Feedback between invasive belowground herbivores and habitat composition in northern hardwood ecosystems

Background/Question/Methods

A complex of invasive root feeding weevils has recently undergone a population eruption in the upper Great Lakes region. Species include Phyllobius oblongus, Polydrusus sericeus, Sciaphilis asperatus, Barypeithes pellucidus and Trachyphloeus aristatus. The larvae feed primarily on fine roots, and completely dominate the belowground mesofauna. Soil cores indicated that significantly more larvae occur in the upper 15 cm of soil, which coincides with the greatest amount of fine root biomass. The larvae experience relatively steady mortality throughout the winter, with highest mortality occurring in spring. Adult emergence peaks in May and early June, and is mostly completed by mid-June. Adults of the various species emerged at different times depending on their larval host plant. Adult weevils are folivores, feeding primarily on sugar maple, ironwood, and raspberry. Adult populations peaked in June, and nearly all were gone by mid-July. Weekly censuses of the two most prevalent species, P. oblongus and P. sericeus, indicated differential temporal patterns of host utilization. Sugar maple was utilized most heavily in early spring and summer, but adults became increasingly concentrated on ironwood and raspberry as the season progressed.

Results/Conclusions

We conducted field and laboratory assays to evaluate the effects of host utilization patterns. In the field, P. oblongus adult emergence was as high in patches of the late-season hosts, ironwood and raspberry, as it was in the early-season host, sugar maple. Emergence of P. sericeus was substantially higher from ironwood and raspberry than sugar maple. Laboratory assays were used to compare adult longevity and fecundity on sugar maple, ironwood, and raspberry. Host plant had minimal effect on adult longevity, but egg production of both weevils was significantly greater when fed raspberry than ironwood or sugar maple. Likewise, both species produced more eggs when fed on ironwood than sugar maple. These results have implications to four general areas of invasion ecology: the intersection of belowground herbivory and exotic insects; the impacts of and interactions among multiple invasive insect species; the effects one invasive complex, in this case rhizophagous insects, may have on a subsequent invasive complex, in this case earthworms which are beginning to occupy these sites; and the effects of forest management practices on invasive herbivores. Specifically, practices such as logging and road construction, and natural processes such as gap formation may indirectly benefit invasive weevil populations due to the positive effects of disturbance on raspberry.