Abstract
Emerging parasitoids of aphids encounter secondary plant chemistry from cues left by the mother parasitoid at oviposition and from the plant-feeding of the host aphid. In practice, however, it is secondary plant chemistry on the surface of the aphid mummy which influences parasitoid olfactory behaviour. Offspring of Aphidius colemani reared on Myzus persicae on artificial diet did not distinguish between the odours of bean and cabbage, but showed a clear preference for cabbage odour if sinigrin had been painted on the back of the mummy. Similarly Aphidius rhopalosiphi reared on Metopolophium dirhodum on wheat preferred the odour of wheat plants grown near tomato plants to odour of wheat alone if the wheat plants on which they had been reared had been exposed to the volatiles of nearby tomato plants. Aphidius rhopalosiphi reared on M. dirhodum, and removed from the mummy before emergence, showed a preference for the odour of a different wheat cultivar if they had contacted a mummy from that cultivar, and similar results were obtained with A. colemani naturally emerged from M. persicae mummies. Aphidius colemani emerged from mummies on one crucifer were allowed to contact in sequence (for 45 min each) mummies from two different crucifers. The number of attacks made in 10 min on M. persicae was always significantly higher when aphids were feeding on the same plant as the origin of the last mummy offered, or on the second plant if aphids feeding on the third plant were not included. Chilling emerged A. cole- mani for 24 h at 5°C appeared to erase the imprint of secondary plant chemistry, and they no longer showed host plant odour prefer- ences in the olfactometer. When the parasitoids were chilled after three successive mummy experiences, memory of the last experience appeared at least temporarily erased and preference was then shown for the chemistry of the second experience.
Highlights
Aphidiine parasitoids of aphids show strong olfactory preferences for what is called the “host plant complex”, the combination of their host aphid on leaves of the host plant (Wickremasinghe & van Emden, 1992; Guerrieri et al, 1993; Du et al, 1996; Powell et al, 1998)
Douloumpaka & van Emden (2003) found that rearing parasitised aphids on artificial diet revealed a maternal cue for the odour of the plant on which the mother parasitoid had developed, but the offspring emerging from excised pupae showed no response to this cue
Under the unnatural circumstances of a parasitoid developing in an aphid on artificial diet, this maternal cue may either re-inforce or cancel out the cue acquired by the host aphid during the 48 h of plant-feeding while exposed to parasitisation (Douloumpaka & van Emden, 2003)
Summary
Aphidiine parasitoids of aphids show strong olfactory preferences for what is called the “host plant complex”, the combination of their host aphid on leaves of the host plant (Wickremasinghe & van Emden, 1992; Guerrieri et al, 1993; Du et al, 1996; Powell et al, 1998). Attraction can often be shown to volatiles from uninfested host plant leaves (Powell & Zhang, 1983; Powell & Wright, 1992; Wickremasinghe & van Emden, 1992) Some of these olfactory responses appear to be innate (Storeck et al, 2000), using the term to denote behavioural responses that do not appear to have been learned, as discussed by Vet et al (1995). The parasitoid would, encounter the chemical cues in both the silk and the aphid cuticle while cutting its way out of the mummy during normal emergence (Fig. 1) This acquisition of behavioural responses to olfactory stimuli encountered during adult eclosion and emergence from the mummy has been termed “emergence conditioning” (Storeck et al, 2000), the process could be regarded as a form of “imprinting” (see discussion of conditioning and imprinting processes by Hollis et al, 1991). Work is described showing evidence of memory acquisition by the parasitoids, and how that memory may be temporarily erased by chilling, as previously demonstrated for a number of other insects, for example Drosophila melanogaster Meigen (Diptera: Drosophilidae) by Xia et al (1999)
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