Susceptibility of Different Instars of the Spruce Budworm (Lepidoptera: Tortricidae) to Bacillus thuringiensis var. kurstaki Estimated with a Droplet-Feeding Method

Abstract

We modified a neonate droplet-feeding technique to investigate lethal dose requirements for the spruce budworm, Choristoneura fumiferana (Clemens), without the confounding influence of instar-specific feeding rates and feeding behavior. Individual 4th, 5th, and 6th instars consumed known doses of 3 commercial Bacillus thuringiensis variety kurstaki products suspended in a buffer containing sucrose. The droplet volume was adjusted for each instar to ensure complete uptake. Experiments with 32p indicated that 98-99% of the presented volume was taken up during imbibing and that 89-98% of this was actually ingested. The LD50s expressed in international units per larva ranged from 1.6 to 1.8 for 4th, 2.2 to 2.9 for 5th, and 5.1 to 7.5 for 6th instars. The decrease in larval susceptibility in later instars was not caused by an increase in larval weight; on a per unit weight basis, 6th instars were 4.5- fold more susceptible than 4th instars (0.32 versus 1.44 IU/mg fresh weight). The relationships between larval stage and susceptibility was the same for Dipel 48AF, Foray 48B, and Foray 76B. Droplet sizes that are theoretically required to deliver a lethal dose in the form of 1 droplet per spruce needle ranged from 55 to 96 µm for an LD50 and from 129 to 192 µm for an LD95, depending on instar and potency of the product. These calculations suggest that droplets in the size range that is most commonly encountered on coniferous foliage after aerial application contain at best little more than an LD50.