Interspecific Variation Within the Genus Asclepias in Response to Herbivory by a Phloem-feeding Insect Herbivore

Abstract

Induced plant responses to leaf-chewing insects have been well studied, but considerably less is known about the effects of phloem-feedings insects on induction. In a set of laboratory experiments, we examined density-dependent induction by the milkweed-oleander aphid, Aphis nerii, of putative defenses in four milkweed species (Asclepias incarnata, Asclepias syriaca, Asclepias tuberosa, and Asclepias viridis). We hypothesized that high aphid density would lead to increased cardenolide expression in species with low constitutive levels of cardenolides (e.g., A. tuberosa), but that there would be no induction in high constitutive cardenolide species (e.g., A. viridis). Based on previous studies, we did not expect cardenolide induction in A. incarnata. Contrary to our predictions, we observed feeding-induced declines of cardenolide concentrations in A. viridis. Cardenolide concentrations did not respond to aphid feeding in the other three milkweed species. Aphids also caused reductions in biomass accumulation by two of four Asclepias species, A. viridis and A. incarnata. High aphid density led to a decrease in A. viridis foliar nitrogen concentration. However, aphids had no effect on the defensive chemistry, growth, or nutritional quality of either A. syriaca or A. tuberosa. Our results highlight that congeneric plant species may respond differently to the same levels of herbivore damage.

Physiological factors for tolerance of Kosteletzkya virginica (L.) Presl to one-instar bollworms of Helicoverpa armigera (Hubner)

“…The induced responses are broadly defined as any modification in the plant following damage, including changes of toxic or antinutritional compounds, other physiological characteristics and morphological trait (Zehnder and Hunter 2007). Except toxic or autinutritional factor, such as phenolic (Kudo 2003) and cardenolide (Zehnder and Hunter 2007), some metabolic changes, such as plant protein (Inbar et al 1999), sugar (Liu et al 1998), respiration and photosynthesis (Chen et al 2003a, b), are clearly part of the repair process to injured plant tissue (Edwards and Wratten 1983). In previous cases, it was shown that insect stress not only caused the defoliation, but also affected the growth rate and nutrition uptake (Feng and He 1995;Kosola et al 2001;Masters 1995;Xia et al 1993;Zehnder and Hunter 2007).…”

Secondary Defense Chemicals in Milkweed Reduce Parasite Infection in Monarch Butterflies, Danaus plexippus

“…For a total of 15 replicates per treatment, six leaf disks were placed together in a vial containing 1ml of methanol and stored at −80°C prior to analysis. We measured the Bcardenolide dose^provided to caterpillars on leaf disks using methods described by Zehnder and Hunter (2007).…”

Mycorrhizae Alter Toxin Sequestration and Performance of Two Specialist Herbivores

“…We assessed the cardenolides that herbivores sequestered following wellestablished methods (Zehnder and Hunter, 2007;. Aphids and caterpillars were ground for 3 min in methanol, sonicated for 1 h, and then centrifuged for 6 min.…”

“…Using these models, we evaluated the effects of AMF inoculum availability on three measures of cardenolides sequestered by herbivores; total cardenolide concentration (sum of all cardenolide peaks), cardenolide diversity (using Shannon's index), and cardenolide polarity (relative representation of lipophilic cardenolides), calculated by summing the relative peak areas multiplied by each peaks' retention time (Rasmann and Agrawal, 2011;Sternberg et al, 2012). A greater diversity of cardenolides and more lipophilic cardenolides are considered more toxic than lower diversity or more polar mixes (Fordyce and Malcolm, 2000;Zehnder and Hunter, 2007;Sternberg et al, 2012). Because herbivores feeding upon A. incarnata rarely sequestered cardenolides (Table S1), they were excluded from all sequestration analyses.…”