P.G. Fast, E.G. Kettela, C.J. Wiesner
The objectives of this study were to assess the efficacy against spruce budworm of three different concentrations of B.t. products, by determining regression lines of foliar deposit vs. efficacy for each concentration. Each concentration was sprayed on a 100 ha. block consisting of thinned 20 year old balsam fir. At each of 180 sample sites per block, the following factors were evaluated:
(a) prespray population,
(b) foliar deposit (drops/needle),
(c) post-spray surviving population,
(d) post-spray defoliation.
Regression lines of foliar deposit vs. post--pray survival and defoliation were established for each block.
Good correlations between increasing deposits and increased field mortality were obtained; however, correlations with decreased defoliation were even stronger. Higher concentrations gave clearly greater efficacy. The highest concentration tested (48 BIU/gal.) gave marked reduction in survival as well as foliage saved, even at low deposit levels (< 1 drop/2 needles). The data suggest that concentrations greater than approximately 50 BIU/gal. may not provide commensurate increases in efficacy.
The experiment should be repeated in 1985 and include a commercially available 64 BIU/gal. preparation in order to: (1) further confirm the observed trend, (2) test whether 64 BIU/gal. concentrations yield any benefit, (3) test year to year variation.