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1. Top-down effects from parasitoids may mediate plant defence and plant fitness

   https://doi.org/DOI: 10.1111/1365-2435.13617  

   Tan, C.-W.; Peiffer, Michelle; Ali, Jared G.; Luthe, D.S.; Felton, Gary w. (October 2020, Functional ecology)

   null (Ed.)

   1. Plants face many environmental stresses that can impact their survival, development and fitness. Insects are the most diverse, abundant and threatening herbivores in nature. As a consequence, plants produce direct chemical and physical defences to reduce herbivory. They also release volatiles to recruit natural enemies that indirectly protect them from herbivory. The recruitment of parasitic wasps can benefit plant fitness because they ultimately kill their insect hosts. 2. Recently, studies showed that parasitoids can indirectly mediate plant defences by modulating herbivore oral secretions. In addition to the direct benefits of parasitoids in terms of reducing herbivore survival, we tested if the reduction in induced defences by parasitized caterpillars compared to non-parasitized caterpillars may reduce the costs associated with defence expression. 3. We provide evidence that tomato plants treated with saliva from parasitized caterpillars have significantly higher fitness parameters including increased flower numbers (16.3%) and heavier fruit weight (13.5%), compared to plants treated with saliva from non-parasitized caterpillars. Since plants were grown without actual herbivores, the higher values for these fitness parameters were due to lower costs of induced defences and not due to reduced herbivory by parasitized caterpillars. Furthermore, the resulting seed germination time was shorter and the germination rate was higher when the maternal plants were previously exposed to parasitized herbivore treatment compared to control (non-treated) plants. 4. Overall, application of saliva did not result in transgenerational priming of offspring defence responses. However, offspring of parents exposed to caterpillar saliva had lower constitutive levels and higher induced levels of trypsin inhibitor than offspring from unexposed parents. 5. This study shows that the saliva of parasitized caterpillars can modulate plant defences and further demonstrates that the lower induction of plant defences is associated with elevated plant fitness in the absence of herbivore feeding, suggesting that induced plant defences are costly. 

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2. Trade‐offs in non‐native plant herbivore defences enhance performance

   https://doi.org/10.1111/ele.14283  

   Sun, Xiao; Sun, Yumei; Cao, Xueyao; Zhai, Xincong; Callaway, Ragan M.; Wan, Jinlong; Flory, S. Luke; Huang, Wei; Ding, Jianqing (September 2023, Ecology Letters)

   Abstract 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. 

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3. Complementary effects of two plant defence traits on behaviour and performance of monarch butterfly caterpillars

   https://doi.org/10.1111/een.13403  

   Craig, Emma J; Agrawal, Anurag A (November 2024, Ecological Entomology)

   Abstract Plants have evolved multiple defensive traits in response to herbivory; in turn, herbivore specialists evolved adaptive behaviours to avoid or tolerate such defences. Here, we employ milkweeds (Asclepiasspp.) to test two defences, latex and trichomes, for their independent and interactive effects on behaviour and performance of monarch caterpillars (Danaus plexippus).Latex exuded upon damage and the density of leaf trichomes positively correlate across milkweed species, suggesting they may have evolved together as synergistic defences. Nonetheless, the complementary roles of these two traits have been little‐studied. We focus on two behaviours: shaving, or the removal of trichomes, and chewing, which encompasses both deactivation of latex and leaf consumption.In an experiment with seven milkweed species, with and without manipulated latex flow, we found latex to be the primary determinant of reducing chewing, while both defences positively predicted shaving behaviour in the first instar. Next, we conducted a factorial experiment throughout the first three instars, manipulating latex and trichomes on a high‐latex, high‐trichome species, the woolly milkweedAsclepias vestita. On plants with latex and trichomes intact, caterpillars spent the most time shaving and least time chewing of all treatment groups, suggesting a possible synergism. These defence‐driven behavioural effects decreased later in larval development.Latex and trichomes both impacted monarch performance, additively increasing mortality and reducing growth of survivors. Thus, latex and trichomes represent two important plant defences with effects on specialist herbivore behaviour and implications for insect fitness. 

