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Invasive Species and Biological Control: the Role of Facultative Inherited Bacterial Symbionts
Department of Entomology
In recent years, increased global commerce and travel have also increased the frequency at which organisms become introduced and established in new habitats, sometimes with devastating consequences. Invasive insects such as the emerald ash borer and the European corn borer cost billions of dollars annually, and threaten the security of our food supply when they attack crop species.
One method for controlling such invasive insects is biological control, which uses natural enemies of the pest (predators, parasitoids and pathogens) to reduce pest numbers. Biological control is an attractive alternative to pesticides because, once established, natural enemies are self-sustaining and do not require the monetary and environmental costs associated with repeated chemical applications. However, a substantial proportion of biological control releases are unsuccessful, increasing both the cost associated with biological control and the potential for unintended ecological effects.
To maximize benefit while reducing risk, we need to know as much as possible about the biology of both pest and natural enemy, to determine which natural enemies will be most capable of pest suppression. One aspect of insect biology that is just being clarified is the role of bacterial endosymbionts. Advances in molecular techniques have allowed greater investigation of these mysterious bacteria that are found within many insects, and it has become clear that they can affect many aspects of insect biology, including reproduction, dispersal, host choice, and susceptibility to natural enemies; these factors, in turn, likely influence the establishment and spread of introduced species and the effectiveness of biological control.
In this project I will investigate how symbionts might affect biological control.
First, I will use molecular techniques to identify bacterial symbionts within a series of introduced species collected from native versus exotic populations. Broad patterns of symbiont loss versus retention across species would suggest that symbionts routinely influence and/or are influenced by the process of introduction. Thus, symbionts may represent a new avenue for exploring the causes and consequences of invasiveness.
Second, I will investigate whether symbionts influence the effectiveness of a specific introduced biological control agent, the parasitoid wasp Encarsia inaron. This parasitoid is naturally infected with two bacterial endosymbionts, but can be cured of one or both symbionts. I will compare the effectiveness of parasitoids infected with different combinations of symbionts to determine whether symbionts improve or harm the parasitoid?s ability to control populations of the pest.
This model system provides a greater understanding of symbionts in introduced species, and may ultimately lead to improved biological control, either through selection of target pest species that are most vulnerable to introduced natural enemies, or through selection of biological control agents with the greatest chance for success.
2010 Project Description
We are currently evaluating symbiont infection of native and introduced populations of 12 species. We have run DGGE bacterial community analysis on all of them. Of these, 3 species are receiving more in-depth investigation.
In the soybean aphid, Aphis glycines, we are characterizing a symbiont that is prevalent in both native and introduced portions of its range, we have experimentally cured the aphid of its symbiont using antibiotics, and are now testing symbiont function in the aphid. In the cowpea aphid, Aphis craccivora, we are broadening our investigation of symbiont community composition to multiple introduced and native populations. At present, we have 12 populations, and we are in the process of acquiring additional worldwide populations. In the whitefly parasitoid, Encarsia inaron, we have also examined multiple native and introduced populations, and undertaken a collaboration with a systematist to determine whether a species complex, rather than single parasitoid species is present. We have given 3 oral presentations and two posters on this project, and have one manuscript in preparation.
With Encarsia inaron, we have also investigated whether symbionts influence the outcome of larval competition, parasitism success in late instar hosts, and sex ratio of offspring, as mediated by wasp size. We have given 4 oral presentations on these experiments, and have 2 manuscripts in preparation.
We have found symbionts in 3 species. The soybean aphid has what appears to be the symbiont Arsenophonus in both native and introduced portions of its range. In the cowpea aphid, the symbiont Hamiltonella is prevalent in some portions of the introduced range, but not the native range, and a single individual from the introduced range had a different symbiont, Serratia. In Encarsia inaron, all individuals from the introduced range had the symbiont Wolbachia, but none of the individuals from the native range had it. Some individuals from both populations also had the symbiont Cardinium.
We did not find symbionts in the remaining 9 species, but are using additional protocols to substantiate the apparently negative results. Thus far, the results do not support a conclusion of symbiont loss in invasive species, but certainly seem to suggest that the symbiont composition of invasive species frequently differs between native and introduced populations. Because symbionts can affect the fitness and susceptibility of insects to natural enemies, these data will be useful to biologists studying the spread and biological control of invasive species.
We have found that the presence of symbionts does not apparently alter the competitive ability of Encarsia inaron larvae within a parasitized host. However, using symbionts as a marker, we have learned larvae from the superparasitizing wasp often do win the competition, particularly when the first and second eggs are laid within a few hours of one another. Moreover, offspring from the second egg are more likely to be male. This information will be useful to biological control scientists and practitioners; superparasitism and male production are inefficient in biological control organisms, but might prove a stabilizing force in the population dynamics between host and parasitoid thus improving long-term biological control.