Mortality, feeding inhibition, and recovery of spruce budworm (Lepidoptera: Tortricidae) larvae following aerial application of a high-potency formulation of Bacillus thuringiensis subsp Kurstaki

A larval population of spruce budworm, Choristoneura fumiferana (Clemens), was monitored for 5 d following aerial application of a commercial formulat ion of Bacillus thuringiensis Berliner subsp. kurstaki to investigate dose acquisition and expression (larval mortality, recovery, feeding, and growth ) in relation to spray deposition and persistence of spray deposits. The ma in objective was to test if previous laboratory observations on how B. thur ingiensis affects feeding and dose ingestion by spruce budworm larvae hold true under field conditions. About 40% of the treated population ingested a lethal dose within 1 d after spray application. Lethally dosed larvae died without further feeding upon transfer from treated foliage to (untreated) artificial diet. Resumption of feeding by larvae that survived the treatmen t was delayed relative to larvae from the control population during 3 d fol lowing spray application: during that time, normal feeding activity and lar val weight gain were suppressed. Inhibited feeding by survivors appeared to prevent further dose uptake because the proportion of lethally dosed larva e in daily collections did not increase despite significant residual spray deposits in budworm feeding sites. Restoration of "normal" recovery times b y the fourth day coincided with a 65-85% reduction in persistence of the pa thogen on the foliage and did not result in further lethal dose acquisition , as treatment-induced mortality dropped to about 20% on the 4th and 5th da ys. The observations are consistent with previous laboratory observations o f how B. thuringiensis affects larval feeding and with the hypothesis that feeding inhibition may be a limiting factor in the acquisition of a lethal dose.