SYNERGISM BETWEEN NATURAL ENEMIES AND BIOPESTICIDES - A TEST-CASE USING THE STINKBUG PERILLUS-BIOCULATUS (HEMIPTERA, PENTATOMIDAE) AND BACILLUS-THURINGIENSIS-TENEBRIONIS AGAINST COLORADO POTATO BEETLE (COLEOPTERA, CHRYSOMELIDAE)

Sublethal effects of Bacillus thuringiensis on pests could synergize w ith natural enemies through extended larval development and reduced de fense against predators. Mortality ranged from 18 to 76% for 4th insta r Colorado potato beetles feeding on potato foliage dipped in M-Trak a t 0.2-3.2 ml/liter H2O. Survivors resumed feeding: in 48 h, but feedin g was prolonged and time to pupation was extended 33-46%. Predation su ccess by small nymphs of Perillus bioculatus was 40-77% on 4th instar potato beetle larvae that survived M-Trak exposure compared to only 3- 40% on healthy lih instars. Thus, susceptibility to predation as well as extended development of larvae constitute mechanisms Ly which B. t. tenebrionis could synergize with predators in increasing mortality. T his prediction was tested in 1994-1995 using field releases of P. bioc ulatus followed by applications of low doses of B. t. tenebrionis. At rates of 1-4 per plant in both years, P, bioculatus destroyed 17-55% o f potato beetle eggs compared to 1.6-4.9% in control plots. In 1994, p otato beetle larval mortality could not be related to rates of either control or their combination due to early P. bioculatus dispersal at l ow larval prey density. However, interaction between P, bioculatus and B. I tenebrionis was significant in 1995 in combinations of P. biocul atus (1-2 per plant) and Novodor (2 liters/ha). the interactive effect causing 31% more larval mortality than expected from simple additive effects. The 2-yr field results indicate that synergism of B, thuringi ensis with the egg-larval predator P. bioculatus in killing potato bee tle larvae would critically depend on enough beetle eggs escaping pred ation to locally retain the predator during larval development.