Everything that kills me, makes me feel alive: The paradox of plant nutrition and defense

Induced plant chemical defenses against insect herbivores can be classified as direct and indirect defenses. These defenses are not constitutively present because they incur a high metabolic cost to the host plant. As a result, these defenses are shown to be more affected by resources available to the plant. Insects consume plants for nutrition but while doing so, face defenses. Therefore, an insect’s ability to consume a plant hangs in a delicate balance between available nutrition and the defenses induced by the plant. We lack information on which of these two factors play a more decisive role for the insect herbivore when choosing a host. By conducting insect preference and performance assays coupled with measurement of plant direct and indirect defenses in controlled nutrition environment, we try to unravel the dynamics between plant nutrition, defense and insect behavior. We hypothesized that less defended plants would be a preferred host for insects; and plants with access to less nutrition would have lower defenses due to lack of resources. After measuring herbivory induced benzoxazinoid abundance (direct defense), headspace volatiles and predator preference (indirect defense) we found that plants growing in high nutrition soil had higher defenses than plants in low nutrition soil. Despite having high defenses, insect herbivore preferred to consume these plants and contrary to our hypotheses, performed better feeding on highly defended plants growing in high nutrition environment. Our results suggest that plant defense can be a formidable factor over plant defense for insect herbivores when choosing a host plant.