The influence of aphid parasitoids on the development of their hosts

Abstract

Changes in wing development and development through the stadia of Aphis fabae are examined after parasitisation by Aphidius colemani along with the underlying mechanisms. Parasitoid oviposition into nymphs of winged gynoparae of A. fabae shows that wing development is inhibited and the earlier parasitism takes place, the stronger the effect. Consideration of the timing of events indicates that the active component(s) is most likely injected during oviposition and is not related to hatching and growing of the parasitoid larva. When extracts of venom glands are injected into late-second stadium aphids, many develop to fourth-stadium possessing rudimentary wingbuds, show developmental arrest and often die while moulting to the adult stage. Injections with extracts of female or male parasitoids into later stadia give similar results with regard to development to the adult but aphids injected in the late-fourth stadium develop normally. Experiments with long-day winged virginoparae reveal similar results in development to the adult but injections into long-day wingless aphids, which develop more rapidly, show this effect only when performed early in the third stadium. The earlier the injection before the final moult the greater the effect of the extract on preventing adult development. The results indicate that there is an active factor(s) in the female parasitoid’s venom that disrupts wing development and/or inhibits development to the adult; the loss of activity after treatment at 100 °C or with protease makes it likely that the factor(s) is a protein. Surprisingly, injections of extracts from male parasitoids have similar effects. The location and function of such a factor(s) in males are unknown. Fast Protein Liquid Chromatography is used to fractionate the parasitoid extracts and bioassays reveal activity in fractions 16-25. Aphidius ervi venom also inhibits development of A. fabae (a non-host) whereas A. colemani venom has no effect on the development of Megoura viciae (a non-host). Injection with lipopolysaccharides and infection with a fungal pathogen are used to test whether stimulation of the immune response affects wing development in the host and the possibility that the host is in control of inhibiting its own wing development in order to support its immune system is discussed.