Brood-size and sex-ratio variation in field populations of three species of solitary aphid parasitoids (Hymenoptera: Braconidae, Aphidiinae)

Abstract

Solitary parasitoids of colony-forming hosts may produce quasi-gregarious broods, which favours sibmating on the natal patch and local mate competition (LMC). We examined seasonal variations in brood size and sex ratio in three species of solitary parasitoids of aphids associated with trophobiotic ants. Adialytus arvicola, a parasitoid of Sipha agropyrella on grasses, had the smallest broods (mean=4.2, maximum 19), while Lysiphlebus hirticornis, a parasitoid of Metopeurum fuscoviride on tansy, had the largest broods (mean=32.0, maximum 265). In Pauesia pini, a parasitoid of Cinara piceicola on Norway spruce, broods comprised an average of 5.8 (maximum 41) individuals; brood size increased during early summer when hosts became more available but remained the same later in the season. In all three species the sex ratio at eclosion was female-biased, with broods containing approximately two daughters for each son in both A. arvicola and L. hirticornis; the degree of female-bias was least in P. pini. The sex ratio did not vary with brood size. In A. arvicola, the variance of the number of sons declined with an increase in brood size, consistent with "precise" sex allocation. In contrast, in L. hirticornis, the overall sex-ratio variance was greater than its binomial expectation, while it did not differ from binomial in P. pini. A large proportion of broods contained only sons or only daughters, especially in A. arvicola. An excess of male-only broods is expected if constrained females (which can produce only sons) contribute offspring prior to mating. The number of male-only broods, however, did not differ from the number expected if all females are mated and allocate offspring sexes binomially, except in P. pini. In the latter species, broods with two daughters (as opposed to two sons) exceeded binomial expectations. We propose that P. pini is largely outbreeding, while the strongly female-biased sex ratio in A. arvicola and L. hirticornis is consistent with partial sibmating and LMC. Ant-parasitoid interactions could account for a different population mating structure in the three parasitoid species. Both A. arvicola and L. hirticornis mimic the epicuticular hydrocarbon pattern of their aphid hosts. Eclosing wasps are ignored by honeydew-collecting ants and hence can mate and forage on the natal patch. In contrast, P. pini generally depart the mummy area to avoid attacks by trophobiotic ants and mate off patch.

Consumption of snowdrop lectin (Galanthus nivalis agglutinin) causes direct effects on adult parasitic wasps

“…However, in both A. colemani and T. brassicae, progeny emergence was not affected by GNA ingestion, indicating that the lectin is either not passed on to the parasitoid eggs or that it has no effect on the developing parasitoid. Similarly, progeny sex ratio by GNA-fed wasps was within the range reported earlier for aphid parasitoids (Mackauer and Völkl 2002) and T. brassicae (Bigler et al 1987) indicating that the ingested lectin does not interfere with the sex allocation in the female wasps. These results imply that GNA feeding by the adult females does not cause any detrimental effect on the next generation.…”

Parasitoid developmental mortality in the field: patterns, causes and consequences for sex ratio and virginity

“…Previous field‐based estimates of parasitoid brood sex ratio variances have variously reported under‐dispersion, binomiality and over‐dispersion but were either unable to provide associated estimates of developmental mortality (Mackauer & Volkl 2002) or constrained to laboratory estimates of the mortality of broods collected in the field (Hardy et al. 1998).…”

“…We used yucca leaves infested with 20–28 day‐old brown soft scales; each leaf typically bore 200–300 individuals of variable size and developmental stage. Because there were always substantially more soft scales present on each of the yucca leaves than the numbers that were eventually parasitized (usually 15–20% of hosts on any one leaf), our design minimizes, rather than excludes, potential superparasitism (see also Mackauer & Volkl 2002). Each leaf was placed in a water‐filled vial fixed on top of a one‐metre‐high stake, beneath the canopy of a citrus tree.…”

Temporal genetic structuring of a specialist parasitoid, Lysiphlebus hirticornis Mackauer (Hymenoptera: Braconidae) attacking a specialist aphid on tansy

“…In primary hymenopteran wasp parasitoids, especially gregarious species as well as the solitary species attacking gregarious hosts, local mate competition as a result of mating between siblings and brothers competing for mates at natal/ emergence patches is known to occur (Godfray, 1994). In solitary parasitoids attacking colony forming hosts, quasi‐gregarious broods of parasitoids are produced, which favours sib‐mating or inbreeding on the natal patch (Mackauer & Völkl, 2002). Inbreeding enhances reduced levels of genetic diversity and increases the probability of extinction of local populations (Saccheri et al ., 1998).…”

“…it is multivoltine with approximately 14–17 generations per annum). Female parasitoids have been observed to oviposit large number of eggs (40–70), and often to forage until all available hosts are parasitized, independent of aphid colony size on a tansy ramet (Mackauer & Völkl, 2002). Extinctions of the aphid, and, consequently, the parasitoid as a result of predation at the level of tansy genets and ramets, are common in a single season (Weisser, 2000; Weisser & Härri, 2005).…”