How non-native plants affect host-finding in native insects: Evidence of associational resistance against specialist herbivores
Date
2016
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Publisher
University of Delaware
Abstract
In addition to their well-known direct impacts on ecosystems, non-native plants may have indirect effects on native insect populations. I established experimental plots in southeastern Pennsylvania to investigate whether the matrix of plants (native or non-native) within which a native plant exists impacts insect use of that plant. Each experimental plot consisted of a cluster of common milkweed (Asclepias syriaca L.) surrounded by an eight meter circle of either native or non-native plants, other common milkweeds, or mowed field. I surveyed the insects on the focal milkweed plants once a week from mid-June to mid-August in 2009 and 2010. The number of insects per leaf was calculated at each sample and the largest value was used in a two-way ANOVA to determine if the treatments differed from one another. Insect community composition was also analyzed for differences using non-metric multidimensional scaling (NMDS). Herbivore damage on focal milkweed plants was surveyed once in mid-summer and this was also compared among treatments via a two-way ANOVA. Statistically significant differences among matrices were found in both the insect community and the amount of herbivory on focal milkweed. There were two- three times more milkweed beetles (Tetraopes tetropthalmus Forster) per leaf of focal milkweed plants in treatments not surrounded by a non-native. This supports the hypothesis that non-native plants provide a degree of associational resistance to the native plants they surround. This should be an important consideration when evaluating the impact that non-native plants have on invaded ecosystems.