A COMPARISON OF DISCRIMINATION AND OF DENSITY RESPONSES DURING OVIPOSITION BY EXENTERUS AMICTORIUS AND E. DIPRIONIS (HYMENOPTERA: ICHNEUMONIDAE), PARASITES OF NEODIPRION SWAINEI (HYMENOPTERA: DIPRIONIDAE)

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Abstract
The oviposition behavior of Exenterus amictorius Panzer, an introduced parasite of pre-spinning eonymphs of the Swaine jack pine sawfly, Neodiprion swainei Middleton, was studied in a series of field experiments over a 5-year period in the Province of Quebec. The ability of this parasite to discriminate against hosts with previously deposited progeny varied significantly: discrimination was lacking at the beginning of the host’s spinning period, but was rapidly acquired and persisted to the end of the spinning period. The relationships were described by negative power functions of the form Iδ = aX−b, where Iδ = Morisita’s Index of Dispersion, and X = the number of days from the beginning of the host’s spinning period. The response of E. amictorius to changing host densities, however, was positive at all host densities and described by functions of the form Y = aXb, where Y = number of parasite progeny per host and X = number of hosts dropping into funnel traps at 2-day intervals during the spinning period. Over 67% of the variation in the number of progeny deposited by E. amictorius during a field experiment in 1969 was explained by changes in adult parasite density, and an additional 19% by changes in host numbers. This indicates strongly that the observed change in discrimination against previously deposited progeny bears no direct relationship to either host or parasite density. It was suggested that the change might be influenced either by a reduction in the parasite’s egg complement in time, or by an associative learning process.The density response of an indigenous parasite, Exenterus diprionis Rohwer, was comparatively much weaker and it seemed to lack the adaptive changes in discrimination against previously deposited progeny possessed by E. amictorius. Although attacks of the one species occurred independently of the other, in the event of multiparasitism E. amictorius always survived because of its faster rate of development in the host. The role of discrimination and of density response in the dominance of E. amictorius among the Exenterus spp. attacking N. swainei, and its successful establishment on a variety of North American diprionids, is discussed.