Experimental insect suppression causes loss of induced, but not constitutive, resistance in Solanum carolinense

A mechanistic framework of enemy release

The enemy release hypothesis (ERH) is the best-known hypothesis explaining high performance (e.g. rapid population growth) of exotic species. However, the current framing of the ERH does not explicitly link evidence of enemy release with exotic performance. This leads to uncertainty regarding the role of enemy release in biological invasions. Here, we demonstrate that the effect of enemy release on exotic performance is the product of three factors: enemy impact, enemy diversity, and host adaptation. These factors are modulated by seven contexts: time since introduction, resource availability, phylogenetic relatedness of exotic and native species, host-enemy asynchronicity, number of introduction events, type of enemy, and strength of growth-defence trade-offs. ERH-focused studies frequently test different factors under different contexts. This can lead to inconsistent findings, which typifies current evidence for the ERH. For example, over 80% of meta-analyses fail to consider ecological contexts which can alter study findings; we demonstrate this by re-analysing a recent ERH synthesis. Structuring the ERH around factors and contexts promotes generalisable predictions about when and where exotic species may benefit from enemy release, empowering effective management. Our mechanistic factor-context framework clearly lays out the evidence required to support the ERH, unifies many enemy-related invasion hypotheses, and enhances predictive capacity.

Trade-offs in non-native plant herbivore defences enhance performance

Non-native plants are typically released from specialist enemies but continue to be attacked by generalists, albeit at lower intensities. This reduced herbivory may lead to less investment in constitutive defences and greater investment in induced defences, potentially reducing defence costs. We compared herbivory on 27 non-native and 59 native species in the field and conducted bioassays and chemical analyses on 12 pairs of non-native and native congeners. Non-natives suffered less damage and had weaker constitutive defences, but stronger induced defences than natives. For non-natives, the strength of constitutive defences was correlated with the intensity of herbivory experienced, whereas induced defences showed the reverse. Investment in induced defences correlated positively with growth, suggesting a novel mechanism for the evolution of increased competitive ability. To our knowledge, these are the first linkages reported among trade-offs in plant defences related to the intensity of herbivory, allocation to constitutive versus induced defences, and growth.

Nutrient stress can have opposite effects on the ability of plants to tolerate foliar herbivory and floral herbivory

Discovering how organisms respond to the combinations of stressors they face in their environment is an enduring challenge for ecologists. A particular focus has been how natural enemies and abiotic stressors faced by plants may interact in their effect on the ecology and evolution of plant defense strategies. Here, we report on the results of an experiment measuring how reproduction in the clonal herbaceous plant horsenettle (Solanum carolinense) is affected by damage by leaf-feeding and by flower-feeding herbivores-as well as how horsenettle's tolerance of these different types of herbivory may be altered by nutrient stress. Leaf herbivory by lace bugs reduced horsenettle's seed production and root growth, and the relative impacts were greater in fertilized than in nutrient-stressed plants. In contrast, simulated-floral herbivory reduced seed production to a similar degree in fertilized and nutrient-stressed plants. However, compensation for floral herbivory through increased root growth occurred to a much greater extent in the fertilized than in the nutrient-stressed plants. These results can be explained in terms of the limiting resource model of plant tolerance, with leaf damage interpreted as exacerbating carbon limitation in the fertilized plants and floral damage ameliorating carbon limitation in the fertilized plants. These results can be extended to predicting patterns in the field: Although plants in a nutrient-poor environment may have overall low fitness, they are likely to be more tolerant of leaf herbivores-though this benefit may be countered by lower tolerance of any floral herbivores that share the environment.

Revisiting plant defense-fitness trade-off hypotheses using Solanum as a model genus

Plants possess physical and chemical defenses which have been found to deter herbivores that feed and oviposit on them. Despite having wide variety of defenses which can be constitutive or induced, plants are attacked and damaged by insects associated with different mouthparts and feeding habits. Since these defenses are costly, trade-offs for growth and defense traits play an important role in warding off the herbivores, with consequences for plant and herbivore growth, development and fitness. Solanum is a diverse and rich genus comprising of over 1,500 species with economic and ecological importance. Although a large number of studies on Solanum species with different herbivores have been carried out to understand plant defenses and herbivore counter defenses, they have primarily focused on pairwise interactions, and a few species of economic and ecological importance. Therefore, a detailed and updated understanding of the integrated defense system (sum of total defenses and trade-offs) is still lacking. Through this review, we take a closer look at the most common plant defense hypotheses, their assumptions and trade-offs and also a comprehensive evaluation of studies that use the genus Solanum as their host plant, and their generalist and specialist herbivores from different feeding guilds. Overall, review emphasizes on using ubiquitous Solanum genus and working toward building an integrated model which can predict defense-fitness-trade-offs in various systems with maximum accuracy and minimum deviations from realistic results.

Experimental insect suppression causes loss of induced, but not constitutive, resistance in Solanum carolinense

Spatiotemporal variation in herbivory is a major driver of intraspecific variation in plant defense. Comparatively little is known, however, about how changes in herbivory regime affect the balance of constitutive and induced resistance, which are often considered alternative defensive strategies. Here, we investigated how nearly a decade of insect herbivore suppression affected constitutive and induced resistance in horsenettle (Solanum carolinense), a widespread herbaceous perennial. We allowed replicated horsenettle populations to respond to the presence or absence of herbivores by applying insecticide to all plants in half of 16 field plots. Horsenettle density rapidly increased in response to insecticide treatment, and this effect persisted for at least four years after the cessation of herbivore suppression. We subsequently grew half-sibling families from seeds collected during and shortly after insecticide treatment in a common garden and found strong effects of insect suppression on induced resistance. Feeding trials in field mesocosms with false Colorado potato beetles (Leptinotarsa juncta), a common specialist herbivore, revealed that multi-year herbivore suppression drove rapid attenuation of induced resistance: offspring of plants from insect-suppression plots exhibited a near-complete loss of induced resistance to beetles, while those from control plots incurred ~70% less damage after experimental induction. Plants from insect-suppression plots also had ~40% greater constitutive resistance than those from control plots, although this difference was not statistically significant. We nonetheless detected a strong trade-off between constitutive and induced resistance across families. In contrast, the constitutive expression of trypsin inhibitors (TI), an important chemical defense trait in horsenettle, was reduced by 20% in the offspring of plants from insect-suppression plots relative to those from control plots. However, TIs were induced to an equal extent whether or not insect herbivores had been historically suppressed. While several defense and performance traits (prickle density, TI concentration, resistance against false Colorado potato beetles and flea beetles, biomass, and seed mass) varied markedly across families, no traits exhibited significant pairwise correlations. Overall, our results indicate that, while the divergent responses of multiple defense traits to insect suppression led to comparatively small changes in overall constitutive resistance, they significantly reduced induced resistance against false Colorado potato beetle.