Invasive Spotted Knapweed Alters Insect Communities on Native Common Milkweed and May Confer Associational Resistance
Dr. Mark Hunter, University of Michigan
Several aspects of the interactions between the invasive plant species spotted knapweed (Centaurea maculosa) and the native species common milkweed (Asclepias syriaca) were investigated. Our results indicate a negative direct effect, and an even greater positive indirect effect, of knapweed on milkweed. The presence of knapweed significantly altered the composition of the insect communities found on milkweed, and under certain conditions milkweed benefited from association with knapweed. Abundances of two major milkweed herbivores decreased with increasing knapweed density. Likely as a result of decreased herbivory, milkweed height and leaf number increased with increasing exposure to knapweed, indicating a positive indirect effect of knapweed exposure on milkweed health and growth. However, when insect abundances were observed to be naturally low, knapweed had a negative direct effect on milkweed and reduced the rate of plant growth over time. These results emphasize the complexity of natural systems and of interactions between native and invasive species.
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