PREMISE:Clonality, a form of asexual reproduction and spread, is common among invasive plants, though sexual reproduction via seeds is often still important for their long-range dispersal. In small populations, clonality has been hypothesized to interfere with sexual reproduction by limiting outcrossing opportunities of a plant. METHODS:We developed a structural equation model based on estimates of genetic diversity and seed production of Lepidium draba, a problematic invasive clonal plant, at 26 sites in Colorado to test whether site characteristics relating to small founder populations resulted in low genetic diversity and sexual reproduction. The next year, in pollen supplementation experiments at six sites (three with high genetic diversity, three with low), we tested whether populations with low genetic diversity were limited by non-self pollen. RESULTS:Large populations and populations associated with rivers tended to have higher genetic diversity. Percentage seed fill and total seed production were considerably higher at sites with higher genetic diversity. At populations with low genetic diversity, supplementation with pollen from outside of the site, but not from within the site, increased seed production. At populations with high genetic diversity, pollen supplementation from off-site did not increase seed production. CONCLUSIONS:Our study shows that, in low-diversity populations that are dominated by a few large clones, L. draba produces few seeds compared to high-diversity populations and that this appears to be due to limited availability of non-self pollen. The data indicate that low genetic diversity decreases sexual reproduction, which may greatly reduce long-distance dispersal from these populations.

Hormesis, the phenomenon in which low doses of toxins promote beneficial biological responses and higher doses compromise these responses, offers an underexplored framework for understanding herbivore eco-immunology. Here, we explore how insect herbivores might exploit plant secondary metabolites to enhance immune function. We propose that herbivores experience a 'window of enhanced immunity,' where toxins confer immune benefits at low concentrations, but suppress immune responses at higher concentrations. This concept bridges the interplay between bottom-up (plant defense) and top-down (natural enemy) pressures, providing insights into how herbivores balance challenges posed by exposure to plant toxins and exposure to their natural enemies. We discuss how both generalist and specialist herbivores navigate this balance, highlighting the evolutionary adaptations that influence their strategies. We suggest that the immune systems of specialist and generalist herbivores may both exhibit hormetic responses to plant toxins, although the shape of this relationship likely differs depending on their ability to detoxify and sequester plant toxins.

Patterns of phytochemistry localisation in plant tissues are diverse within and across leaves. These spatial heterogeneities are important to the fitness of herbivores, but their effects on herbivore foraging and dietary experience remain elusive. We manipulated the spatial variance and clusteredness of a plant toxin in a synthetic diet landscape on which individual caterpillars fed. We monitored caterpillars with cameras across most of their larval development. Caterpillars that fed on diets with a lower spatial variance and more clustered arrangement of toxins had overall worse performance, mostly because those caterpillars ate less, moved more, ingested more toxin, or failed to physiologically acclimate. Using empirically parameterised individual-based models, we found that differences in movement away from, not towards, less toxic food drove a body size-dependent effect of clusteredness. Hence, the spatial pattern of phytochemicals itself, beyond mean concentration, can have important consequences for herbivores through complex interactions with herbivore foraging.

Patterns of phytochemistry localisation in plant tissues are diverse within and across leaves. These spatial heterogeneities are important to the fitness of herbivores, but their effects on herbivore foraging and dietary experience remain elusive. We manipulated the spatial variance and clusteredness of a plant toxin in a synthetic diet landscape on which individual caterpillars fed. We monitored caterpillars with cameras across most of their larval development. Caterpillars that fed on diets with a lower spatial variance and more clustered arrangement of toxins had overall worse performance, mostly because those caterpillars ate less, moved more, ingested more toxin, or failed to physiologically acclimate. Using empirically parameterised individual-based models, we found that differences in movement away from, not towards, less toxic food drove a body size-dependent effect of clusteredness. Hence, the spatial pattern of phytochemicals itself, beyond mean concentration, can have important consequences for herbivores through complex interactions with herbivore foraging.

