Fourth International Bemisia Workshop International Whitefly Genomics Workshop
Novel Candidates for the Development of Biopesticides to Control Whitefly Pests
1 Insect Biocontrol Laboratory, USDA, ARS. Beltsville, MD, USA. Correspondence: gelmand@ba.ars.usda.gov
2 Western Cotton Research Laboratory, USDA, ARS, Phoenix, AZ, USA
The sweet potato whitefly, Bemisia tabaci (Biotype B) causes hundreds of millions of dollars in crop losses each year both in field and greenhouse settings. Reports of pesticide resistance in whiteflies and environmental concerns related to pesticide usage have made the reduction of pesticide application a primary goal for agriculture and have increased the emphasis on the use of cost-effective biological control strategies and insect specific biopesticides in IPM programs. Characteristics associated with the designation biopesticide include: greater safety than conventional pesticides and usually no residue problems. The two classes of biopesticides are biochemical and microbial. The use of biochemical and bacterially-produced insecticidal compounds as control agents for whiteflies and other sucking insects are worthwhile alternative strategies that should be examined. Recently, several monosaccharides as well as toxins produced by Photorhabdus luminescens and a newly discovered bacterium, Chromobacterium substugae, were found to be toxic to whiteflies. Arabinose, mannose, ribose and xylose, when added to artificial diet, were found to kill sweet potato whitefly (SPWF) nymphs and adults. Results showed that there was no substantial inhibition of alpha glucosidase (converts sucrose to glucose and fructose) or trehalulose synthase (converts sucrose to trehalulose) activity by arabinose, mannose or xylose. When adult B. tabaci were fed for 24 h on diets containing U- 14C sucrose, L-[methyl-3H] methionine or inulin-[14C]-carboxylic acid, and one or none (control) of the toxic sugars, the four insecticidal sugars, significantly reduced (by 70 – 80%) the amount of radioactivity incorporated and excreted in honeydew. Thus, we conclude that the four sugars act as antifeedants. When U- 14C sucrose and either arabinose, xylose, or mannose were fed to B. tabaci adults, less radiolabeled carbon dioxide was excreted; thus, respiration also appears to be reduced. Interestingly, melibiose and trehalose, two sugars that are not toxic to whiteflies, also reduced (by 30 – 40%) U- 14C sucrose uptake but not carbon dioxide release. In other experiments, a high molecular weight insecticidal protein complex (Tca) produced by the entomopathogenic bacterium P. luminescens, has been reported to be orally toxic to B. tabaci. When artificially fed Tca, newly emerged nymphs developed poorly, or not at all. Tca concentrations of between 0.1 and 0.2 ppm reduced the number of nymphs reaching the second instar by 50%. In addition, a preparation of Tca missing two prominent subunits, TcaAii and TcaAiii, was found to be at least as toxic to B. tabaci nymphs as Tca itself, indicating that the activity of Tca is not dependant on the presence of these subunits at the time of ingestion. Tca (−Aii and Aiii) administered at 4 ppm to adult whiteflies also reduced mean day survival from 6.7 to 2.6 days. In addition, a recently discovered species of Chromobacterium (C. subtsugae) was found to produce toxins that are insecticidal to whiteflies and other insect pest species. When fed on a diet containing 10% of a C. subtsugae extract, the number of 2nd, 3rd and 4th instar nymphs and emerged adults was significantly lower (15 –34%) in experimental groups as compared to control groups. When adult B. tabaci were fed on artificial diet containing 5% of a cell-free bacterial preparation, 100% mortality was observed after 4 days. Upon fractionation through molecular weight cut-off filters, activity was present in the 3–10 kDa and greater than 300 kDa fractions. Thus, data support the existence of at least two toxins, and although the identities of these toxins have not yet been determined, results show that the toxins are not protein in nature.