Some factors influencing the efficiency of Apanteles medicaginis Muesebeck (Hymenoptera: Braconidae) as a parasite of the alfalfa caterpillar, Colias philodice eurytheme Boisduval

It is in high-temperature areas that A. medicaginis becomes an important factor in the control of C. pb, eurytheme. Apantelcs requires a much shorter time for development than does Colias, and should be able to respond rapidly to changes in host density. Its ability to respond rapidly is limited, however, because alfalfa is cut every 30 to 35 days, thus making it difficult for the parasite to complete two generations in a field between cuttings. Consequently, Apanteles females emerging from a field must either seek out another Colias population in a parasitizable stage or wait for emergence and resulting progeny of Colias from the same field. This difference in emergence time, coupled with the difference in flight characteristics and host habitat selection, at times results in concentration of host and parasite in different fields. To effect control under these circumstances, a small number of Apanteles females must cope with very large numbers of Colias larvae.