Parasitoid wasp larvae engage in intense interspecific competition when sharing a host insect, with lethal consequences for the losers. Larval parasitoid competition is generally thought to be resolved by physical traits, such as enlarged mandibles. Although solitary parasitoid larvae are typically superior competitors against gregarious larvae, such physical traits are unlikely to be effective against large numbers of competitors. Larval competition may instead be mediated by maternal factors that increase the survival of offspring facing interspecific competition, thus increasing parental fitness.When laying eggs inside a host, many female parasitoids also inject oviposition fluids that are known to suppress host immune responses, permitting successful offspring development. We explored whether the oviposition fluids of two wasp species in the genus Cotesia (Cotesia rubecula, a strong competitor that almost always wins interspecific competition against a weaker competitor, Cotesia glomerata) might also represent maternal factors that improve the interspecific competitive ability of their offspring.We demonstrate that injections of both venom and calyx fluid from C. rubecula can inhibit egg hatching and larval development of C. glomerata. Venom from C. rubecula decreased C. glomerata egg development by 2.7 times, and calyx fluid injections caused deformities in 23% of developing C. glomerata. In contrast, reciprocal injections of the oviposition fluids from C. glomerata did not inhibit the development of C. rubecula.Our results show that maternal factors can improve the interspecific competitive ability of parasitoid larvae, challenging the previous assumptions that larval competition was resolved primarily through physical combat or larval secretions.

Insect herbivore eco-immunology involves complex interactions between herbivore immunity and their natural enemies, and the responses of these interactions to environmental factors including plant anti-herbivore toxins. Plant toxins can affect herbivore immunity, leading to either immunoenhancement or immunosuppression, which in turn influences their vulnerability to parasitoids and pathogens. Herbivore immune responses differ among species regionally, reflecting adaptations to local environmental conditions and natural enemy pressures. Additionally, anthropogenic factors including like climate change, plant domestication, and invasive species are altering these eco-immunological dynamics. Such changes can ripple through food webs, affecting not only herbivores and their natural enemies but also broader community structures. By understanding these complex interactions, we can better predict ecosystem responses to environmental change.

Climate warming is considered to be among the most serious of anthropogenic stresses to the environment, because it not only has direct effects on biodiversity, but it also exacerbates the harmful effects of other human-mediated threats. The associated consequences are potentially severe, particularly in terms of threats to species preservation, as well as in the preservation of an array of ecosystem services provided by biodiversity. Among the most affected groups of animals are insects-central components of many ecosystems-for which climate change has pervasive effects from individuals to communities. In this contribution to the scientists' warning series, we summarize the effect of the gradual global surface temperature increase on insects, in terms of physiology, behavior, phenology, distribution, and species interactions, as well as the effect of increased frequency and duration of extreme events such as hot and cold spells, fires, droughts, and floods on these parameters. We warn that, if no action is taken to better understand and reduce the action of climate change on insects, we will drastically reduce our ability to build a sustainable future based on healthy, functional ecosystems. We discuss perspectives on relevant ways to conserve insects in the face of climate change, and we offer several key recommendations on management approaches that can be adopted, on policies that should be pursued, and on the involvement of the general public in the protection effort.

Insect herbivores frequently encounter plant defense molecules, but the physiological and ecological consequences for their immune systems are not fully understood. The majority of studies attempting to relate levels of plant defensive chemistry to herbivore immune responses have used natural population or species-level variation in plant defensive chemistry. Yet, this potentially confounds the effects of plant defense chemistry with other potential plant trait differences that may affect the expression of herbivore immunity. We used an artificial diet containing known quantities of a plant toxin (4-methylsulfinylbutyl isothiocyanate; 4MSOB-ITC or ITC, a breakdown product of the glucosinolate glucoraphanin upon herbivory) to explicitly explore the effects of a plant toxin on the cellular and humoral immune responses of the generalist herbivore Trichoplusia ni (Lepidoptera: Noctuidae) that frequently feeds on glucosinolate-containing plants. Caterpillars feeding on diets with high concentrations of ITC experienced reduced survivorship and growth rates. High concentrations of ITC suppressed the appearance of several types of hemocytes and melanization activity, which are critical defenses against parasitic Hymenoptera and microbial pathogens. In terms of T. ni humoral immunity, only the antimicrobial peptide (AMP) genes lebocin and gallerimycin were significantly upregulated in caterpillars fed on diets containing high levels of ITC relative to caterpillars that were provided with ITC-free diet. Surprisingly, challenging caterpillars with a non-pathogenic strain of Escherichia coli resulted in the upregulation of the AMP gene cecropin. Feeding on high concentrations of plant toxins hindered caterpillar development, decreased cellular immunity, but conferred mixed effects on humoral immunity. Our findings provide novel insights into the effects of herbivore diet composition on insect performance demonstrating the role of specific plant defense toxins that shape herbivore immunity and trophic interactions.

