Chemical Control Studies of the Walnut Caterpillar¹

Materials and Methods

Experiments 1 and 2 were conducted at the Dixon Springs Agricultural Center, Simpson, III. Experiment 1 consisted of selecting 3 lower branches on each of 5 black walnut trees ca. 40 ft tall. The selected branches were shaken vigorously to remove any insect predators. Foliage infested with a specific number of 2nd instars was placed on each branch. The distal ca. 6 ft of each branch containing the larvae was then enclosed in a fiberglass bag. The open end of each bag was closed and tied to the branch with string. The larvae were allowed to feed for 2 days. On Aug. 10, 1976 each bag was removed prior to treatment. Using a 1-gal knapsack sprayer, foliar sprays were applied onto the selected branches and larvae using the following insecticides with formulation rate indicated for 100 gal water: acephate (Orthene) 75SP 1.0 lb, Bacillus thuringiensis Berliner (Dipel) WP 0.5 lb, diazinon 25EC 1.0 qt, malathion 50EC 1.0 qt. Each treatment, including a check, was applied to 3 branches. The bags were then retied. On Aug. 12 each bag was removed and the number of live and dead larvae was counted on the branches, foliage, and in each bag.

Experiment 2 was the same as Experiment 1 except that different branches were selected and 4th instars were used. The larvae were placed on the branches on Aug. 13. Treatments were applied on Aug. 20. On Aug. 24 the number of live and dead larvae were counted in each bag.

Results and Discussion

In Experiment 1, larvae treated with acephate, malathion, and diazinon sprays immediately became very active inside the bags and many dropped from the foliage. Some larvae which dropped were able to exit the bags through the screen mesh. B. thuringiensis (Dipel) treated larvae ceased feeding and did not show increased activity. Two days after treatments were applied, all larvae sprayed with insecticides were dead (Table 1). Unaccounted larvae probably escaped through the screen mesh.

In Experiment 2, larvae reacted as in Experiment 1. All larvae were dead 1 day after being treated with acephate (Orthene), malathion, and diazinon (Table 2). Some larvae were observed to be alive in the B. thuringiensis (Dipel) treatments 3 days after being sprayed, but all were dead on the 4th day. Wheel bugs, Arilus cristatus (L.) were often observed resting and occasionally seen ex tending their mouthparts through the bags feeding on the larvae. The dead larvae in the check treatments were probably attacked by these bugs.

None of the insecticide treatments produced evidence of phytotoxicity on black walnut. Presently, of the 4 foliar sprays used, only acephate (Orthene) is approved by USEPA for use on black walnut for walnut caterpillar control.