Pretty Picky for a Generalist: Impacts of Toxicity and Nutritional Quality on Mantid Prey Processing.

Prey have evolved a number of defenses against predation, and predators have developed means of countering these protective measures. Although caterpillars of the monarch butterfly, Danaus plexippus L., are defended by cardenolides sequestered from their host plants, the Chinese mantid Tenodera sinensis Saussure guts the caterpillar before consuming the rest of the body. We hypothesized that this gutting behavior might be driven by the heterogeneous quality of prey tissue with respect to toxicity and/or nutrients. We conducted behavioral trials in which mantids were offered cardenolide-containing and cardenolide-free D. plexippus caterpillars and butterflies. In addition, we fed mantids starved and unstarved D. plexippus caterpillars from each cardenolide treatment and nontoxic Ostrinia nubilalis Hübner caterpillars. These trials were coupled with elemental analysis of the gut and body tissues of both D. plexippus caterpillars and corn borers. Cardenolides did not affect mantid behavior: mantids gutted both cardenolide-containing and cardenolide-free caterpillars. In contrast, mantids consumed both O. nubilalis and starved D. plexippus caterpillars entirely. Danaus plexippus body tissue has a lower C:N ratio than their gut contents, while O. nubilalis have similar ratios; gutting may reflect the mantid's ability to regulate nutrient uptake. Our results suggest that post-capture prey processing by mantids is likely driven by a sophisticated assessment of resource quality.

Impact of Consuming 'Toxic' Monarch Caterpillars on Adult Chinese Mantid Mass Gain and Fecundity.

Predators that feed on chemically-defended prey often experience non-lethal effects that result in learned avoidance of the prey species. Some predators are able to consume toxic prey without ill-effect. The Chinese mantid is able to consume cardenolide-containing monarch caterpillars without immediate adverse effects. Although they discard the caterpillars' gut contents, mantids consume sequestered cardenolides. Although consumption of these cardenolides does not elicit an acute response, there may be long-term costs associated with cardenolide consumption. We tested the hypothesis that consumption of monarch caterpillars will adversely affect adult mantid biomass gain and reproductive condition. We reared mantids from egg to adult and assigned them to one of four toxicity groups that differed in the number of monarch caterpillars offered over a 15-day period. Mantids consumed similar amounts of prey biomass during the experiment. Yet, mantids in the high-toxicity group had a higher conversion efficiency and gained more biomass than mantids in other groups. Mantids in all treatment groups produced similar numbers of eggs. However, mantids in the high-toxicity group produced heavier eggs and devoted a greater portion of their biomass toward egg production than those in the control group. This increase in reproductive condition is probably driven by variation in prey nutritional value and/or the nutritional advantages inherent in eating multiple food types. Our results demonstrate the mantid is able to incorporate 'toxic' monarch prey into its diet without acute or chronic ill-effects.

Are exotic herbivores better competitors? A meta-analysis.

Competition plays an important role in structuring the community dynamics of phytophagous insects. As the number and impact of biological invasions increase, it has become increasingly important to determine whether competitive differences exist between native and exotic insects. We conducted a meta-analysis to test the hypothesis that native/ exotic status affects the outcome of herbivore competition. Specifically, we used data from 160 published studies to assess plant-mediated competition in phytophagous insects. For each pair of competing herbivores, we determined the native range and coevolutionary history of each herbivore and host plant. Plant-mediated competition occurred frequently, but neither native nor exotic insects were consistently better competitors. Spatial separation reduced competition in native insects but showed little effect on exotics. Temporal separation negatively impacted native insects but did not affect competition in exotics. Insects that coevolved with their host plant were more affected by interspecific competition than herbivores that lacked a coevolutionary history. Insects that have not coevolved with their host plant may be at a competitive advantage if they overcome plant defenses. As native/exotic status does not consistently predict outcomes of competitive interactions, plant-insect coevolutionary history should be considered in studies of competition.

False ring formation in eastern hemlock branches: impacts of hemlock woolly adelgid and elongate hemlock scale.

Herbivores can alter plant physiology through the induction of abnormal wood formation. Feeding by some insects induces the formation of false rings, a band of thick-walled latewood cells within the earlywood portion of the tree ring that reduces water transport. Hemlock woolly adelgid (Adelges tsugae Annand) and elongate hemlock scale (Fiorinia externa Ferris) are invasive insects that both feed on eastern hemlock [Tsuga canadensis (L.) Carrière]. Adelges tsugae has a greater effect on tree health than F. externa, but the mechanism underlying their differential effect is unknown. We explored the effects of these herbivores by assessing growth ring formation in branches of trees that had been experimentally infested for 4 yr with A. tsugae, F. externa, or neither insect. We measured false ring density, ring growth, and earlywood: latewood ratios in the two most recently deposited growth rings. Branches from A. tsugae-infested trees had 30% more false rings than branches from F. externa-infested trees and 50% more than branches from uninfested trees. In contrast, branches from F. externa-infested trees and control trees did not differ in false ring formation. Radial growth and earlywood: latewood ratios did not differ among treatments. Our results show that two invasive herbivores with piercing-sucking mouth parts have differing effects on false ring formation in eastern hemlock. These false rings may be the product of a systemic plant hypersensitive response to feeding by A. tsugae on hemlock stems. If false rings are responsible for or symptomatic of hemlock water stress, this may provide a potential explanation for the relatively large effect of A. tsugae infestations on tree health.