Secondary effects of the pesticide Bacillus thuringiensis kurstaki on chicks of spruce grouse (Dendragapus canadensis).

The objective of this study was to determine if application of the biological pesticide Bacillus thuringiensis kurstaki (B.t.k.) had secondary effects on spruce grouse (Dendragapus canadensis) chicks in the southern boreal forest of Ontario, Canada. B.t.k. kills larvae of Lepidoptera that might otherwise be food. This may affect types of food ingested and influence chick growth rates by reducing the availability of a high-protein food. Amount of ingested grit, which is diet-dependent, may also be influenced, which could alter exposure to anthropogenic toxic metals, such as cadmium (Cd) and lead (Pb), associated with the grit. Chick growth rate from 0 to 14 days of age was determined, and gizzard contents were analyzed for types of foods and levels of macronutrients and trace metals in chicks raised in a plot treated with B.t.k. compared to chicks raised on a control plot. Chick growth rates were significantly different (p < 0.05); chicks from the treatment plot increased body mass 30% less as compared to chicks raised on the control plot. Control chicks contained on average 2.5-fold the mass (in g) of grit and nearly twofold the mass of food, which included larvae, in their gizzards compared to those exposed to B.t.k. whose diets were dominated by spiders and ants. Concentrations of magnesium (Mg), calcium (Ca), phosphorous (P), iron (Fe), manganese (Mn), sulfur (S) and the trace metals zinc (Zn), copper (Cu), nickel (Ni), and chromium (Cr) did not differ between experimental and control birds (p > 0.05). In contrast, control chicks had higher levels of aluminum (Al), Cd, cobalt (Co), and Pb in their gizzards than chicks from the sprayed area (p < 0.05), attributed to greater amounts of consumed grit. Although birds from the sprayed area had lower levels of the toxic metals Pb and Cd in their gizzards, a protein-deficient diet could have been a factor that contributed to decreased rates of growth in B.t.k.-exposed birds as compared to those birds held on nonsprayed sites. Reduced growth during the first 2 weeks of avian development has important consequences at the population level, and it is recommended that this secondary effect be considered for all species who rely on Lepidoptera larvae as a food resource prior to wide spread application of B.t.k. for insect control.