Chemical defense acquired via pharmacophagy can lead to protection from predation for conspecifics in a sawfly.

Chemical defense is a widespread anti-predator strategy exhibited by organisms, with individuals either synthesizing or extrinsically acquiring defensive chemicals. In some species, such defences can also be transferred among conspecifics. Here, we tested the effects of pharmacophagy on the defense capability of the turnip sawfly, Athalia rosae, which can acquire neo-clerodane diterpenoids (clerodanoids) via pharmacophagy when having access to the plant Ajuga reptans. We show that clerodanoid access mediates protection against predation by mantids for the sawflies, both in a no-choice feeding assay and a microcosm setup. Even indirect access to clerodanoids, via nibbling on conspecifics that had access to the plant, resulted in protection against predation albeit to a lower degree than direct access. Furthermore, sawflies that had no direct access to clerodanoids were consumed less frequently by mantids when they were grouped with conspecifics that had direct access. Most, but not all, of such initially undefended sawflies could acquire clerodanoids from conspecifics that had direct access to the plant, although in low quantities. Together our results demonstrate that clerodanoids serve as a chemical defense that can also be transferred by interactions among conspecifics. Moreover, the presence of chemically defended individuals in a group can confer protection onto conspecifics that had no direct access to clerodanoids.

Ecological conditions alter cooperative behaviour and its costs in a chemically defended sawfly.

The evolution of cooperation and social behaviour is often studied in isolation from the ecology of organisms. Yet, the selective environment under which individuals evolve is much more complex in nature, consisting of ecological and abiotic interactions in addition to social ones. Here, we measured the life-history costs of cooperative chemical defence in a gregarious social herbivore, Diprion pini pine sawfly larvae, and how these costs vary under different ecological conditions. We ran a rearing experiment where we manipulated diet (resin content) and attack intensity by repeatedly harassing larvae to produce a chemical defence. We show that forcing individuals to allocate more to cooperative defence (high attack intensity) incurred a clear cost by decreasing individual survival and potency of chemical defence. Cooperative behaviour and the magnitude of its costs were further shaped by host plant quality. The number of individuals participating in group defence, immune responses and female growth decreased on a high resin diet under high attack intensity. We also found some benefits of cheating: non-defending males had higher growth rates across treatments. Taken together, these results suggest that ecological interactions can shape the adaptive value of cooperative behaviour and maintain variation in the frequency of cooperation and cheating.

Bacterial diversity and community structure shapes pederin polymorphism but lacks association with host genotype specificity in the rove beetle, Paederus fuscipes.

Paederus beetles are notorious for their irritant haemolymph toxin pederin, which is synthesised by an unculturable bacterial symbiont, causing dermatitis when in contact with human skin. Pederin polymorphism is observed when (+) females carrying the toxin pederin and (-) females lacking this co-occur in natural populations. Despite reports detecting pederin polymorphism in several Paederina beetles, symbiont infection frequencies in their natural populations and the bacterial diversity differences underlying the polymorphism between these female types are understudied. Herein we report a high prevalence (>80%) of female Paederus fuscipes carrying bacterial ped genes in all six study populations. This finding suggests that selection pressure favouring pederin-producing females is crucial for survival in the natural environment. 16S rRNA metabarcoding analysis revealed significant dominance of the unculturable pederin-producing Pseudomonas-like bacterium (Gammaproteobacteria) in (+) females, consistent with previous studies. The microbial diversity of the (-) females revealed a significant abundance of Apibacter (Bacteroidia), previously undocumented, suggesting its importance in the functionality of (-) females. Predicted functions related to metabolisms are enriched in (-) females, suggesting fitness advantage possibilities in sustaining the population in the absence of predation. Further investigations on the possible genetic basis of the host genotype revealed no association of host mtDNA and pederin polymorphism in Pa. fuscipes.

Intragroup and intragenomic conflict over chemical defense against predators.

