Abundance of Herbivores on Six Milkweed Species in Illinois

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Abstract
To aid the understanding of the role of insect herbivores as selective agents in the evolution of their host plants a survey of herbivore abundance and impact was undertaken in central Illinois [USA] on the 6 milkweed species: Asclepias incarnata, A. sullivantii, A. syriaca, A. verticillata, A. amplexicaulis and A. tuberosa. These species occur in this order on a moisture gradient from wet to dry soil conditions. This survey revealed that 12 spp. occurred at an abundance of at least 1 individual/100 host stems in 1 plot-year on 1 host species: Oncopeltus fasciatus*, Lygaeus kalmii*, Aphis nerii*, Labidomera clivicollis*, Tetraopes tetrophthalmus*, T. femoratus, T. quinquemaculatus, Rhyssomatus lineaticollis*, Danaus plexippus, Cycnia tenera*, Euchaetias egle and a leafminer. All but L. kalmii are specific to milkweeds in Illinois. Seven of these species, marked with asterisks, were abundant enough to act as major selective forces on the life history patterns of the milkweed species, populations and clones concerned. Results support the hypothesis that major differences in herbivore species and abundance on milkweeds are largely a result of differences in the environments that milkweeds successfully colonize and morphological (allocational) differences between milkweeds. This hypothesis related to differences in herbivores between milkweed species, populations within a species and clones within a population. The herbivores were very patchily distributed on milkweed species; and, or all the possible herbivore-host plant relationships, in only 17% of the pair-wise comparisons in the 12 .times. 6 matrix were herbivores abundant on host plants. When all plot-years and all pair-wise comparisons were considered, in only 10% of the 252 possible situations in the 12 .times. 21 matrix were herbivores abundant on host plants. Such patchiness of herbivores supports recent theory on plant defense strategies, which argues that early successional plants, such as the milkweeds studied, are hard to find by specialized herbivores.