Food and the Future Environment

Understanding Economic Thresholds


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. 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. 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.

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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?

IPM graph showing the pest population density over time
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

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Line A: Damage- economic injury level (too late for control), Line B: Prevent damage- economic threshold (control needed), Line C: population equilibrium (control not needed) in balance with natural enemies
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?

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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 (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?

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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.