1. Insect herbivores simultaneously experience bottom-up effects of plant defensive chemistry and the top-down effects of natural enemies. At the intersection of these effects are herbivore immune systems, herbivore traits that have largely been overlooked in studies of plant-insect interactions. Most previous studies have demonstrated compromised immunity of herbivores that feed on plants with higher defensive chemistry. Many studies have used embedded microfilaments or silica beads as proxies for parasitoid eggs. Yet, parasitoids may evade or suppress host immune responses by injecting venom and calyx fluid, or through modifications of their egg surface structure, necessitating studies that include all three trophic levels to obtain a complete picture of how plant traits may modulate herbivore immunity. 2. Here, we examined the effect of host plant species that differ in glucosinolate (anti-herbivore compounds produced by plants in the Brassicaceae) concentrations on the immune status of an herbivore and its consequences for two species of parasitoids with different life history traits. 3. We found that larvae of the butterfly Pieris rapae that fed on field mustard Brassica rapa, which contain 52-fold higher glucosinolate concentrations than collards B. oleracea, attained lower body weights and experienced prolonged development to adulthood. 4. Yet, caterpillars that fed on B. rapa had enhanced cellular immunity, as measured by total and differential haemocyte counts as well as melanization capacity, compared to larvae that fed on B. oleracea. 5. In turn, the likelihood that at least some eggs in clutches of the gregarious endoparasitoid Cotesia glomerata would be encapsulated, leading to a reduction in brood size, were three times greater when their host caterpillars fed on B. rapa compared to B. oleracea. 6. Interestingly, eggs of the solitary endoparasitoid Cotesia rubecula were rarely encapsulated irrespective of the host plant on which their host caterpillar fed. Therefore, our results suggest that plant defence metabolites can influence the expression of herbivore immunity, but the effectiveness of this response strongly depends on the identity of the parasitoid and its ability to evade the caterpillar immune response, and possibly the evolution of these trophic interactions in non-native systems.

Identifying traits that are associated with success of introduced natural ene-mies in establishing and controlling pest insects has occupied researchers and biological control practitioners for decades. Unfortunately, consistent gen-eral relationships have been difficult to detect, preventing a priori ranking of candidate biological control agents based on their traits. We summarise previ-ous efforts and propose a series of potential explanations for the lack of clear patterns. We argue that the quality of current datasets is insufficient to detect complex trait-efficacy relationships and suggest several measures by which current limitations may be overcome. We conclude that efforts to address this elusive issue have not yet been exhausted and that further explorations are likely to be worthwhile.

Trap cropping involves the use of plant species or genotypes to attract pest insects away from the main crop to avoid pest damage. In this study, we evaluated the potential of using winter triticale (x Triticosecale) as a trap crop for the wheat stem sawfly (Cephus cinctus Norton), an economically devastating pest of wheat (Triticum aestivum L.). The wheat stem sawfly larvae consume parenchyma tissue within the wheat stem and cut the stem at the base causing it to lodge. Triticale is, on average taller and has a larger stem diameter than winter wheat. These traits are considered attractive to adult females when choosing hosts for oviposition. We conducted a two-year field study of one winter wheat and one winter triticale genotype combination for its potential as a trap crop. To complement the field study, we grew three genotypes of winter triticale and one winter wheat genotype in cone-tainers and infested them in the field. The cone-tainer and field studies revealed that the chosen winter triticale genotypes were not more attractive than the winter wheat genotypes for adult wheat stem sawflies. The field study also evaluated the average larval position in the stem and found the average position was variable between sampling dates in both crops. Thus, determining the precise timing of field swathing could destroy significant portions of larval populations. Future research should focus on genotype selection to establish triticale-wheat cultivar combinations to create a push-pull system.

