Although the economic injury level (EIL) and the economic threshold (ET) are fundamental integrated pest management (IPM) concepts, they are often confused. This is understandable, as the concepts are closely related and the definitions of EIL and ET can be expressed differently depending on context.  You will see various versions of the following definitions throughout the literature.

Economic injury level. The smallest number of insects (amount of injury) that will cause yield losses equal to the insect management costs.

Economic threshold. The pest density at which management action should be taken to prevent an increasing pest population from reaching the economic injury level."

Source: Entomological Society of America "Handbook of Soybean Insect Pests," Leon G. Higley and David J. Boethel, eds

EILs are usually expressed as a pest density and are developed from yield-loss relationships derived from field research studies. The EIL has been described as the break-even point, the level of pest a plant can tolerate, among other things. The main thing is that we want to manage the pest population before it reaches the EIL. That is where the ET comes in.

The ET is the practical rule used to determine when to take management action. In fact, some refer to the ET as the action threshold. It is essentially a prediction of when a pest population is going to reach the EIL.  It is assumed that once the ET is reached, there is a high probability that the pest population will reach the EIL if no management action is taken.

Establishing an ET is complex. It incorporates the EIL as well as other factors which require a thorough understanding of the pest's population dynamics in the specific crop. Pest and crop phenology, pest population growth and injury rates, and the practical aspects of management tactics all have to be considered when establishing ETs.

Unfortunately, a complete understanding of the pest and crop biology and ecology is rarely available, and a certain level of subjectivity is part of establishing ETs. For example, sometimes the ET is simply set at 80% of the EIL, as it is with bean leaf beetle on soybean. In this case the ET is relatively close to the EIL.  On the other hand, for an insect such as the soybean aphid that has an exponential population growth rate, the ET is well below the EIL and at a level where no damage occurs. 

Because ET development is complex and dependent on the specific agro-ecosystem, one should have an understanding of how ETs are determined before altering them. Simply raising or lowering ETs to account for changes in the market may not be appropriate.

For a discussion on the use of economic thresholds, go to "Economic Thresholds for Today's Commodity Values," which is an article adapted from the Proceedings of the UNL Crop Production Clinics 2009. And for an excellent and thorough discussion of economic thresholds, see the book "Economic Thresholds for Integrated Pest Management," Leon G. Higley and Larry P. Pedigo (eds).

Tom Hunt
Extension Entomologist, Haskell Ag Lab, Concord