THE IMPORTANCE OF COINCIDENCE IN THE FUNCTIONAL AND NUMERICAL RESPONSES OF TWO PARASITES OF THE EUROPEAN PINE SAWFLY, NEODIPRION SERTIFER

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Abstract
The possibility of imperfect coincidence between the appropriate stages of Neodiprion sertifer (Geoff.) and two of its important parasites was demonstrated. One of the parasites, the indigenous ichneumonid Exenterus canadensis Prov., which attacks late-stage larvae, has good spatial coincidence; but some members of each generation suffer from imperfect temporal coincidence, or asynchrony, caused by the interaction of temperature influence on parasite development rate and temperature variability between development sites in the litter. The second parasite, Pleolophus basizonus (Grav.), is an introduced, multivoltine ichneumonid cocoon parasite. It may be imperfectly synchronized in its first generation each year and may show imperfect spatial coincidence in all generations through its inability to attack host cocoons beneath approximately 1 in. or more of litter.The intricate relations between parasite and host density, time, attack, and coincidence were investigated using the basic functional response submodel developed by Holling, a submodel that describes changes in oviposition behaviour with time, and a submodel that predicts the number of hosts attacked, given the number of eggs laid and data on the distribution of eggs among hosts. In the two species studied, the effect of asynchrony in one generation cannot be considered without considering the influence of superparasitism. At low host densities, superparasitism largely buffers the effects of decreased synchrony. This buffering effect decreases as host density increases until when each parasite is attacking all the hosts it can, it is almost eliminated. Imperfect spatial coincidence in one generation merely lowers the usable host density. Thus its effect can be seen in the functional response of the parasite to host density. When host–parasite interactions over 25 to 35 host generations were simulated, using initial conditions resembling those ensuing when small numbers of both host and parasite invade a previously unattacked stand, populations became stable after passing through one or more oscillations. Decreasing temporal or spatial coincidence increased host and parasite densities at the peaks of oscillations and increased the ultimate steady density of host and parasite, until coincidence was reduced to nearly half. At this level, the host escaped the regulating ability of both species of parasites.