Thinking Outside the Asclepias Box: Oleander Aphids and Honeyvine Milkweed

Abstract

The oleander aphid, Aphis nerii Boyer de Fonscolombe, feeds on milkweeds in the Apocynaceae family. In Kentucky, A.nerii colonizes several common Asclepias species and honeyvine milkweed, Cynanchum laeve (Michaux). Previous studies investigating whether and how host plant characteristics influence A.nerii fitness have produced conflicting results. Our observations suggested that A.nerii more readily colonized C.laeve than Asclepias species. We hypothesized that aphid fitness differed among host plant species having different characteristics, and specifically, that A.nerii performance would be greatest on host plants having low cardenolide concentrations and few physical defenses. Physical and chemical characteristics were measured on C.laeve and three Asclepias species. Laboratory and field experiments were conducted to compare A.nerii performance among these four milkweed species in the absence of natural enemies. Changes in aphid densities over time were examined in the presence of natural enemies. Plant characteristics and aphid performance were found to differ among milkweed species. In the absence of natural enemies, A.nerii performance was best on C.laeve. When exposed to natural enemies, A.nerii densities remained higher on A.incarnata and C. laeve than on A.syriaca and A.tuberosa. Physical differences between C.laeve, a vining milkweed, and tested Asclepias species may increase suitability of honeyvine milkweed for A.nerii colonization. Our results suggest that C.laeve may rely more on tolerance and less on nonpreference or antibiosis resistance to herbivores.

Transcriptional profile and differential fitness in a specialist milkweed insect across host plants varying in toxicity

“…Some studies have demonstrated no effect of cardenolides on A. nerii survival, fecundity, development, density and growth rate, but these studies focused on low‐toxicity plant species or experimental applications to vary cardenolide production within a species (Mooney, Jones, & Agrawal, ; Mooney et al., ; Zehnder, Parris, & Hunter, ). Comparing milkweed species that vary more substantially in cardenolide toxicity, we found that host plants with increased and more nonpolar cardenolides have negative effects on A. nerii development and fecundity, consistent with other studies that have demonstrated that plant species characterized by more toxic cardenolides are negatively associated with A. nerii population growth (Agrawal, ; Colvin, Snyder, & Thacker, ; de Roode et al., ). In the first fitness experiment, there was evidence of greater longevity on more highly defended plants (Figure e), perhaps indicative of trade‐offs between fecundity and longevity (Blacher, Huggins, & Bourke, ; Miyatake, ).…”

“…While cardenolide variation is the most likely reason for the fitness effects seen here and in other studies (Agrawal, ; Colvin et al., ; de Roode et al., ), milkweeds utilize a suite of defences and it is possible that other plant defences could also play a role in influencing these insects’ fitness. However, increased cardenolide production across various milkweed species, including A. incarnata , A. curassavica and Gomphocarpus spp., is not correlated with increases in other defences (i.e., latex, trichomes or phenolics) (Agrawal & Fishbein, ; Agrawal, Salminen et al., ).…”

Mycorrhizae Alter Toxin Sequestration and Performance of Two Specialist Herbivores

“…Monarch caterpillars, in contrast, have NA + /K + -ATPases that are insensitive to cardenolides (Dobler et al, 2012;Petschenka and Agrawal, 2015). Despite being able to tolerate cardenolides, both oleander aphids and monarch caterpillars exhibit reduced performance on host plants with high concentrations of cardenolides (Zalucki et al, 2001a;Agrawal, 2004Agrawal, , 2005Rasmann et al, 2009;de Roode et al, 2011;Colvin et al, 2013;Tao et al, 2016b;Birnbaum et al, 2017). Furthermore, both oleander aphids and monarch caterpillars sequester cardenolides (Rothschild et al, 1970;Malcolm and Brower, 1989;Malcolm, 1990;Züst and Agrawal, 2016b), providing an effective defense against aphid predators (Pasteels, 1978;Malcolm, 1989Malcolm, , 1992Pappas et al, 2007;Mooney et al, 2008) and monarch predators and parasites (Brower et al, 1968;Reichstein et al, 1968;Brower and Moffitt, 1974;Sternberg et al, 2012).…”