J. Régnière, E.G. Kettela, K. van Frankenhuyzen
SERG Project # 1999/05 (2000)
Aerial applications of Foray® 96B at high rates (30, 60, and 120 BIU/ha) were made against fifth instar spruce budworm populations, Choristoneura fumiferana (Clem.), in north-central Ontario in the summer of 2000. The objective was to determine the efficacy of such high application rates in terms of population reduction and foliage protection. The trials also provided an opportunity to measure rates of SBW immigration into treated populations from neighboring untreated areas. Deposit was measured, foliage bioassayed, and 48-hour dose-acquisition tests conducted. Actual deposit corresponded well with nominal application rates in all but two of the spray blocks. There was a strong and consistent inverse relationship between survival and actual deposit in 7day bioassays (90% mortality at 200 droplets per gram of current-year foliage). Bacillus thuringiensis (B.t.) was the main cause of mortality in rearing, with parasites and disease representing a small and near constant proportion of deaths. None of the mortality recorded in the field between the prespray and 9-day postspray samples could be attributed to the treatments, implying that mortality from B.t. in these trials took an unusual amount of time to occur. After 30 days, five of the nine treated populations exhibited nearly 90% mortality attributable to treatment. In the remaining four blocks, however, mortality seemed unaffected by the treatments. There was no obvious relationship between mortality due to treatment and actual deposit. On the other hand, foliage protection was highly related to the combination of initial density and actual deposit. End-of season defoliation was directly proportional to initial population density and inversely proportional to deposit. The weather may have been too cold and wet for rapid dose acquisition in the field, and many insects may have entered feeding inhibition and ingested sub-lethal doses of B.t., at least in some of the treated plots. In the plots where it could be detected at all, mortality was very high, regardless of application rate. There was a very strong inverse relationship between apparent fecundity of residual (survivor) populations at the end of the season and egg density. This allowed us to estimate immigration at about 0.204 eggs per bud, which is a high immigration rate considering that 0.2 eggs per bud would lead to defoliation in the vicinity of 20% in the following year. There was also an inverse relationship between deposit and egg density. However, the apparent fecundity data do not support the hypothesis that B. t. had a significant deleterious effect on the reproduction of the survivors.