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4. Patterns of constitutive and induced herbivore defence are complex, but share a common genetic basis in annual and perennial monkeyflower

   https://doi.org/10.1111/1365-2435.14601  

   Blanchard, Megan L; Lowry, David B; Holeski, Liza M (August 2024, Functional Ecology)

   Abstract Despite multiple ecological and evolutionary hypotheses that predict patterns of phenotypic relationships between plant growth, reproduction and constitutive and/or induced resistance to herbivores, these hypotheses do not make any predictions about the underlying molecular genetic mechanisms that mediate these relationships.We investigated how divergent plant life‐history strategies in the yellow monkeyflower and a life‐history altering locus,DIV1, influence plasticity of phytochemical herbivory resistance traits in response to attack by two herbivore species with different diet breadth.Life‐history strategy (annual vs. perennial) and theDIV1locus significantly influenced levels of constitutive herbivory resistance, as well as resistance induction following both generalist and specialist herbivory. Perennial plants had higher total levels of univariate constitutive and induced defence than annuals, regardless of herbivore type. Annuals induced less in response to generalist herbivory than did perennials, while induction response was equivalent across the ecotypes for specialist herbivory.The effects of theDIV1locus on levels of constitutive and induced defence were dependent on genetic background, the annual versus perennial haplotype ofDIV1and herbivore identity. The patterns of univariate induction due toDIV1were non‐additive and did not always match expectations based on patterns of divergence for annual/perennial parents. For example, perennial plants had higher levels of constitutive and induced defence than did annuals, but when the annualDIV1was present in the perennial genetic background induction response to herbivory was higher than for the perennial parent lines.Patterns for multivariate defence arsenals generally echoed those of univariate, with annual and perennial monkeyflowers and those with alternative versions ofDIV1differing significantly in constitutive and induced resistance. Like univariate resistance, induced multivariate defence arsenals were affected by herbivore identity.Our results highlight the complexity of the genetic mechanisms underlying plastic response to herbivory. While a genetic locus underlying substantial phenotypic variation in life‐history strategy and constitutive defence also influences defence plasticity, the induction response also depends on genetic background. This result demonstrates the potential for some degree of evolutionary independence between constitutive and induced defence, or induced defence and life‐history strategy, in monkeyflowers. Read the freePlain Language Summaryfor this article on the Journal blog. 

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5. Woody seeds and seedlings are unresponsive to herbivore kairomones

   https://doi.org/10.1093/aobpla/plag013  

   Headrick, Kevin C; Pellegrini, Brooke A; Santiago, Eirette M; Overstrum, Katherine M; Bhavanam, Santhi P; Orians, Colin M; Orrock, John L; Preisser, Evan L (March 2026, AoB PLANTS)

   Burns, Jean; Gorchov, David (Ed.)

   Abstract Herbivory is particularly threatening to young plants that lack the resources needed to survive an attack. Seeds and seedlings should thus benefit greatly from using pre-attack cues to induce defence before damage. Locomotion mucus from slugs, generalist herbivores that consume young plants, has been shown to speed germination, slow growth, and increase both chemical defences and resistance to herbivores in several herbaceous plants. Whether woody species exhibit similar responses has not been tested. Arion subfuscus, an invasive slug in the eastern USA, is a major herbivore of young sugar maples (Acer saccharum); we explored the effect of its locomotion mucus on Ac. saccharum seeds and seedlings. We exposed sugar maple seeds and seedlings to mucus and measured germination speed and rate, seed susceptibility to slugs, seedling emergence, growth, chemical defences, and foliar susceptibility to Lymantria dispar and Ar. subfuscus. Contrary to our expectations and previous findings with herbaceous species, we found that mucus had no effect on these performance or resistance traits. Habituation to repeated cue exposure or the limited coevolutionary history between Ac. saccharum and Ar. subfuscus could be the reason for the lack of an observed response by the plants. Further studies should investigate the effects of kairomones using a short-term cue exposure procedure or by using a woody plant species and native slug herbivore with a coevolutionary history. Understanding how woody plants respond to kairomones would provide insight into the risk and defence strategies used by long-lived species in crucial early life stages. 

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