HORTICULTURAL ENTOMOLOGY Impact of Insecticide Efficacy on Developing Action Thresholds for Pest Management: A Case Study of Onion Thrips (Thysanoptera: Thripidae) on Onion BRIAN A. NAULT 1 AND ANTHONY M. SHELTON Department of Entomology, New York State Agricultural Experiment Station, Cornell University, 630 W. North Street, Geneva, NY 14456 J. Econ. Entomol. 103(4): 1315Ð1326 (2010); DOI: 10.1603/EC10096 ABSTRACT An action threshold (AT) is one of the most important decision-making elements in integrated pest management. Unlike economic thresholds, ATs are not typically derived from an economic injury level model, but they are more commonly used. ATs may be identiÞed from research-based, pestÐ crop relationships, but they also may be based on experience. ATs may be adjusted depending on, e.g., weather and plant variety, but modifying ATs to accommodate differences in insecticide efÞcacy has received little attention. To examine this point, several combinations of ATs and insecticides were evaluated against onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripi- dae), a major pest of onion (Allium cepa L.). Studies were conducted in New York onion Þelds from 2006 to 2008 by using registered insecticides for T. tabaci on onions. We hypothesized that the most efÞcacious insecticides would provide acceptable control of thrips populations regardless of AT (one, three, and Þve thrips per leaf), whereas less effective products would only control populations using the lowest AT (one thrips per leaf). Results indicated that T. tabaci infestations were managed effectively when spinetoram was applied after a three larvae per leaf threshold, but not when using lambda-cyhalothrin, methomyl or formetanate hydrochloride. However, T. tabaci infestations were managed well when methomyl and formetanate hydrochloride were applied after a one larva per leaf threshold. T. tabaci infestations were never controlled using lambda-cyhalothrin, regardless of the AT used. None of the products reduced T. tabaci populations to an acceptable level when applied at a Þve larvae per leaf threshold. Implications of adjusting ATs based on efÞcacy of different insecticides are discussed. KEY WORDS Thrips tabaci, action threshold, insecticide, Allium cepa More than 50 yr have passed since Stern et al. (1959) published their seminal article on the economic injury level (EIL) concept as a basis for integrated pest management (IPM). The EIL is deÞned as the lowest population density that will cause economic damage (Pedigo et al. 1986). Advancements in the theory and practice of EILs in IPM have been described in several prominent publications (Stone and Pedigo 1972; Nor- ton 1976; Poston et al. 1983; Pedigo et al. 1986; Onstad 1987; Higley and Wintersteen 1992; Pedigo and Higley 1992; Higley and Pedigo 1993, 1996; Peterson and Hunt 2003). Over 225 articles on EILs were published by the early 1990s (Peterson 1996), and many more have been published since, reßecting the importance of the EIL concept on IPM. The EIL model most commonly referred to is EIL = C/VIDK, where C is cost of the management tactic, V is the market value per production unit, I is injury units per pest, D is damage per injury unit, and K is the proportional reduction in the pest population (Pedigo et al. 1986). Calculating EILs is difÞcult for many reasons (Hammond 1996), but one of the most prob- lematic reason is describing and interpreting the re- lationship between crop yield loss and insect density or feeding damage (components I and D) (Nault and Kennedy 1998). Many factors can affect this relation- ship, including weather, crop variety, partial plant resistance, crop phenology, and the insect stage tar- geted (Ostlie and Pedigo 1985, Bystrak et al. 1994, Fournier et al. 1995, Rueda et al. 2007). Models have been used to resolve uncertainty in levels of param- eters in the EIL model (probabilistic EIL) (Peterson and Hunt 2003), and they have been modiÞed to consider impacts of additional factors, such as the environment (environmental EIL; Higley and Win- tersteen 1992, Pedigo and Higley 1992, Higley and Pedigo 1993), the potential control by natural enemies (Naranjo et al. 2002, Musser et al. 2006), esthetics (Sadof and Raupp 1986), and plant quality (Hutchins 1986). The economic threshold (ET) is a pest density or injury level below the EIL. Curative control tactics are 1 Corresponding author, e-mail: ban6@cornell.edu. 0022-0493/10/1315Ð1326$04.00/0  2010 Entomological Society of America