Economic Injury Levels in Theory and Practice

The topic of economic decision levels probably is most often-discussed issue in economic entomology and insect -pest management today. The fun­ damental questions addressed are: How many insects cause how much and is damage significant? Most entomologists agree that a common decision rule or threshold should answer such questions and that such decision rules are backbone of progressive pest control (3, 46, 64, 79). At issue is form such rules should take and how they should be developed. To date, most widely accepted form is that of economic threshold as presented by Stern et al (80). These authors developed their decision rule from basic principles of economic damage and economic injury level (ElL), and it is from their ElL concept that much of our present theory is derived. The major advantage of this concept is its simplicity and practicality in most situations. The ElL, defined as the lowest population density that will cause economic damage, has been used most often to support management decisions with short-range objectives, i.e. a one-season or less planning horizon at farm level. Additionally, concept has been applied primarily where man­ agement tactics are responsive rather than preventative. Consequently, ElLs have been most developed for occasional and perennial pests [sensu Stern et al (80)] where scouting, evaluation, and subsequent therapy are possible. ElLs have application for all pests, however, when used as criteria of management success. In this regard, they are as applicable to severe pests as to occasional pests and as applicable to preventative tactics as to curative ones. This is because most lPM strategies call for reduction of pest populations to a level below that which is economic; i.e. familiar principle of tolerating pest presence, albeit at noneconomic levels.