The Importance of Host-Feeding by Adult Parasites in the Reduction of Host Populations1

Abstract

Host-feeding, or host predatism, by the adult [female][female], is known to be fairly common among the parasitic Hymenoptera. That its quantitative importance in the reduction of host populations may be as great as that of parasitization, or greater, is emphasized by the present findings. Adult [female][female] of the parasite Metaphycus helvolus kill black scale, Saissetia oleae, by parasitization, by mutilation with the ovipositor, and by host-feeding at ovipositor wounds. Field tests show that 70-97% of a black-scale infestation may be killed by M. helvolus over a period of several months. About 20-25% of this mortality is caused by parasitization; the rest is attributed to host-feeding. The relative importance of host-feeding is highest during the early phases of an infestation; that of parasitization is higher later, as the scale increases in size. Laboratory tests, each involving one parasite generation, show that, within limits, % parasitization of scale is not markedly affected by parasite numbers. The % of scale killed by host-feeding is affected by parasite numbers, however, and follows a general upward trend as the parasite density increases. Total host-feeding mortality is much greater than parasitization mortality. Per cent parasitization increases with host density at parasite densities of 1, 5, and 10 [female][female] per scale-infested potato sprout; % host-feeding mortality decreases. Host-feeding mortality is the greater in all cases and increases, with respect to parasitization, at the higher parasite densities. Parasitized scales, per [female] parasite generation, increase as host density increases. Parasite fertility is highest with 1 [female] parasite per scale-infested potato sprout, and, because of the limitation of the egg-laying capacity, reaches a maximum of 70 progeny at a host density of about 400 scale. Fertility is much lower at parasite densities of 5 and 10 [female][female] per sprout, probably because of competition between ? parasites. Host-feeding, per [female] parasite, increases slightly as host density increases, although not so rapidly as parasitization. Host-feeding mortality, per [female] parasite, is greatest with 1 parasite per sprout, less with 5 parasites per sprout, and least with 10 parasites per sprout. In every case host-feeding is greater than parasitization. In general, as scale size increases, total parasite-caused mortality increases. Per cent parasitization increases rapidly from very small scales to medium-sized scales, and then increases more slowly from medium-sized to large-sized scales. Per cent host-feeding mortality decreases from small scales to medium-sized scales, and then increases with large-sized scales. Per cent host-feeding mortality is much greater than % parasitization at the small scale size, slightly greater at the large scale size, but somewhat less at the medium scale size. These studies indicate that scales may be parasitized between the limits of 0.4 and 2 mm. in length.