Michael Garvey, Justin Bredlau, Karen Kester, Curtis Creighton, Ian Kaplan
Nicotine, a chemical derived from tobacco, is one of the most toxic and addictive compounds known in the natural world. This chemical has potent antimicrobial effects and also can be lethal if ingested or contacted. Even though tobacco plants produce this potent toxin (presumably for defense against their enemies), some specialized animals like certain insect species have evolved the ability to tolerate this chemical in their diet and eat nicotine-laced plants. For example, the tobacco hornworm caterpillar, Manduca sexta, is an insect native to North America that specializes on plants in the nightshade family—containing tobacco and close relatives like tomato—and is seemingly unphased by nicotine.
In this study, we employed a series of experiments feeding hornworm caterpillars diets varying widely in nicotine concentrations. To do so, we use tobacco mutant plants that genetically vary in nicotine content in their leaves; we also infused pure nicotine into artificial diets to test the direct effects of this chemical, independent from living plants whose leaves contain hundreds of other co-occurring chemicals. Interestingly, we found that nicotine-treated caterpillars exhibited a more robust immune response than control individuals fed a nicotine-free diet. A comparable response was not observed on hornworms fed tomato plants varying in their defensive chemistry. These data suggest that nicotine uniquely acts as an immunotherapeutic chemical and further implies that caterpillars reared on a nicotine-rich diet are better protected against foreign invaders (e.g., pathogens and parasites). Although it is well-known that specialist herbivores have evolved tolerance to the defensive chemistry of their food plants, these findings illustrate that herbivorous insects can also use these chemicals to their advantage, potentially improving their chances of survival in nature.