The Role of Insect Parasitoids in Population Cycles of the Spruce Needleminer in Denmark

The spruce needleminer, Epinotia tedella (Cl.) (Lepidoptera: Tortricidae), is a small and abundant moth associated with Norway spruce (Picea abies Karst.). Larvae mine spruce needles, usually those more than 1 year old, and each requires about 35 needles to meet its food demands. In central Europe, the spruce needleminer is regarded as a temporary, serious pest when densities reach several thousand per square meter. However, it seldom causes significant damage in Scandinavian countries. An exception was the heavy infestation in southern Denmark in 1960-61. The spruce needleminer has one generation per year. Adults emerge in June and deposit eggs singly on spruce needles. Larvae mine the needles from July through October and then descend on silken threads in November to hibernate in the forest litter as prepupal larvae in cocoons. Pupation occurs in early May and lasts 3-4 weeks. Like many other forest defoliators, spruce needleminers are associated with a diverse fauna of parasitic Hymenoptera (parasitoids) (Münster-Swendsen 1979). Eggs are attacked by a minute wasp (Trichogramma sp.) that kills the embryo and emerges as an adult a few weeks later. Because spruce needleminer eggs have all hatched by this time, the parasitoids must oviposit in the eggs of other insect species. In other words, this parasitoid is not host-specific and therefore not expected to show a numerical response to spruce needleminer population changes. Newly hatched moth larvae immediately bore into needles and, because of this, are fairly well protected against weather and predators. However, specialized parasitic wasps (parasitoids) are able to deposit their eggs inside a larva by penetrating the needle with their ovipositor. Two species, Apanteles tedellae (Nix.) and Pimplopterus dubius (Hgn.), dominate the parasitoid guild and sometimes attack a large percentage of the larvae (Münster -Swendsen 1985). Parasitized larvae continue to feed and, in November, descend to the forest floor to overwinter with unparasitized individuals. In late April, however, the parasitoids take over and kill their hosts. Besides mortality from endoparasitoids, up to 2% of the larvae die within the mine due to an ectoparasitoid and a predatory cecidomyid larva.