Laboratory evaluation of Gryon pennsylvanicum (Ashmead) (Hymenoptera, Platygastridae) as a biological control agent of Leptoglossus occidentalis Heidemann (Heteroptera, Coreidae)

Abstract

Leptoglossus occidentalis Heidemann (Heteroptera, Coreidae), the western conifer seed bug, is a serious conifer seed pest in North America; after its accidental introduction in Italy, it has spread to several European countries. In Central Italy, severe seed loss was recorded in stone pine forests (Pinus pinea). Classical biological control against L. occidentalis was evaluated by performing laboratory tests on Gryon pennsylvanicum (Ashmead), the most important hymenopteran egg parasitoid of the pest in North America, to acquire biological and demographical parameters. Tests were performed in climatic chambers using glass tubes to house specimens at 26±1°C, 75±5% RH and 16:8 L:D and supplying honey drops as food for adults. G. pennsylvanicum reared on eggs of L. occidentalis has a high fecundity rate (144.55 eggs/female) and 90% of the total number of eggs are laid in the first two weeks of oviposition. Mean oviposition period is 23days. Parasitoid females live a mean of 40days, and longevity is higher in host deprived females. The intrinsic rate of increase (rm) was calculated to be 0.206. Mean sex ratio throughout a female’s life span was 70.42% females, though at the start of oviposition it was over 87% females. Adults survived only four days without honey as food, but up to 50days with food; water did not influence longevity. Females lived significantly longer than males. Fecundity was negatively affected by duration of host deprivation, nevertheless, host-deprived females retained the ability to parasitize host eggs, and without a substantial sex ratio change in offspring. On the whole, G. pennsylvanicum shows potential for use in classical biological control against L. occidentalis. Several factors contribute to its suitability for biological control like high reproductive rate, high sex ratio, long-lived adults, and the ability to maintain reproductive potential during periods of host deprivation with a no substantial change in the sex ratio.