Host-Parasite Relationship Studies of the Larval Alfalfa Weevil and the Ichneumonid Parasite Bathyplectes curculionis (Thomson)

The parasitic effects of the ichneumonid Bathyplectes curculionis (Thomson) on the larval alfalfa weevil, Hypera postica (Gyllenhal), were studied. Results of experiments on the rates of parasitism of the four host larval instars indicated that the first three are either preferred by the parasite over the fourth instar larvae or are more susceptible to the parasite\u27s attack. Survival of the younger weevil larvae after their exposure to female parasites was markedly poorer than that of unparasitized larvae. Premature death of host larvae was probably from both the puncturing by the parasite\u27s ovipositor and the feeding and other activities of parasite larvae within the hosts. The incidence of premature mortality of host larvae following ovipostion by Bathyplectes increased with multiple stinging and decreased with host age. The effects of parasitism on host development and activity were studied at 25-26 C. Larvae of each ins tar were parasitized and the number of days required for development to the cocoon stage was compared with that of unparasitized larvae of the same age. The development time for larvae parasitized during the third or fourth instar was significantly longer than that for unparasitized larvae. No significant difference existed between the length of development time for larvae parasitized during the first or second instar, and that for unparasitized larvae. There was no significant difference in the activity of unparasitized and parasitized larvae. The influence of parasitism on growth, food consumption and food utilization by H. postica larvae during the third and fourth instars was studied at 22.2 C and 30 C. At both temperatures, total growth was significantly higher with unparasitized than with parasitized larvae. At 22.2 C, the total food consumption by unparasitized larvae was significantly higher than that by parasitized larvae. At 30 C, there was no significant difference between the total food consumption by unparasitized and parasitized larvae, although unparasitized larvae consumed more food. The food consumption per larva per day was significantly higher for unparasitized larvae at both temperatures. There was, however, no significant difference in the dry weight-fresh weight consumption index between parasitized and unparasitized larvae at either temperature. No significant difference existed, at either 22.2 C or 30 C, in the approximate digestibility of alfalfa by parasitized and unparasitized larvae. The efficiency of food digestion by both types of larvae decreased with age, the decline being more pronounced at 30 C than at 22.2 C. The net efficiency of conversion of ingested food to body matter was higher for parasitized than for unparas tized larvae at 22.2 C. The difference was marginally significant at the 5 percent level. At 30 C, there was no significant difference between the net efficiencies of conversion of ingested food to body matter by parasitized and unparasitized larvae. Also no significant difference existed between the net efficiencies of conversion of digested food to body matter by parasitized and unparasitized larvae at either temperature. Efforts to discover a practical method to distinguish parasitized from unparasitized larvae without dissection were unsuccessful