Bioassay, Bacillus Thuringiensis Applied in Different Amounts of Water, 1993

Using a leaf-disk bioassay, B. thuringiensis products applied in different amts of water per acre were evaluated for their effect on PLR larvae. B. thuringiensis products were applied to 10-yr-old spur type ‘Red Delicious’ (Oregon Spur II) on seedling roots that had not been sprayed with conventional insecticides while leaves were present. Treatments were applied at their recommended field rates (see table), carried in water of 400, 100 and 50 gal/acre (gpa), using a conventional speed sprayer operating at 200 psi. Each treatment was replicated three times with four trees in each. Leaves were collected from the interior canopy of treated trees at 1, 3 and 7 d after treatment. One punch (2.3 cm diameter) was taken from each of 20 leaves per treatment on each date. Four punches were placed in a petri dish (Falcon 1006, 50 × 9 mm). Petri dishes were chosen randomly, and five 1- to 2-d-old leafroller larvae were placed on the leaf disks. The petri dish lids were put in place, and dishes were placed inside a food storage container with a moist paper towel to maintain high humidity and kept at 75°F (±2°F) constant temperature and 16:8 photoperiod. Petri dishes were examined after 7 d and larval survival recorded. Five dishes were set up in each replicate (75 larvae per treatment).