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Issue No. 1, Article 3/March 21, 2008

Will Economic Thresholds for Making Insect Control Decisions Be Lower in 2008?

Determining whether an insect control product (usually an insecticide) is "needed" is one of the basic principles of insect pest management. Need can be defined in a number of ways, but most people associate the need for an insecticide with economics. In other words, most people ask some form of these questions: "If I elect to use an insecticide, will I get a return on my investment?" or "Will the value of yield protection with an insecticide offset the cost of control?"

Over several decades, these questions helped form the foundation for the principles of economic injury level and economic threshold. In simple terms, the economic injury level is the point (of injury or insects) at which the value of expected crop loss equals the cost of control. Economic thresholds are the practical cousins of economic injury levels; they have been in wide use in IPM programs for a long time. One widely accepted definition for economic threshold is the population density (or level of injury) at which control action should be initiated to prevent an increasing pest population (or injury) from reaching the economic injury level.

Over time, some authors have chosen to use the term "action threshold" to describe the nature of the principle more directly. If you want to read a well-developed explanation of these basic IPM principles, refer to L. P. Pedigo's chapter "Economic Thresholds and Economic Injury Levels" in Radcliffe's IPM World Textbook at ipmworld.umn.edu/chapters/pedigo.htm.

Pedigo explains the variables that are essential for determining an economic injury level (EIL), which can be determined by the equation EIL = C/VIDK, where:

  • C = cost of control (e.g., per acre)
  • V = market value of the crop (e.g., per acre)
  • I = injury units per insect
  • D = damage (proportion of yield loss) per injury unit
  • K = proportional reduction in insect injury

The question of whether economic thresholds for making insect control decisions will be lower in 2008 refers directly to V, the market value of the crop. Most people know that as the value of the crop increases, economic thresholds decrease. If you are so inclined, you can run the figures in the equation for yourself to see how crop value influences an economic injury level. Or you can simply ponder what triggers pest control actions for high-value crops such as apples, sweet corn, and other fruits and vegetables. Commodity prices for most of our field and forage crops in the Midwest have increased significantly over the past year, so many people are wondering whether economic thresholds should be lowered.

The answer would be so simple if we could state that if the commodity price doubles (e.g., the value of soybeans increases from $6 to $12 per bushel), the economic threshold should be halved. Unfortunately, the relationship between insect numbers and amount of economic damage typically is not linear, so simple math doesn't usually address the issue appropriately. Consider, for example, that the economic threshold for soybean aphids is 250 per plant and the economic injury level is approximately 675 per plant. The conservative economic threshold takes into account aphid population doubling time, allowing time for a grower to schedule an insecticide application. However, if you divide the economic injury level by 2 to accommodate the increased value of soybeans, the "revised" economic injury level is about 340 aphids per plant, still greater than the economic threshold of 250 aphids per plant.

So what are we entomologists to do in this period of high prices for alfalfa, corn, soybeans, and wheat? This is not simple to answer. As indicated by a respected colleague, "lowering thresholds nominally into ranges for which we have no damage relationships (pest density:yield loss) is not a light decision." The truth is, the research required to develop an economic injury level and an associated economic threshold is demanding, requiring significant time (i.e., more than one growing season) and resources, and many of the variables often cannot be assessed with precision. We also know that research conducted to develop economic thresholds may be time sensitive, meaning that the research is a snapshot at the time it is conducted. After economic thresholds are established, many variables may change, including yield potential, plant populations, and other input costs. Consequently, most economic thresholds are not updated regularly, if at all.

Our work is cut out for us. We understand the driving forces associated with insect control decisions when commodity prices are high, but we also are aware of unintended consequences, such as ecological disruptions, that might occur if applications of insecticides justified solely by economics become common and widespread. During the week of March 24, entomologists from the Midwest will gather in Columbus, Ohio, at the annual meeting of the North Central Branch of the Entomological Society of America. Questions about commodity prices and economic thresholds will be the focus of many discussions. We hope to gain insights and share some of the discussions with you in future articles of the Bulletin.--Kevin Steffey and Mike Gray

Kevin Steffey
Mike Gray

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