Monitoring Whitefly Susceptibility to Applaud

A bioassay developed by one of the authors (MY) in 1993 was used to monitor susceptibility of sweetpotato whitefly to Applaud in five different field locations. Whitefly populations were exposed to from 0 (untreated fields) to 4 (small plot trial) applications of Applaud. Susceptibilities of whiteflies, as measured by LC50s and LC95s, did not increase with exposure to Applaud (0 to 4 applications) nor since baseline measurements of susceptibility were made in 1993. Under current patterns of use (single use), risk of resistance to Applaud appears to be minimal. Introduction Applaud® (buprofezin) was discovered in 1977 by Nihon Nohyaku, first registered in Japan in 1983, and is now registered in over 60 countries (Yasui, 1993). Prior to 1996, Applaud had only been evaluated in the U.S. in experimental plots for efficacy (e.g., Akey & Henneberry 1994; Ellsworth et al. 1994; Natwick 1994a b Palumbo 1994; Watson et al. 1994). In 1996 Arizona received a Section 18 emergency exemption allowing use of Applaud against sweetpotato whitefly [ Bemisia tabaci (Genn.) (Strain B) = Bemisia argentifolii (Bellows & Perring); (a.k.a. silverleaf whitefly)] in cotton. This was the first time Arizona whitefly populations had been exposed to Applaud on a relatively broad scale (ca. 70,000 A). In 1993 while conducting research at The University of Arizona, we developed a simplified Bemisia susceptibility monitoring technique for Applaud (Yasui, unpubl. data). In 1996 in cooperation with The Univ. of Ariz. (PCE) and AgrEvo USA Company (JL & DC), , Nihon Nohyaku (MY) further refined and used this technique to establish baseline data prior to wide scale use of Applaud. Methods Bioassay Methods (brief) As an insect growth regulator, Applaud has no lethal effect on whitefly adults. Applaud, a chitin biosynthesis inhibitor, interferes with the normal molts of whitefly nymphs. Monitoring of nymphal mortality, therefore, is needed, instead of the more common and more convenient adult mortality bioassays (e.g., Prabhaker et al. 1992; Simmons & Dennehy 1996). Nymphal assays are difficult, especially when host material and specialized insect rearing facilities are necessary. In 1993, Yasui developed a simplified susceptibility monitoring technique which obviated the need for "clean" host material or any specialized equipment. With further refinement this past year, we established a protocol that successfully uses foliage and whitefly eggs collected from the field of interest. We determined that the second leaf below the terminal (i.e., 1 leaf below the first folded leaf) contains the greatest concentration of similarly -aged whitefly eggs. Furthermore, we found that holding these leaves for 6 -8 days at 25 °C yielded nearly uniform cohorts of settled 1st instars, the ideal age for initiating the assay. With very simple tools (vials, scissors, water), a dissecting microscope, & reasonable control over near "room" temperature conditions, anyone with interest in a particular field population can conduct this bioassay, though it does require about 21 days to complete.