Superparasitism and sex ratio adjustment in a wasp parasitoid: results at variance with Local Mate Competition?

Abstract

Anaphes nitens is a solitary parasitoid of the egg capsules of the Eucalyptus snout beetle, Gonipterus scutellatus. Some traits of its natural history suggest that Local Mate Competition (LMC) could account for sex ratio adjustment in this species. We tested whether males emerged early, a prerequisite for fully local mating, and investigated the occurrence and effect of superparasitism on adult size and pre-emergence mortality, factors that might influence sex ratio adjustment. We found in field-collected egg capsules that males emerged first. To investigate the effects of superparasitism on adult size, we compared the sizes of parasitoids that emerged early and late from egg capsules collected in the field, and from egg capsules parasitized and superparasitized in the laboratory. Superparasitism reduced parasitoid size, affecting females more strongly than males, and increased pre-emergence mortality. We estimated A. nitens sex ratio and parasitism rate in the field during 2 years in five localities and during 4 years in a sixth. Following LMC we expected an increase in sex ratio (proportion of males) with increasing parasitism rate (assumed to reflect parasitoid density). We found that sex ratio decreased from 0.38 when the parasitism rate was low (0-20%) to 0.21 when parasitism was high (80-100%). In contrast with field results, a laboratory experiment showed that: (1) at a low parasitism level sex ratio was clearly female biased (0.28+/-0.04), (2) at a high parasitism level sex ratio increased (0.40+/-0.07), (3) male larval survivorship was not lower than female survivorship, and (4) low-quality hosts (i.e. superparasitized) were allocated more males. We conclude that LMC cannot explain the sex ratio adjustment observed in the field, even at low parasitism rates, and alternative explications for highly female-biased sex ratios must be found. One such alternative is female-biased dispersal.

Discrimination of the age of conspecific eggs by an ovipositing ectoparasitic wasp

“…This discrimination ability has been demonstrated in most parasitoid species tested (Gauthier et al, 1996;Santolamazza et al, 2004;Darrouzet et al, 2007). However, A. calandrae is a solitary species, and it is possible that females do not adjust their sex ratio as predicted by the LMC theory, as already observed in Anaphes nitens Girault (Santolamazza & Rivera, 2003). The sex ratios produced by isolated females when in indirect competition are female-biased and not different from those produced by females that are not in a competition context.…”

The sex ratio distortion in the human head louse is conserved over time

“…However, in the female-biased parasitoid wasp Nasonia vitripennis , which is often used in textbooks as an example of local mate competition, mothers cannot discriminate between kin [25]. In many cases in which the female bias could not be attributed to local mate competition, local resource competition among females or female-biased dispersal have been put forward as alternative explanations [eg., [26,27]]. Local resource competition and differential dispersal can be ruled out in head lice for the same reasons discussed earlier for local mate competition.…”

Winter and spring ecology of Anaphes nitens, a solitary egg-parasitoid of the Eucalyptus snout-beetle Gonipterus scutellatus

“…The early emergence of males in A. nitens has been studied in the laboratory (Santolamazza Carbone and Cordero Rivera 2003) but not in the field. We recorded this behaviour under all environmental conditions.…”

“…The potential of A. nitens for snout-beetle control during the whole year has been well-documented (Cordero Rivera et al 1999;Santolamazza Carbone and Cordero Rivera 2003;Santolamazza Carbone et al 2006), as also has its physiological time in the laboratory (Santolamazza Carbone et al 2006), but until now there were no details about its overwintering mode and seasonal biology under natural conditions. In nature, parasitoids are regularly exposed to changes of temperatures, day-length, and humidity, but, surprisingly, very few data are available on parasitoid behaviour under the action of fluctuating abiotic factors.…”