Specificity in the chemically-mediated interactions between milkweed and its herbivores

The classic view of herbivores caught between plant defenses and predation, suggests that trade-offs in the ability to exploit plant resources with resistance to predation may allow coexistence. We have recently found high levels of variation for the sequestration ability of different herbivores on milkweed (see Figure). We are currently continuing to complete the sequestration survey (an additional five species of herbivores and several parasitoids), and plan to utilize these data on relative sequestration to predict resistance to predation. In addition, in collaboration with Susanne Dobler, we are sequencing all these species for the target ATPase gene that is responsible for monarch caterpillar insensitivity to cardenolides.

Recent evidence has shown that many specialist herbivores that are attracted to plants with high levels of ‘defense’ and that sequester these compounds, may still be negatively impacted by them. We are thus testing the novel hypothesis that herbivores that cause strongest induced plant responses may benefit the most from sequestration, yet may be the slowest growing or least competitively dominant. We have dubbed this relationship and its association with plant exploitation ability the ‘specific induction - specific sequestration’ hypothesis for herbivore coexistence.

This hypothesis will be tested by 1) collecting sequestration data on the rest of the community, 2) calculating interaction strengths of predators on each herbivore species, and 3) utilizing data on the specificity of induction of both defense compounds and effects on the focal herbivore.