Brood sex ratios of the solitary parasitoid wasp, Coccophagus atratus

Abstract.

• 1 Female eggs of Coccophagus atratus are deposited within the haemolymph of coccoid scale insects. Male eggs are deposited on to late larval and prepupal stages of parasitoids of scale insects, including conspecifics.
• 2 When presented with either one host type or a combination of both host types, female C.atratus deposit all their available eggs, assigning the appropriate sex egg to each host encountered. Brood sizes are not adjusted for different combinations of hosts.
• 3 Behavioural observations show that females do not move away from patches of hosts until all their eggs are laid, regardless of the host type.
• 4 Brood sex ratios varied with changes in the relative availability of hosts for males and hosts for females. When both host-types were present in equal numbers, male biased sex ratios resulted (mean ±SEM =0.71 ± 0.009) and when 70% of hosts provided were suitable for female eggs, mostly female-biased sex ratios resulted (mean ± SEM = 0.37±0.01).
• 5 Our results do not fit predictions based on the assumption that a sex ratio of 0.5 should be expected in C.atratus. Observed sex ratios indicate that the unusual life histories of these parasitoids need to be taken into account in explanations of their sex ratios.

     

Host population structure affects field sex ratios of the heteronomous hyperparasitoid, Coccophagus atratus

Abstract.

• 1 Second instar Filippia gemina de Lotto scale insects are the preferred hosts of female Coccophugus atrutus Compere larvae. These scale insects were found on their host plants, Chrysanthemoides monilifera Norlindh and Cliffortia strobilifera Mettenius, only at certain times during a 1 year sampling programme.
• 2 Late larval instars and prepupae of C.atratus, and a Metaphycus species, are the preferred hosts of male C.atratus larvae. These hosts, although they occurred on the same host plants as hosts for female C.atratus, were most numerous at different times during the sampling period.
• 3 The ratio of hosts suitable for C.atratus varied from a predominance of hosts suitable for females through to a predominance of hosts suitable for males. Sex ratios of adult C.atratus followed a similar trend but did not reflect, exactly, the ratio of available hosts. Differences in mortality between sexes and hyperparasitism may account for this anomaly.
• 4 Variable population sex ratios observed in C.atratus apparently result from the behaviour of individual females in which brood sex ratios are dependent on the relative availability of hosts for males and hosts for females. This behaviour, in turn, may result from variability in the host population structure but may also result from selection pressures operating at the time that heteronomous hyperparasitism evolved.

     

Hyperparasitism by an exotic autoparasitoid: secondary host selection and the window of vulnerability of conspecific and native heterospecific hosts

Encarsia transvena is an 'autoparasitoid' in the hymenopteran family Aphelinidae. In this species, female eggs are laid in whitefly nymphs. Male eggs are laid externally on immature parasitoids enclosed within the whitefly integument, either their own species, or other primary parasitoids. We explored parasitism by E. transvena of conspecific female immatures and those of a native primary parasitoid, Eretmocerus eremicus, in laboratory experiments. In the first experiment, female E. transvena were offered different combinations of two stages of E. transvena (late larvae – prepupae (ET2), and early pupae (ET3)), and one stage of E. eremicus (prepupae – early pupae (EE2)) in paired choice tests. The results indicated very little parasitism of ET3 relative to the host it was paired with, either EE2 or ET2. However, when EE2 was offered with ET2, there was no statistically significant difference in parasitism. In a no-choice experiment in which oviposition patterns and male progeny development were examined in four stages of both species of wasp, clear differences were observed between the host species. Only one stage of E. transvena (ET2) was parasitized and supported development of male E. transvena to any significant degree. In contrast, in E. eremicus, EE2, EE3 (red-eyed pupae), and EE4 (late pupae) were all parasitized, and male E. transvena emerged from all three stages, although fewer males emerged from EE4. In both species, wasp larvae that were still enclosed within the wet whitefly remains (ET1 and EE1) were parasitized at a very low rate. Lastly, an experiment that determined the length of the later developmental stages of E. transvena and E. eremicus suggested that the duration of the period in which E. transvena is susceptible to parasitism by conspecific females is less than half the period of susceptibility of E. eremicus. These results taken together suggest the potential for interference of E. eremicus by E. transvena, but other factors not examined here may also influence the outcome of interactions in the field.     

Vestiges of an ancestral host plant: preference and performance in the butterfly Polygonia faunus and its sister species P. c‐album

1. In the study of the evolution of insect–host plant interactions, important information is provided by host ranking correspondences among female preference, offspring preference, and offspring performance. Here, we contrast such patterns in two polyphagous sister species in the butterfly family Nymphalidae, the Nearctic Polygonia faunus, and the Palearctic P. c‐album. 2. These two species have similar host ranges, but according to the literature P. faunus does not use the ancestral host plant clade – the ‘urticalean rosids’. Comparisons of the species can thus test the effects of a change in insect–plant associations over a long time scale. Cage experiments confirmed that P. faunus females avoid laying eggs on Urtica dioica (the preferred host of P. c‐album), instead preferring Salix, Betula, and Ribes. 3. However, newly hatched larvae of both species readily accept and grow well on U. dioica, supporting the general theory that evolutionary changes in host range are initiated through shifts in female host preferences, whereas larvae are more conservative and also can retain the capacity to perform well on ancestral hosts over long time spans. 4. Similar rankings of host plants among female preference, offspring preference, and offspring performance were observed in P. c‐album but not in P. faunus. This is probably a result of vestiges of larval adaptations to the lost ancestral host taxon in the latter species. 5. Female and larval preferences seem to be largely free to evolve independently, and consequently larval preferences warrant more attention.