4.What were the most significant accomplishments this past year?
This report serves to document research conducted under a trust fund agreement between ARS and the Rural Development Administration of South Korea. Seventy-six strains of the bacterium, Bacillus thuringiensis (Bt) have been screened for toxicity against the sweet potato whitefly (SPWF), Bemisia tabaci and 39 of these have been tested for activity against Colorado potato beetle (CPB). Bt strains were cultured on agar plates, sub-cultured to check for contamination, examined for the presence of crystals and then collected in water. In bioassays, filtered preparations were added to the whitefly and CPB diets, and percent survival after 2, 4, 6, 8 and 10 days of incubation (SPWF) or, after 24, 48, 72 and 96 hours (CPB) was recorded. Of the 76 strains of Bt tested, two were mildly toxic to CPBs and 12 produced compounds that were highly toxic to whiteflies. For these 12 Bt preparations, percent survival of whiteflies on day 4 of treatment ranged between one and 27% while percent survival of control whiteflies was typically greater than 50%. Expressed as percent of control, whitefly survival was less than 10 % after treatment with four of the Bt strains and between 12 and 18% after treatment with five of the strains. For the remaining three toxic Bt preparations, whitefly survival was between 35 and 56 percent of control values. For active Bt strains, percent whitefly survival was also assessed in experiments in which the Bt preparations were boiled prior to addition to the whitefly diet. Boiling destroyed the toxicity of 10 of the Bt strains indicating that the active toxins in these strains were not exotoxins, ATP analogs that universally inhibit the release of energy when ATP is converted to AMP and therefore, are not suitable for use in insect control. Therefore, 10 of the strains contain compounds that may be useful as biopesticides to manage populations of whiteflies and other sucking insects. Experiments to continue the characterization of active molecules and to test additional strains for activity against CPB are in progress.