Impact of plant-beneficial soil microbes on aboveground parasitism of insect herbivores

Aphids are important plant sap-feeding insects, affecting plant ecology and crop yields by direct feeding and transmission of plant viruses. These small soft-bodied insects are well-known pests of agricultural crops, as well as ornamental plants such as roses and tulips. Populations of aphids are naturally controlled by (i) host plant resistance and (ii) aphid natural enemies (including ladybirds, lacewings, syrphids, and parasitoid wasps). With a deeper understanding of how plants deal with herbivores in natural populations, we can exploit these beneficial interactions and implement sustainable pest control methods. Under current climate predictions, insect pest outbreaks are likely to increase with consequences for reduced crop yields and future impacts on the natural environment through increased conversion of land for agriculture or increased application of pesticides.

We have shown that host plant 'anti-aphid' defences can be induced by inoculating barley plant roots with naturally occurring beneficial bacteria, which reduces aphid reproduction and lifespan, and thus population sizes. We recently discovered that aphids on microbe-inoculated plants are parasitized more and earlier by parasitoid wasps. Parasitoid wasps lay an egg inside the aphid host and, after egg-hatching, the larva feeds on the aphid before emerging as an adult leaving only a dry husk of the aphid body behind (an 'aphid mummy'). We hypothesise that plant inoculation with defence-inducing bacteria increases the attractiveness of plants to parasitoid wasps (via plant volatile chemicals), and that aphids feeding on inoculated plants are more susceptible to parasitism (reduced immune responses). Aphids also host a variety of bacterial endosymbionts that help aphids to feed on nutritionally poor plant sap, but they can also confer resistance to parasitoid wasps. We will use a set of behavioural studies combined with chemical analysis of the plant and aphid to understand how plant inoculation with beneficial bacteria can lead to increased parasitism of aphids.

This work will contribute to our understanding of how plant root microbiomes can have strong impacts on aboveground interactions, and across multiple trophic levels (from soil bacteria to parasitoid wasps of herbivorous insects). Our results will improve understanding of aphid parasitism and herbivore regulation in both (i) natural ecosystems, and (ii) agroecosystems, enabling the development of sustainable insect pest management strategies.