The Biology, Behavior, and Morphology of Trioxys (Trioxys) utilis, an Internal Parasite of the Spotted Alfalfa Aphid, Therioaphis maculata (Hymenoptera: Braconidae, Aphidiinae)1

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Abstract
Descriptions and illustrations are given of the adult, the egg, the three larval instars, and the larval head structures, together with illustrations of the male genitalia and of the ovaries and associated structures. Under constant conditions in the laboratory, (1) unmated females produced male progeny only, mated females produced male progeny only, mated females produced both sexes in a 1:1 ratio, and multimated males supplied lesser amounts of sperm in successive matings so that the male-female ratio of progeny went as high as 24:1; (2) though oviposition occurred in aphids of all stages, those of the first three instars were preferred; (3) maximum adult longevity of honey-fed females was 27 days at 50° F.; (4) superparasitism was common, the female apparently being unable to detect a parasitized host, and multiple strikes also were common; (5) reproductive capacity, based on number of eggs per female, ranged between 141 and 179; (6) durations of the various life-cycle stages were: egg 52-53 hours, first-instar larva 9-12 hours, second instar 32-34 hours, third instar 69-71 hours, pupa about 73 hours; (7) first-instar larvae fed almost exclusively on the trophamnion cells and feeding of second-instar larvae was nearly the same, and all contents of the aphid were consumed during the first half of the third instar; (8) with the complete life cycle at 70° F. requiring about 10 days, there are possibly 17 generations per year in the field; (9) a facultative diapause, usually lasting about 120 days, was found during the summer and fall months; (10) aphid defenses against parasitization included the jumping habit of the aphid when disturbed, and production of copious amounts of honeydew which often hindered or trapped the parasite. The jumping habit is partially overcome by the use of the ventral prongs on the parasite's abdomen, which serve to hold the aphid during oviposition. Various factors which may limit the effectiveness of Trioxys utilis against its host are discussed, and its present distribution, hosts, and hyperparasites are noted.

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