The timing of egg maturation in insects: ovigeny index and initial egg load as measures of fitness and of resource allocation

Both the ovigeny index (OI) - defined as the proportion of the potential lifetime complement of eggs that is mature upon female emergence, and egg load - defined as the number of mature eggs carried by a female at a given moment in her lifetime, have been identified as significant fitness variables in insects. In discussions of egg maturation strategies initial egg load (IEL) and OI are often considered together, and they have been assumed to be strongly correlated. The purpose of this review is: (i) to summarise what is known about variation in OI and IEL both in relation to one another and in relation to other fitness variables (notably life-span and body size), (ii) to assess the merits of OI and IEL as measures of resource allocation to reproduction, and (iii) to contrast knowledge of the mechanisms underlying cross-species variation in these two variables with what is known concerning the mechanisms underlying intraspecific variation. Our review concentrates upon parasitoid wasps because most information on OI and IEL has been obtained for these animals, but most of the issues we address are pertinent to other insects, at least holometabolous ones. We conclude that, despite certain limitations, OI is, for insect evolutionary ecology, more useful than IEL for two reasons: (i) OI is a true measure of the degree to which lifetime egg production is concentrated into the early phase of adult life, (ii) the observed intraspecific and interspecific declines in OI with increasing body size are consistent with theoretical predictions concerning body size-related changes in reproductive effort (when the proportional change in the allocation to reproduction is considered), whereas the intraspecific alteration in IEL is not (iii) OI is a measure of the relative extent of resource allocation between the juvenile and the adult stages of the female. Lastly, we discuss the possible application of OI to insect population dynamics and management.