SUSCEPTIBILITY OF DIFFERENT INSTARS OF THE SPRUCE BUDWORM (LEPIDOPTERA, TORTRICIDAE) TO BACILLUS-THURINGIENSIS VAR KURSTAKI ESTIMATED WITH A DROPLET-FEEDING METHOD

We modified a neonate droplet-feeding technique to investigate lethal dose requirements for the spruce budworm, Choristoneura fumiferana (Cl emens), without the confounding influence of instar-specific feeding r ates and feeding behavior. Individual 4th, 5th, and 6th instars consum ed 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 w ith P-32 indicated that 98-99% of the presented volume was taken up du ring imbibing and that 89-98% of this was actually ingested. The LD(50 )s expressed in international units per larva ranged from 1.6 to 1.8 f or 4th, 2.2 to 2.9 for 5th, and 5.1 to 7.5 for 6th instars. The decrea se in larval susceptibility in later instars was not caused by an incr ease 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 wa s the same for Dipel 48AF, Foray 48B, and Foray 76B. Droplet sizes tha t are theoretically required to deliver a lethal dose in the form of 1 droplet per spruce needle ranged from 55 to 96 mu m for an LD50 and f rom 129 to 192 mu m for an LD95, depending on instar and potency of th e product. These calculations suggest that droplets in the size range that is most commonly encountered on coniferous foliage after aerial a pplication contain at best little more than an LD50.