Nicotine moderates the effects of macronutrient balance on nutrient intake by parasitized Manduca sexta L

Effect of Plant Defenses and Plant Nutrients on the Performance of Specialist and Generalist Herbivores of Datura: A Macroevolutionary Study

Macroevolutionary patterns in the association between plant species and their herbivores result from ecological divergence promoted by, among other factors, plants' defenses and nutritional quality, and herbivore adaptations. Here, we assessed the performance of the herbivores Lema trilineata daturaphila, a trophic specialist on Datura, and Spodoptera frugiperda, a polyphagous pest herbivore, when fed with species of Datura. We used comparative phylogenetics and multivariate methods to examine the effects of Datura species' tropane alkaloids, leaf trichomes, and plant macronutrients on the two herbivores´ performances (amount of food consumed, number of damaged leaves, larval biomass increment, and larval growth efficiency). The results indicate that species of Datura do vary in their general suitability as food host for the two herbivores. Overall, the specialist performs better than the generalist herbivore across Datura species, and performance of both herbivores is associated with suites of plant defenses and nutrient characteristics. Leaf trichomes and major alkaloids of the Datura species are strongly related to herbivores' food consumption and biomass increase. Although hyoscyamine better predicts the key components of the performance of the specialist herbivore, scopolamine better predicts the performance of the generalist; however, only leaf trichomes are implicated in most performance components of the two herbivores. Nutrient quality more widely predicts the performance of the generalist herbivore. The contrasting effects of plant traits and the performances of herbivores could be related to adaptive differences to cope with plant toxins and achieve nutrient balance and evolutionary trade-offs and synergisms between plant traits to deal with a diverse community of herbivores.

Manduca sexta experience high parasitoid pressures in the field but minor fitness costs of consuming plant secondary compounds

Plant-herbivore coevolutionary interactions have led to a range of plant defenses that minimize insect damage and a suite of counter adaptations that allow herbivores to feed on defended plants. Consuming plant secondary compounds results in herbivore growth and developmental costs but can have beneficial effects such as deterrence or harm of parasitoid enemies. Therefore, the role of secondary compounds on herbivore fitness must be considered in the context of the abundance and level of harm from natural enemies and the costs herbivores incur feeding on plant secondary compounds.In this study, I combined field measurements of Cotesia congregata wasp parasitism pressure with detailed measurements of the costs of plant secondary compounds across developmental stages in the herbivore host, Manduca sexta.I show that C. congregata parasitoids exert large negative selective pressures, killing 31%-57% of M. sexta larvae in the field. Manduca sexta developed fastest during instars most at risk for parasitoid oviposition but growth was slowed by consumption of plant secondary compounds. The negative effects of consuming plant secondary compounds as larvae influenced adult size traits but there were no immune, survival, or fecundity costs.These results suggest that developmental costs experienced by M. sexta herbivores consuming defensive compounds are minor in comparison to the strong negative survival pressures from abundant parasitoid enemies.

Learned and naïve natural enemy responses and the interpretation of volatile organic compounds as cues or signals

In response to arthropod herbivory, plants release volatile organic compounds (VOCs), which are attractive to natural enemies. Consequently, VOCs have been interpreted as co-evolved plant-natural enemy signals. This review argues that, while these data are necessary, they are not sufficient to demonstrate a VOC plant-natural enemy signaling function. We propose that evidence that (1) plant fitness is increased as a consequence of natural enemy recruitment, and either (2A) natural enemies preferentially learn prey-induced VOCs or (2B) natural enemies respond innately to the VOCs of the prey-host plant complex, is also required. Whereas there are too few studies to rigorously test hypotheses 1 and 2A, numerous studies are available to test hypothesis 2B. Of 293 tests of natural enemy responses to VOCs, we identified only 74 that were unambiguous tests of naïve natural enemies; in the remainder of the tests either natural enemies were experienced with their host in the presence of VOCs, or experience could not be ruled out. Of those 74 tests with naïve natural enemies, attraction was observed in 41 and not in 33. This review demonstrates that empirical support for the hypothesized VOC plant-natural enemy signaling function is not universal and presents alternative hypotheses for VOC production.