DEFINING THE WESTERN SPRUCE BUDWORMS NUTRITIONAL NICHE WITH RESPONSE-SURFACE METHODOLOGY

A response surface design was used to help define the ''nutritional niche'' of the western spruce budworm, Choristoneura occidentalis. We evaluated how calcium, magnesium, and phosphorus interacted to influence budworm fitness on artificial diets containing five different levels of the minerals. We quantified survival rates for several life stages using a three-generation bioassay. Data from the bioassay were used to estimate population growth over three complete generations. Performance of the budworm was affected by Mg (linear response for F1 generation; 0.83-1.95 mg/g tested) and P (quadratic response for P1 [parental] and F1 generations; 2.74-4.95 mg/g tested) but was largely unaffected by Ca (0.50-6.81 mg/g tested). Overall, the results suggested budworm performance is best when Mg is at low concentrations and P is at moderate concentrations, and that very high levels of Mg and P are detrimental to the budworm. On average, host trees had too much Mg and too little P for optimal response by budworms. Interactions between Mg and P imply that balances or ratios of minerals are important in budworm nutritional ecology. However, detectable effects from Mg, P, and Ca in the diets diminished as the bioassay continued into the F2 generation, and, consequently, the estimated number of larvae alive at the beginning of the F2 and F3 generations showed no relationship to concentrations of the test minerals. This result probably reflected: (1) the loss of some design points in the second and third generations of the bioassay because larvae did not survive on diets that were markedly suboptimal and (2) increased variability in the population growth response as the experiment continued into the F2 and F3 generations.