Estimating Effect of Megastigmus spermotrophus (Hymenoptera: Torymidae) on Douglas-Fir Seed Production: The New Paradigm

Abstract

In a pollen exclusion experiment performed on the cones of five Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco) trees, the number of seeds infested by a seed chalcid, Megastigmus spermotrophus Wachtl, did not differ significantly between pollinated and unpollinated cones from the same tree. This finding led to a revision of the formula used to calculate M. spermotrophus effect on Douglas-fir seed production because the traditional formula, which is based on the assumption that only pollinated seeds are infested by these chalcids, exaggerates their effect. The relationship between the new formula and the traditional formula is nonlinear, varying with both pollination rate and infestation level. To assist other researchers in estimating the error in past chalcid studies, the discrepancies for a range of pollination rates were calculated. Past assessments were strongly biased only where pollination rates were <70% and chalcid attack rates were from 50 to 85% using the traditional formula. For low pollination rates, the discrepancy can exceed 50%. Evidence is presented to explain how the unfertilized female gametophyte, which is small and is normally resorbed in the absence of pollination, can support the development of chalcid larvae. In this study, there was a strong correlation (-0.93) between cone length and chalcid attack rate.