Leaf-Dip Bioassays of Chitin Inhibitors and Abg-6178 for Control of Colorado Potato Beetle on Potatoes, 1991

All bioassays used larvae aged 36 ± 12 h from a laboratory strain of Colorado potato beetles reared on greenhouse-grown ‘Norland’ potatoes. The entomopathogen Beauveria bassiana (Balsamo) Vuillemin was assayed as both a technical powder (ABG-6178) and as aerial conidia grown in the laboratory. The fungus was cultured for 10 to 20 d on Sabouraud‘s dextrose agar after isolation from adult Colorado potato beetle cadavers previously infected with ABG-6178. Treatments were applied by submersing terminal potato leaflets (of about 10 cm2) for 4 s in a test concentration. A wetting agent, Tween 80 (0.05% concentration), was used to prepare all dilutions. Controls were treated with 0.05% Tween 80 alone. Treated leaves were allowed to drain on absorbent paper for about 60 s and placed into individual assay chambers to air dry. Four to five larvae were then placed on each treated leaf with a camel hair brush. Assays were repeated 2 to 4 times. Assay chambers were made of two stacked 60 x 20 mm plastic petri dishes. The top petri dish was lined with moist filter paper. A hole drilled through the dish base allowed the leaf petiole to extend into the lower petri dish which held a distilled water reservoir to maintain leaf turgor. Assays were kept in a growth chamber at 23°C, 70-80% RH and 18:6 L:D. Leaves were replaced after 48 h and, thereafter, as necessary. Mortality was counted at 24 h intervals after 3 to 7 d of exposure. Individuals not responding to tactile stimulation were considered dead. Data were corrected with Abbott‘s formula and analyzed using probit analysis (POLO-PC). Not all treatments were assayed on each day.