Bioassay, Tests of B. Thuringiensis Products Against Overwintering and Non-Overwintering Leafroller Larvae, 1993

Abstract

Abstract Different B. thuringiensis products were evaluated for their effect on PLR larvae from two sources, a non-diapausing laboratory colony and one collected from the field and destined to enter diapause. Egg masses were collected from the field in Sep, and larvae hatching from these were destined to stop development in the second or third instar and produce hibernacula for overwintering. These larvae were used in tests 1-2 d after hatching. The other source of larvae was from a laboratory colony that was reared continuously in a long-d photoperiod of 16:8 (L:D) h. Larvae from this colony would not stop feeding but continue development. Treatments were prepared by diluting the appropriate amt of product (see table) in 500 ml water in a glass beaker. A small amt (about 2 u,l) of wetting agent, X-77*, was added to each treatment. An untreated control was prepared using water plus the wetting agent only. Untreated apple leaves were collected from ‘Red Delicious’ trees at the WSU Tree Fruit Research and Extension Center, Wenatchee. Leaves were dipped, then allowed to dry. Two punches (2.3 cm diameter) were taken from each leaf. Four punches were placed in each 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 and 10 d and larval survival recorded. Five dishes were used for each treatment (25 larvae per treatment per test). Larvae surviving in each test were collected after the last evaluation and stored in alcohol until head capsule widths could be measured. The average head capsule width is an indication of larval age and is thus a measure of sublethal effects of a treatment on larval development. Three tests were conducted using these methods. Rates in the first test were higher than those used in the final two.