Relationship Between Japanese Beetle (Popillia japonica Newman) Larval Density, and Microbial Community Structure in Soil Microcosms
Invasive species are a serious economic and ecological threat throughout the world. When it comes to aboveground insects like the emerald ash borer or the gypsy moth, the ecological impacts caused by their presence has been well documented. However, in the case of invasive insects that spend the majority of their life underground, their effects on soil processes are far less apparent. Since its introduction almost a century ago, the Japanese beetle Popillia japonica Newman (JB) has managed to invade most of the eastern United States. The larval stage is a serious pest, feeding on the soil, thatch, and roots of turfgrass and numerous horticultural crops. In order to break down the relatively nutrient poor soil matrix, the larval hindgut is modified into a multi-lobed, bulbous, microbe-rich fermentation chamber where symbiotic microbes aid digestion. However, the origin and composition of these gut microbes, and whether or not they modify interactions between JB larvae and the soil is unclear. The goal of this study is to gain a better understanding of the interactions between Japanese beetle (JB) larvae and their subterranean environment. Specific objectives include (1) quantifying the impact of JB larvae on soil microbial activity and functional diversity, (2) characterizing the microbiota associated with the JB larval gut and (3) identifying linkages between soil and gut microbes. Using GC/MS, CO2 emissions from microcosms containing JB-infested and uninfested soil were measured as a surrogate for the impact of JB larvae on soil microbial activity. Coarse changes in the functional diversity of soil microbes were also examined using phospholipid fatty acid analysis (PLFA). Findings of the microcosm study support the idea that JB larvae cause significant changes to soil microbial activity resulting in increased soil respiration. These changes were accompanied by changes in microbial diversity that was characterized by shifts in soil microbial populations toward bacteria and away from fungi and protozoa. In contrast to the information provided by PLFA analysis, which broadly characterized the functional composition of the soil microbial community by the percentages of bacteria, fungi, and protozoa, 16S rRNA sequencing of JB-infested and uninfested soil provide little evidence that JB larval infestation have a major influence on soil bacteria. Bacterial communities of JB-infested and uninfested soils share similar taxonomic and functional profiles. Taken together these findings suggest that changes in soil microbial communities resulting from JB larval infestation may be largely due to effects on eukaryotic microbes. Characterizations of JB larval gut bacteria reveal that the whole gut bacterial community experiences major taxonomic shifts during larval development. These differences were likely due to in-gut selection; first instar larval gut microbiota was similar to that of the soil, 3rd instar larval gut bacterial communities were less diverse and dominated by groups with greater cellulolytic competence. Although the functional profiles of 1st and 3rd instar JB larval gut microbiota paralleled their taxonomic trends there appeared to be a higher degree of functional convergence. This finding supports the idea that despite the increased microbial diversity that may result from the community’s environmental sourcing of gut microbes, functional convergence of these microbial communities may occur in response to the physiological needs of the host.
Richmond, Purdue University.
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