Whitefly Growth Regulators: A Field Sampling Protocal for Nymphs

We developed a sampling protocol for nymphs of the sweetpotato whitefly for timing the application of insect growth regulators. These plans require counting the number of large, visible nymphs within a 3.88 cm2 leaf disk on the fifth main stem node leaf down from the terminal. Use of insect growth regulators for white fly control is recommended when whitefly densities from 30 plants average 0.5 1.0 large nymphs per disk and 3 -5 adults per leaf. We evaluated these sampling and decisionmaking plans within a large -scale field experiment. Precision was adequate for densities of 1.0 large nymph per leaf and greater, but sample sizes greater than 30 are needed for lower densities. The ability of samplers to detect and categorize nymphal instars and sampler -tosampler variation in this ability were significant sources of variation. A binomial (presence /absence) sampling plan may diminish sampler -sampler variation while increasing efficiency and accuracy of decisionmaking. Introduction Careful monitoring of pest density for timing control is a key to whitefly management. Since 1994, Arizona pest managers have used a leaf -turn' method for sampling whitefly adults (Ellsworth et al. 1995; Naranjo et al. 1996). This method for sampling adult whiteflies is a reliable and efficient technique for estimating whitefly abundance and timing control activities (Diehl et al. 1995; Ellsworth et al. 1996a; Naranjo, 1995; Naranjo et al. 1995). However, with the introduction of two new insect growth regulators (IGRs) for whitefly control, sampling needs in Arizona cotton have changed. Because the IGRs ApplaudTM and Knack® target immature whiteflies, an estimate of nymph densities in addition to adult densities is needed for timing use of these compounds. We developed and evaluated sampling plans for whitefly nymphs in cotton. These plans were adapted from Naranjo & Flint (1994) and are outlined in Ellsworth et al. (1996b). A provisional threshold of 0.5 -1.0 large nymphs per 3.88 cm2 leaf disk and 3 -5 adults per fifth main stem leaf was derived from efficacy testing in Arizona (for example, Ellsworth et al. 1994) and compared to experience with these IGRs in Israel (Horowitz, personal communication). We evaluated these sampling and decision -making plans as part of a commercial -scale whitefly management trial in 1996.