ACTIVITY PATTERNS OF PARASITOIDS ON THE SWAINE JACK PINE SAWFLY, NEODIPRION SWAINEI (HYMENOPTERA: DIPRIONIDAE), AND PARASITOID IMPACT ON THE HOST

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Abstract
Field studies enabled the identification of some limiting factors to which parasitoids were exposed in exploiting the sawfly host, Neodiprion swainei Middleton, and aspects of their quantitative and qualitative impact on the host population. Emergence patterns and flight activity of parasitoids in the spring indicated that larval parasitoids were vulnerable to hyperparasitism by cocoon parasitoids. Although cocoon parasitoids were limited by a shortage of time in which to exploit the host in the spring they still reduced the overwintered host population by 66% in 1969. All forms of predation caused less than 10% sawfly mortality so parasitoids formed the major mortality factor in the spring. Sawfly in prolonged diapause appeared to be immune from cocoon parasitoid attack and thereby efficiency of these parasitoids was limited. However, cocoon parasitoid activity was selective on sawfly that would normally have emerged early in the season and thus delayed the average development of the host population. The complex food web based on the sawfly host provided a sensitive buffering mechanism to produce a degree of stability in the host–parasitoid interaction. The composition of the parasitoid faunae before and after establishment in N. swainei populations of the introduced Pleolophus basizonus (Gravenhorst) indicates the rapidity with which the parasitoid became influential in the complex.