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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.
2011 Project Description
We have now evaluated symbiont infection in paired native and introduced populations of 18 species, with particular focus on aphids. We have also begun a broader scale survey of aphids to determine within and among population variation.
We have collected approximately 400 populations of aphids representing at least 100 different species. We have used diagnostic PCR to determine symbiont status of approximately 30 species. We continue to pursue in-depth investigations of Aphis glycines (soybean aphid), Aphis craccivora (cowpea aphid) and the whitefly parasitoid Encarsia inaron.
We now have cured multiple populations of A. glycines, and are investigating interactions with host plants and stress. This year we have given 2 presentations on this aspect of the project, and have a manuscript in preparation.
For Aphis craccivora, we have now acquired more than 30 worldwide introduced and native populations. This year we have given 2 presentations on this aspect of the project and have 2 manuscripts in preparation.
For Encarsia inaron, we have completed our work on the outcome of larval competition, parasitism success in late instar hosts and sex ratio of offspring. This year we have given 1 presentation on this aspect of the project, have published one paper, and have another manuscript in preparation.
Thus far, we have not found consistent patterns of symbiont gain or loss between native and introduced populations, but are finding that symbiont composition often differs between native and introduced populations.
We have found that most aphid species examined have at least one facultative symbiont. These symbionts are often not fixed within or among populations, suggesting that symbiont-associated variation might have important ecological and evolutionary ramifications for this group.
Aphis glycines has the symbiont Arsenophonus in both native and introduced portions of its range. Our tests of Arsenophonus functionality have not indicated a defensive role for the symbiont, but we are seeing evidence that the symbiont may influence and be influenced by aphid/plant interactions.
In Aphis craccivora, the symbiont Hamiltonella is more prevalent in the introduced range, but overall symbiont diversity is similar between native and introduced ranges. We also see some patterns with respect to symbiont infection and host plant usage. 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. 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.
White, J.A. 2011. Caught in the act: rapid, symbiont-driven evolution. BioEssays 33: 823-829.
White, J. A., C. Hurak, J. A. Wulff, M. S. Hunter, and S. Kelly. 2011. Parasitoid bacterial symbionts as markers of within-host competitive outcomes: superparasitoid advantage and sex ratio bias. Ecological Entomology 36: 786-789.