Fourth International Bemisia Workshop International Whitefly Genomics Workshop
Monitoring Bemisia Resistance to Neonicotinoid Insecticides in Floriculture Crops
Department of Entomology, Texas A&M University, College Station, TX, USA. Correspondence: c-bogran@tamu.edu
Bemisia whiteflies continue to be key pests of floriculture crops, but the availability and efficacy of neonicotinoid-based control programs in the last 10 years have reduced their pest status in the industry. However, specific operational factors in greenhouse production increase the risk of resistance selection to neonicotinoid insecticides. First, the high value of ornamental plants allows growers to use insecticides at frequencies that would be un-economic in other cropping systems. Second, neonicotinoid insecticides can be applied as foliar sprays, root drenches or through irrigation systems. And third, their efficacy as both residual-systemic and as contact-foliar insecticides gives them great versatility but also renders them particularly prone to overuse. The observed patterns of resistance evolution to neonicotinoid insecticides in Bemisia populations affecting greenhouse crops in Europe and the recent introduction of several neonicotinoid insecticides for the floriculture industry indicate the need for resistance management strategies. The long-term objective of our work is to develop an insecticide resistance monitoring program for floricultural crops. The objectives of the present study are to initiate the process by focusing on a high risk pest-host plant complex, Bemisia on Poinsettias. Our specific objectives are to 1) obtain baseline susceptibility data for neonicotinoid insecticides in Bemisia whiteflies attacking floricultural crops; 2) develop and implement neonicotinoid insecticide resistance detection and monitoring programs for floricultural crops; and 3) develop and distribute neonicotinoid resistance management information for floricultural crops using existing extension education programs. Here we report results of preliminary bioassays to determine baseline susceptibility to imidacloprid in a B. argentifolii laboratory colony. A Bemisia argentifolii colony that has been maintained in the laboratory since 2000 without exposure to insecticides was used as a source of susceptible insects. Solutions containing from of 0 to 100 ppm imidacloprid (Marathon® II, OHP Inc. Mainland, PA) were used to drench cotton seedlings growing in soil-less media in 6 inch pots (soil systemic bioassay). Cotton was used as the host plant because it is easily grown from seed under laboratory conditions and for consistency with previously published methods. Seedlings with two true-leaves were cut at the base of the stem at 24, 48 and 72 hours after insecticide application to determine the effect of up-take time on lethal concentration (LC) values. Single treated seedlings were placed into micro-centrifuge tubes filled with distilled water to provide moisture and then placed inside 150 mm (diameter) × 25 mm (depth) Petri dishes (Falcon®, Becton Dickinson and Co. Lincoln Park, NJ). Twenty adults were introduced into each Petri dish and the dishes were sealed with parafilm (Parafilm®, American National Can™, Neenah, WI) to avoid whitefly escape. Each concentration was tested 4 to 8 times (one dish = one replicate) at each of the three up-take time treatments. Whitefly mortality was assessed 24 hours after exposure to treated seedlings. Probit analyses were conducted to determine LC50 and LC90 values. As expected, whitefly mortality increased linearly with imidacloprid concentration. LC50 values were 25.4, 1.2 and 0.2 ppm for bioassays with uptake times of 24, 48 and 72 hours, respectively. Similarly, LC90 values were 69.6, 26.8 and 5.4 ppm for bioassays with uptake times of 24, 48 and 72 hours, respectively. These values are lower than those previously reported from soil systemic bioassays but similar in magnitude to those reported from leaf-disk bioassays. Based on the slope of regression lines and the variability around LC50 value estimations in Probit analyses, an uptake time of 48 hours will be used in subsequent bioassays. The challenges and opportunities of resistance monitoring and development of insecticide resistance management strategies for floricultural crops will be discussed.