Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars

Self-medication is a specific therapeutic behavioral change in response to disease or parasitism. The empirical literature on self-medication has so far focused entirely on identifying cases of self-medication in which particular behaviors are linked to therapeutic outcomes. In this study, we frame self-medication in the broader realm of adaptive plasticity, which provides several testable predictions for verifying self-medication and advancing its conceptual significance. First, self-medication behavior should improve the fitness of animals infected by parasites or pathogens. Second, self-medication behavior in the absence of infection should decrease fitness. Third, infection should induce self-medication behavior. The few rigorous studies of self-medication in non-human animals have not used this theoretical framework and thus have not tested fitness costs of self-medication in the absence of disease or parasitism. Here we use manipulative experiments to test these predictions with the foraging behavior of woolly bear caterpillars (Grammia incorrupta; Lepidoptera: Arctiidae) in response to their lethal endoparasites (tachinid flies). Our experiments show that the ingestion of plant toxins called pyrrolizidine alkaloids improves the survival of parasitized caterpillars by conferring resistance against tachinid flies. Consistent with theoretical prediction, excessive ingestion of these toxins reduces the survival of unparasitized caterpillars. Parasitized caterpillars are more likely than unparasitized caterpillars to specifically ingest large amounts of pyrrolizidine alkaloids. This case challenges the conventional view that self-medication behavior is restricted to animals with advanced cognitive abilities, such as primates, and empowers the science of self-medication by placing it in the domain of adaptive plasticity theory

Crop pests and predators exhibit inconsistent responses to surrounding landscape composition

The idea that noncrop habitat enhances pest control and represents a win–win opportunity to conserve biodiversity and bolster yields has emerged as an agroecological paradigm. However, while noncrop habitat in landscapes surrounding farms sometimes benefits pest predators, natural enemy responses remain heterogeneous across studies and effects on pests are inconclusive. The observed heterogeneity in species responses to noncrop habitat may be biological in origin or could result from variation in how habitat and biocontrol are measured. Here, we use a pest-control database encompassing 132 studies and 6,759 sites worldwide to model natural enemy and pest abundances, predation rates, and crop damage as a function of landscape composition. Our results showed that although landscape composition explained significant variation within studies, pest and enemy abundances, predation rates, crop damage, and yields each exhibited different responses across studies, sometimes increasing and sometimes decreasing in landscapes with more noncrop habitat but overall showing no consistent trend. Thus, models that used landscape-composition variables to predict pest-control dynamics demonstrated little potential to explain variation across studies, though prediction did improve when comparing studies with similar crop and landscape features. Overall, our work shows that surrounding noncrop habitat does not consistently improve pest management, meaning habitat conservation may bolster production in some systems and depress yields in others. Future efforts to develop tools that inform farmers when habitat conservation truly represents a win–win would benefit from increased understanding of how landscape effects are modulated by local farm management and the biology of pests and their enemies

Least square mean (±1 SE) of the total amount of the food block eaten by <i>G. incorrupta</i> caterpillars over 5 days in the feeding choice experiment according to parasitism treatment (0–3 <i>E. mella</i> eggs) and post-assay survival to adulthood (survived, died).

<p>Asterisks denote significant differences among means of survivors and victims within each treatment from a Tukey-Kramer test (see text for statistics).</p

Least square mean (±1 SE) of the total amount of the PA block eaten by <i>G. incorrupta</i> caterpillars over 5 days in the feeding choice experiment according to parasitism treatment (0–3 <i>E. mella</i> eggs) and post-assay survival to adulthood (survived, died).

<p>Asterisks denote significant differences among means of survivors and victims within each treatment from a Tukey-Kramer test (see text for statistics).</p

Univariate (ANCOVA) responses of feeding intake by caterpillars in the choice experiment, quantified in terms of i) the angularly transformed percentage of overall intake from the PA block, ii) the log transformed absolute intake of the PA block, and iii) the log transformed absolute intake of the food block over five days.

<p>Significant <i>P</i> values are marked by boldface type.</p

Least square mean (±1 SE) percentage of overall intake from PA block by <i>G. incorrupta</i> caterpillars over 5 days in the feeding choice experiment according to parasitism treatment (0–3 <i>E. mella</i> eggs) and post-assay survival to adulthood (survived, died).

<p>Letters denote significant differences among treatment means from a Tukey-Kramer test (see text for statistics).</p

ANCOVA responses of feeding intake by caterpillars in the choice experiment, quantified in terms of i) the angularly transformed percentage of overall intake from the PA block, ii) the log transformed absolute intake of the PA block, and iii) the log transformed overall intake (PA+food block) over five days.

<p>Significant <i>P</i> values are marked by boldface type.</p

Number of survivors to adulthood of <i>E. mella</i> flies that developed in PA+ and PA− caterpillars.

<p>Parasitoids had lower survival in PA+ caterpillars (see text for statistics).</p