Survival Mode between the Yearly Reproduction Periods, and Reproductive Biology of Scambus pomorum (Hymenoptera: Ichneumonidae: Pimplinae), a Parasitoid of the Apple Blossom Weevil Anthonomus pomorum (Coleoptera: Curculionidae)

Abstract

First evidence is given for the hitherto unknown survival mode of females [♀♀] of the parasitoid Scambus pomorum (Ratzeburg 1848), which propagates once every spring, on larvae of the apple blossom weevil, Anthonomus pomorum Linnaeus 1758. After mating, the ♂♂ die, and the ♀♀ continue as predators of leafminer larvae, and perhaps other concealed insects, while they remain in a state of reproductive diapause. In captivity, the female parasitoids attack the mines of moths of different families, of agromyzid flies and sawflies, living on various tree species. A similar explicitly predatory life style of a parasitoid wasp has not been reported before. Leafminers as prey are not responsible for the absence of oogenesis before winter, for eggs develop in overwintered ♀♀ on the same food. Single ♀♀ kill, and feed upon, up to two Phyllonorycter blancardella (Fabricius 1781) (Lepidoptera: Gracillariidae) larvae per day at 20°C, both in early summer and autumn. Predation is linearly related to temperature. In captivity, predation by well-fed ♀♀ showed a dip in August. ♀♀ fed this way, overwintered successfully in coniferous trees, like they are known to do in nature. They became active again in late March. ♀♀ are ready to oviposit when the first suitable host stages appear, on the earliest flowering apple varieties, or on pear. One ♀ lays up to 14 eggs per day at 16.5°C, and total egg production may exceed 100 eggs, but about 50 eggs is more usual, and many ♀♀ are even less fecund. Some ♀♀ are still alive, and probably reproducing, when suitable host stages are becoming scarce. Parasitism levels should be determined when approximately 75% of the host weevils have become pupae; dissections of capped blossoms and parasitoid rearings gave similar results. Her extended lifespan makes the female wasp vulnerable to calamities like pesticide applications. Perhaps, more elevated leafminer densities in nearby hedgerows could seduce ♀♀ to leave the apple planting in time.