An account is given of host acceptance, the influence of temperature on fecundity and longevity, and searching capacity of Chrysocharis laricinellae (Raczeburg), a parasite of the larch casebearer, Coleophora laricella (Hübner).
Fourth-instar case-bearing larvae were the preferred stages for attack. The size of the parasite progeny varied directly with the size of the host. Fecundity of progeny reared from small hosts was significantly less than that of progeny reared from large hosts. Not all parasite adults could successfully oviposit through the tough skin of the host pupa. C. laricinellae showed poor searching capacity. Superparasitism was common at low host densities of the casebearer and resulted in either one parasite emerging or in total parasite mortality. A sex ratio where females predominated resulted when sufficient numbers of preferred stages of the host were presented to mated C. laricinellae. The temperature threshold for attacking hosts was 55°F, and for development of the parasite 40°F. At 50°F the adults lived 4 months on the average.
Therefore C. laricinellae is able to survive in the field without alternate hosts and at very low host densities. The parasite seems to depend on a continuing supply of suitable instars of host species in sufficient numbers to be effective. If C. laricella is the only available host species in the ecosystem the biological control value of this chalcid must be rated as poor.