species, and to regularly monitor both populations.
Hugh: In row crops, we use a bed sheet. So we'll go and we shake the plants from meter of row and that shakes all the insects out, or a lot of them out onto the bed sheet and you can count them.
Jane: With IPM leading to lower costs and better environmental outcomes, GRDC views it as an important step forward. Apart from funding IPM Research, GRDC also provides information and training for growers.
David Shannon (GRDC Southern Region Panel Chairman): We have run a series of workshops, IPM workshops. We also work with the grower groups so that grower groups can scale up their grower members on the use of IPM.
Jane: And it's well worth getting up to speed.
Rowan: I find the system of IPM very easy because it's not an almost do nothing, but you just don't worry about it anywhere near as much.
Jane: With IBM's effectiveness in controlling insects, while reducing costs both financial and environmental, it's here to stay.
Rowan: IPM for us has cut down our chemical usage, insecticide usage a long way and you feel better for not using it.
If the video does not load for you, go to GCTV2: Integrated Pest Management [18]
Read the Penn State University Potato Leafhopper on Alfalfa Fact Sheet [19].
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.
Module 8 Formative Assessment Worksheet [20]
Please complete the Module 8 Formative Assessment in Canvas.
Links
[1] http://extension.entm.purdue.edu/401Book/default.php?page=insect_anatomy
[2] http://ento.psu.edu/extension/insect-image-gallery/honey-bees/general-honey-bee-images
[3] https://askabiologist.asu.edu/incomplete-metamorphosis
[4] http://extension.entm.purdue.edu/fieldcropsipm/insects/corn-rootworms.php
[5] https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/pests-and-diseases/cotton-pests/cotton-pest-identifcation
[6] http://npic.orst.edu/envir/beneficial/table.html
[7] https://creativecommons.org/licenses/by-sa/2.0
[8] http://www.troutnut.com
[9] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod8/Krimmel%202011-Omnivorous%20Insects_%20Evolution%20and%20Ecology%20in%20Natural%20and%20Agricultural%20Ecosystems%20_%20Learn%20Science%20at%20Scitable.pdf
[10] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod5/Pesticide%20Development%20-%20A%20Brief%20Look%20at%20the%20History.pdf
[11] https://www.canr.msu.edu/cherries/pest_management/how_pesticide_resistance_develops
[12] http://www.nap.edu/read/619/chapter/4#17
[13] http://www.nap.edu/read/619/chapter/4#28
[14] https://www.nap.edu/read/619/chapter/4#36
[15] https://extension.usu.edu/files/publications/publication/ipm-concept'96.pdf
[16] https://extension.usu.edu/files/publications/publication/economic-injury-level96.pdf
[17] https://www.youtube.com/watch?v=h27UzYpjC3A
[18] https://www.youtube.com/watch?v=OiKuPLCLs9g
[19] http://ento.psu.edu/extension/factsheets/potato-leafhopper-alfalfa
[20] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod8/Module%208_FormativeAssessmentWorksheet_RevisedOct2017.docx