THE NATURE OF VARIATION IN LIFE HISTORY CHARACTERS OF DYSDERCUS BIMACULATUS (HETEROPTERA: PYRRHOCORIDAE), A COLONIZING SPECIES.

The nature of variation in life history attributes has been the subject of conflicting expectations and few empirical studies. For the cotton stainer bug, Dysdercus bimaculatus (Pyrrhocoridae; Heteroptera), I have predicted that phenotypic variation in different reproductive traits will reflect the pattern of certainties and uncertainties in its environment (Derr, 1980). The difficulty lies in constructing a hypothesis about the extent to which phenotypic variation is caused by genetic differences among individuals. The total phenotypic variance for any trait is expressed by: Vp = VA + VN + VE where VA is the additive genetic variance, VN is the nonadditive genetic variance (due to dominance and epistasis), and VE environmental variance (Falconer, 1960). An estimate of heritability (h2 = VA/VP) indicates the potential for a population to change in response to a change in the vectors of natural selection (Dingle et al., 1977; Hoffman, 1978) and may suggest prior emphases of selection (McLaren, 1976; Istock, 1977). This paper examines gene-dependent variation in reproductive characters of a sample of D. bimaculatus. Dysdercus bimaculatus is a colonizing insect. Seed crops on which this insect feeds are short-lived, mature at various times throughout the tropical dry season, and occur on trees which are widely spaced (Derr, 1980). Prior to laying her first clutch, the female cotton stainer bug can migrate between crops and may diapause if no food is available (her wing muscles degenerate upon feeding and oogenesis). The level of environmental moisture stress varies among trees and affects the tendency of a female to break diapause (Derr, 1980). Directional selection should have acted to minimize the age at first reproduction (called a) and maximize clutch size in this colonizing species (Lewontin, 1965). The expectation for the heritability of a trait which has been subject to directional selection depends on the relationship of genotype to phenotype. If phenotypic differences among individuals were largely due to genetic differences, then directional selection would tend to reduce VA in a population. An estimate of the heritability of that trait would be close to 0 (Istock, 1977). Low phenotypic variability among individuals could also be a function of some factor in the environment (environmental buffering [Giesel, 1976; Hoffman, 1978]). The remaining Vp would then be due to genetic differences among individuals, and an estimate of heritability would be close to 1. In contrast to the tendency of directional selection on a and clutch size to reduce Vp for those traits, the tendency for a female D. bimaculatus to break diapause under conditions of moisture stress ought to be more variable. This variability would result from environmental uncertainty and the several options for reproduction in time and space available to an insect capable of migration and diapause (Derr, 1980). In one year or at one site, a "stress-tolerant" female which lays eggs at a stressful site may leave more offspring than a "stress-intolerant" female which waits or flies beyond that site; in another year or at another site, the stress-intolerant female may leave more offspring. Expectations for the heritability of a phenotypically variable trait depend on the predictability of environmental fluctuations and the period of seasonal oscil-