Japanese barberry (Berberis thunbergii DC.) has rarely been observed to experience noticeable defoliation from insect or mammal herbivores, and this freedom from enemies is presumed to contribute to its invasiveness. However, during the summer of 2007, I observed extensive defoliation of current year's shoots on Berberis populations across central and northern New Jersey. Additional reports from volunteer observers showed that similar amounts of herbivory occurred in southern New York. I examined the upper 15 cm on a sample of 4868 current year's shoots on 320 plants in a population in New Jersey; of these shoots, 11.6% were completely defoliated, 65.3% were partially defoliated, and 23.1% showed no defoliation. Only 9% of plants showed no defoliation on any shoot; 87% of these were small plants with less than three stems per root base. The fraction of stems completely or partially defoliated was higher on larger plants (more stems per root base), but did not vary with slope position, plant density, or numbers of current year's shoots per plant. The defoliating agent was shown to be Coryphista meadii, the barberry geometer, a lepidopteran native to all of North America. Further studies will be necessary to determine if the herbivory is recurrent, if it is occurring throughout the introduced range of B. thunbergii, and if it has impacts on barberry's reproduction, growth, and survivorship.
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