SOME POPULATION FEATURES OF REPRODUCTIVE CAPACITY IN NEODIPRION SWAINEI (HYMENOPTERA: DIPRIONIDAE)

Abstract

AbstractEgg production in Neodiprion swainei may be severely reduced by food quality and quantity, tachinid parasitism, and prolonged diapause. Since these and other agents may affect the sawfly both lethally and sublethally and may operate differentially against males and females, population losses are not adequately expressed in terms of numbers of individuals. However, reduction of the potential number of eggs the population could produce provides an integrated measure of these effects. One example, from an experiment in which larvae were fed various quantities and types of foliage, demonstrates that loss of reproductive potential is several times as great as the loss of individuals and indicates that the latter seriously underestimates the importance of food in population events.Cocoon diameter and fecundity are closely related. In a Saguenay Valley population, fecundity, cocoon diameter, and deviations from the regression line were bimodally distributed, indicating a mixture of two types of individuals differing in egg production characteristics. Families within a single population differed from each other by as much as 14 eggs in the elevation of the regression line; this was more than the observed difference in elevation between populations. A single regression equation satisfactorily predicted fecundity in one Quebec and a Wisconsin population, but was unsuitable for two other Quebec populations, each of which required its own equation. Among insects of common size, mean egg length varied inversely with fecundity, but it is uncertain whether differences in egg size were responsible for variation in the fecundity-cocoon diameter relationship.