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Little work has been done to determine the relationship between modularity and chemical defense induction against herbivory in herbaceous plant species. We investigated this topic using Asclepias syriaca, common milkweed, and one of its specialist aphid herbivores, Aphis nerii. We studied milkweeds of varying modularities growing in the prairies of Pierce Cedar Creek Institute in Hastings, MI and introduced A. nerii to the plants, monitoring growth of plants and aphid populations over time. After A. syriaca leaves and A. nerii were harvested from study sites, leaf samples from aphid and control treatments were analyzed to measure variation with degree of modularity in cardenolide concentration, leaf hair length, leaf size, and aphid population growth rate. Although we predicted that plant chemical defense expression would be lower in more modular milkweed genets and aphid performance would be higher in these genets, we were unable to reject the null hypotheses that milkweed modularity has no effect on the level of plant chemical defense expression in response to aphid herbivory or on measures of aphid performance such as population growth rates or cardenolide sequestration. We conclude that the high variability in aphid treatments among replicates was too high to detect responses. We also conclude that our aphid “sinks” for leaf phloem resources were too subtle to induce leaf defenses and detect significant responses in both plants and aphids to variation in milkweed modularity.
Syrotchen, Jill, "The influence of plant modularity on the defense against specialist insect herbivory" (2018). Honors Theses. 3026.
Honors Thesis-Open Access