Effects of Simulated Acid Rain on Bacillus Thumngiensis Residual Against GYPSY Moth, New Haven County, Connecticut, 1990

Three-yr-old potted white oak seedlings were sprayed with SAN 415 SC 32LV formulation of the NRD-12 strain of B. thuringiensis Berliner, at 1.6 and 8.0 billion international units (BIU)/acre equivalent. Bond at 2% v/v was used as a sticker. At 1, 3, and 7 DAT, plants were subjected to simulated acid rain (SAR) episodes (pH 3.0; pH 4.2; none) to determine if insecticidal efficacy of B. thuringiensis against 2nd-instar GM larvae was affected. Each SAR event delivered 1.3 cm (0.5 inch) in 40 min (3.8 cm; 1.5 inch total). SAR solutions were made by adjusting the pH of Millipore grade water with 2 parts H2S04 and 1 part HN03. SAR mineral content was adjusted to reflect average content of ambient rain for the Northeast. Seedlings were sprayed with B. thuringiensis and treated with SAR using an enclosed spray tower (3.5 m diam; 6.0 m ht) equipped with a rotating platform (10 rpm). A modified Beeco Mist spray nozzle rotated at 75-100 rpm and was positioned 3.7 m (12 ft) above the plants. For the 1.6 and 8 BIU dose, the spray droplet size ranged from 640-1800 um in diam; the volume median diam was 1332 urn. Spray deposit density averaged 2.3 drops/cm2 for both doses. Plants were housed in a shaded greenhouse before and after treatment with SAR. At the appropriate DAT date and SAR episode, a small stem containing 5-7 leaves (bouquet) was harvested from each seedling and placed in a bioassay chamber where 10 GM larvae were placed on each bouquet and allowed to feed for 5 d. Percent larval mortality was determined on each bioassayed bouquet and pooled for each B. thuringiensis spray/SAR episode combination. Data were analyzed by ANOVA. Means were separated by DMRT and data transformed by arcsine before analysis.