Tetrastichus incertus, a parasite of the alfalfa weevil (Hypera postica) larva, occurs throughout the northeastern United States, including almost all of New York. Field studies in 1967 and 1968 indicated that the parasite and its host were not closely synchronized in New York; the parasite was uncommon in June when hosts were abundant while the opposite was true in July. In 1966 and 1967, the proportion of parasites entering diapause increased throughout the summer, from 1–31% in June to 67–100% in September. Therefore, most parasites produced in June, when hosts were abundant, emerged in July when hosts were scarce. Winter mortality among parasites in diapause was high (51–64%) in field cages. The survivors of the overwintering generation of T. incertus constituted a small fraction of the annual production of parasites.
In the laboratory the offspring of older T. incertus tended more to enter diapause than did the offspring of younger parasites. This indicated the incidence of diapause may be correlated with host availability; when hosts are uncommon, diapausing parasites are produced.
Field and laboratory data indicated a ratio of one T. incertus to eight alfalfa weevil larvae would be necessary to achieve 50% parasitism of the host population. Such a host:parasite ratio never occurs when the hosts are common.