Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Estimating effect of Megastigmus spermotrophus (Hymenoptera: Torymidae) on Douglas-fir seed production: the new paradigm.

Abstract

In a pollen exclusion experiment performed near Monmouth, Oregon, on the cones of 5 Pseudotsuga menziesii trees, the number of seeds infested by Megastigmus spermotrophus 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 the effect of M. spermotrophus on P. menziesii seed production because the traditional formula, which is based on the assumption that only pollinated seeds are infested by this torymid, 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.