Insect herbivores must simultaneously balance bottom-up effects of plant defensive chemistry and the top-down effects of natural enemies. At the intersection of these effects are herbivore immune systems, an herbivore trait that has largely been overlooked in studies of plant-insect interactions. Counter to the majority of studies showing that herbivores feeding on plants containing higher levels of toxins are immunocompromised, we demonstrate that Pieris rapae caterpillars feeding on more toxic host plants have enhanced cellular immunity at the cost of reduced growth rates and body size. However, whether enhanced immune systems are effective defense against parasitoids depends on parasitoid identity. Whereas enhanced immunity provided increased protection against the parasitoid Cotesia glomerata, it did not provide protection against C. rubecula that suppressed and evaded the host’s immune system. Our study demonstrates that both herbivore immunity and species identity of trophic participants are crucial in determining the structure of multitrophic interactions.

Insect herbivores frequently must balance host plant quality and the risk of attack by their natural enemies when making oviposition decisions. Yet, which factor is more important remains unresolved in plant–insect ecology. Here, we report the oviposition preference and larval performance of the brassicaceous specialist Plutella xylostella, in the context of plant quality (cabbage Brassica oleracea vs. mustard B. juncea) and associated natural enemies. Despite the greater larval weight and adult lifespan on cabbage, ovipositing females strongly preferred mustard. Both the egg parasitoid Trichogrammatoidea bactrae and the larval ectoparasitoid Bracon brevicornis are more likely to attack P. xylostella that feed on cabbage; thus, mustard represents enemy-reduced space from these two parasitoids. However, larval diet had no impact on the parasitism rate of specialist Cotesia vestalis. Feeding on mustard improved larval immune responses. The total hemocyte number, diversity, and phenoloxidase activity were higher in mustard-fed larvae which increased their survival against the entomopathogen, Bacillus thuringiensis. Interestingly, host plants altered the larval body odor profile. Mustard-fed larvae emitted allyl isothiocyanate (AITC) and butyl isothiocyanate (BITC) while cabbage-fed larvae emitted dimethyl disulphide (DMDS) and dimethyl trisulphide (DMTS) that served as short-range cues for larval parasitoids. For B. brevicornis, host body odor guided oviposition choice was crucial as their fitness was affected by the host larval diet. Although C. vestalis showed a clear preference towards volatiles emitted by mustard fed larvae, their fitness was unaltered. Taken together, our results illustrate that P. xylostella prefers to lay eggs on mustard plants providing enemy-reduced space from some, but not all, natural enemies.

Inducible responses to herbivores can be either localized or spread systemically throughout a plant. The ways in which clonal plants integrate their response to herbivores among clonal ramets is not well understood. Yet, this is important to understand the impacts that herbivores may have on clonal plants. We conducted a factorial split-plot greenhouse experiment to determine whether resistance is induced among ramets and how biomass allocation changes among ramets following herbivore damage to one of them. We manipulated the presence of two herbivores, Pieris rapae and Trichoplusia ni, and the root connection of ramets of the clonal invasive weed, Lepidium draba. We found local inducible resistance on the ramet where an herbivore fed, but not in neighboring ramets. Biomass allocation shifted in response to herbivores. Feeding by the generalist caterpillar T. ni resulted in a greater belowground biomass relative to shoot biomass in the local plant, but only when the clonal connection was intact. In contrast, herbivores had little impact on the root mass fraction of neighboring ramets. Herbivory to the local ramet increased the regrowth of neighboring ramets that lacked clonal connection, a trend that was driven by the specialist herbivore P. rapae. Herbivores did not induce systemic resistance among ramets of L. draba, but herbivores, especially the specialist, did alter how neighboring ramets regrow after grazing or mowing. Our observations suggest that individual ramets have fairly autonomous responses to herbivores, and that coordination among ramets, when present, may happen via signals that do not depend on root connections.

The diverse ecology of parasitoids is shaped by extrinsic competition, i.e., exploitative or interference competition among adult females and males for hosts and mates. Adult females use an array of morphological, chemical, and behavioral mechanisms to engage in competition that may be either intra- or interspecific. Weaker competitors are often excluded or, if they persist, use alternate host habitats, host developmental stages, or host species. Competition among adult males for mates is almost exclusively intraspecific and involves visual displays, chemical signals, and even physical combat. Extrinsic competition influences community structure through its role in competitive displacement and apparent competition. Finally, anthropogenic changes such as habitat loss and fragmentation, invasive species, pollutants, and climate change result in phenological mismatches and range expansions within host-parasitoid communities with consequent changes to the strength of competitive interactions. Such changes have important ramifications not only for the success of managed agroecosystems, but also for natural ecosystem functioning.