Indirect interactions and community processes: impacts of hemi-parasitic plants on the structure of insect herbivore communities.

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Yellow-rattle is a parasitic plant that steals resources from other plants (host plants) via root connections. Water and nutrients are diverted from the host into the yellow-rattle, reducing the growth of the host. This may make the host less suitable for insects which also live on the host plant, particularly those that feed on the same water and nutrient supply as the yellow-rattle. Previous experiments in greenhouses have shown that insects which are fed on host plants infected with yellow-rattle are twice as likely to die than those feeding on uninfected hosts, but no one has measured these effects in natural communities where there are many different insects and plants. As well as making host plants less suitable for insects to feed on, yellow-rattle can also reduce the number of host plants for the insects - it prefers to infect grasses so in places where there is a lot of yellow-rattle the amount of grass decreases and grass-feeding insects may also decrease. This has not been tested yet, nor do scientists know whether it is the changes in the amount of grass or the changes in the suitability of the host plants which have the biggest effect on insects. We plan to find out how yellow-rattle affects the number and type of insect herbivores that are living in the same ecological community. We expect that grass-feeding species of insects will be reduced in number, but insects feeding on host plants that are not attacked by yellow-rattle will increase when the yellow-rattle is present. We think yellow-rattle will cause these effects in two ways: (1) by changing the balance of plant species living in the community and (2) by changing the suitability of the infected plants for insects. We will investigate which of these effects is the most important. We will also test the ability of individual insect species to grow and reproduce on plants with and without yellow-rattle present and find out whether insects avoid feeding on infected plants.