ABSTRACT

Summary. Various types of thresholds have been developed for weed management in an attempt to provide a more rational approach to decision making. The economic threshold concept was originally developed for management of arthropod pests, and is based on an understanding of arthropod population biology. Adoption of a management strategy for weeds that was developed for maintaining arthropod populations below a damaging level, referred to as the economic injury level or EIL, is not ecologically sound. Many of the factors that regulate populations of the two types of pest are different. For arthropod management the economic threshold (ET) is defined as the pest population at which treatment should be initiated to stop the population from increasing to the EIL. Weed science has adopted the ET to be the same as the EIL; this leads to maintenance of a relatively high seed bank as weeds at or below the ET density are allowed to produce seed. Research where weed seed production was accurately determined in corn, sugarbeets, alfalfa and other crops is now suggesting that several important weed species should be managed so that they do not produce seed. I am proposing that a new threshold called a no seed threshold, or NST, should be established for such weeds. Application of the ET concept to an invading weed species is disastrous as it leads to establishment of the seedbank before any control action is taken; for an invading species that is expanding its range the use of NST seems more appropriate. Progressive farmers in California have adopted NST for management of weeds. These farmers claim that the strategy is economically superior to that 32using single season ET. In one case adoption of NST for weed management has resulted in decreased reliance on herbicides as weed control can be attained using non-chemical techniques. There is urgent need for weed science to develop improved data on weed population dynamics that is coupled to economics of weed control and crop production; until such data become available it is not feasible to accurately assess the use of thresholds for weed management. [Article copies available for a fee from The Haworth Document Delivery Service: 1-800-342-9678. E-mail address: getinfo@haworthpressinc.com]