Laboratory Studies on Competition Among Three Parasites of the Spotted Alfalfa Aphid Therioaphis Maculata (Buckton)

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Abstract
Three species of solitary endophagous parasites of the spotted alfalfa aphid were tested under constant temperature conditions to obtain information on the reproductive behavior and competitive ability of each in relation to the others. The braconid, Trioxys utilis, was found at both 21 and 27°C to be a very effective competitor because of its ability to find hosts rapidly, to oviposit rapidly and frequently, because of the ability of its larvae to compete successfully with other larvae within the host, and because of its habit of parasitizing young aphids before they become attractive to other parasite species. However, it tends to be wasteful of eggs, in the laboratory at least, since under some situations it does not readily discriminate parasitized from unparasitized hosts. Corresponding with this propensity toward superparasitism is its habit of multiple parasitism. A second braconid, Praon palitans, is somewhat less efficient in its search ability, oviposition rate, and host discrimination than T. utilis. It tends to waste eggs through excessive amounts of superparasitism. It also multiple—parasitizes hosts freely. It parasitizes older host stages, which if already parasitized by other species places it at a disadvantage. However, the larvae of this species are excellent competitors within multiple—parasitized hosts. The third parasite, Aphelinus semiflavus, is inferior to the other species in its slowness to find and parasitize hosts. However, it discriminates to a high degree parasitized from unparatized hosts, and tends to avid wastage of eggs through superparasitism or multiple parasitism. Larvae of this species, when engaged in internal competition with other species through multiple parasitism by the latter, generally fail to survive.

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