Biological Factors Affecting Contarinia oregonensis (Diptera: Cecidomyiidae) Infestations in Douglas-Fir Seed Orchards on Vancouver Island, British Columbia

Abstract

Factors affecting population densities of the Douglas-fir cone gall midge, Contarinia oregonensis Foote, were examined in seven Douglas-fir seed orchards in British Columbia. Variation among trees in the numbers of eggs laid per cone accounted for between 73 and 100% (regression intercept set at zero) of the variation in the numbers of larvae present at cone harvest, and was the most important factor determining numbers of larvae. The incidence of parasitoids was low. Other gall midge mortality factors included interspecific competition, cone abortion, and death of scales. Intraspecific competition between established larvae at high densities reduced the size of resultant adult gall midges.