Insects are often chemically defended against predators. There is considerable evidence for a group-beneficial element to their defenses, and an associated potential for individuals to curtail their own investment in costly defense while benefitting from the investments of others, termed "automimicry." Although females in chemically defended taxa often lay their eggs in clusters, leading to siblings living in close proximity, current models of automimicry have neglected kin-selection effects, which may be expected to curb the evolution of such selfishness. Here, we develop a general theory of automimicry that explicitly incorporates kin selection. We investigate how female promiscuity modulates intragroup and intragenomic conflicts overinvestment into chemical defense, finding that individuals are favored to invest less than is optimal for their group, and that maternal-origin genes favor greater investment than do paternal-origin genes. We translate these conflicts into readily testable predictions concerning gene expression patterns and the phenotypic consequences of genomic perturbations, and discuss how our results may inform gene discovery in relation to economically important agricultural products.

Arthropod predation in a dendrobatid poison frog: does frog life stage matter?

Frogs in the family Dendrobatidae are well known for their conspicuous colors and variable alkaloid-based chemical defenses. The aposematic coloration in dendrobatid frogs appears to deter predators with color vision, but relatively little is known about how these frogs are protected and their defenses are perceived by non-color vision dominated predators. The neotropical bullet ant Paraponera clavata and the red-legged banana spider Cupiennius coccineus are predators that avoid adults of the dendrobatid Oophaga pumilio, but readily consume non-toxic frogs. Juvenile O. pumilio possess the same warning coloration as adult O. pumilio, but may be more palatable given that they have lower quantities of defensive chemicals. This may provide juvenile O. pumilio protection from color-sighted predators, while leaving them susceptible to predators that use chemoreception. To test this hypothesis, we presented juveniles and adults of both O. pumilio and the non-chemically defended frog Craugastor bransfordii to bullet ants and banana spiders. Both bullet ants and banana spiders preyed upon C. bransfordii significantly more than on O. pumilio. Adult and juvenile C. bransfordii experienced similar predation rates by both predators. The life stage of O. pumilio significantly predicted predation by bullet ants, with juveniles being consumed significantly more often than adults. However, the life stage of O. pumilio did not predict predation by banana spiders, as no adults or juveniles were consumed. Our study provides evidence that bullet ants can detect differences in chemical defenses between juvenile and adult O. pumilio, resulting in differential predation on the more palatable juvenile frogs. The avoidance of both adults and juveniles by C. coccineus suggests the alkaloids in O. pumilio act as an effective chemical deterrent to banana spiders, regardless of quantity. Overall, our results suggest that differences in alkaloid defenses among life stages in O. pumilio correspond to differences in relative palatability to at least one arthropod predator.

Pyrrolizidine alkaloid deters ant predators of Utetheisa ornatrix eggs: effects of alkaloid concentration, oxidation state, and prior exposure of ants to alkaloid-laden prey.

To examine the chemical defense of lepidopteran eggs towards ant predators, eggs of the arctiid moth Utetheisa ornatrix were offered to laboratory colonies of the ant Leptothorax longispinosus. The ants rapidly devoured Utetheisa eggs produced by parents reared on an alkaloid-free diet, but left eggs that had been endowed with parental alkaloid largely unmolested. That defense can be attributed directly to the presence of a pyrrolizidine alkaloid since topical application of monocrotaline in either its free base or N-oxide state conferred protection on otherwise palatable eggs. The N-oxide and free base were not equally effective in conferring protection however; free-base treated eggs were subject to greater predation than N-oxide treated eggs after 24 h. The emergence of differential alkaloid effectiveness only after prolonged exposure is consistent with the notion that chemical protection is seldom absolute. That difference was resolved only when the otherwise food-deprived ants exceeded a certain threshold of hunger. The efficacy of applied monocrotaline as a predator deterrent increased with increasing concentration of N-oxide, but was lowest for eggs treated with an intermediate concentration of free base. The latter concentration effect likely represents an artefact of the behaviour of the ant colonies offered eggs treated with an intermediate concentration, as those colonies were the most voracious predators in reference tests with palatable eggs. In addition to the immediate deterrent value of pyrrolizidine alkaloid, ant colonies that had been exposed to alkaloid-laden eggs subsequently avoided even palatable, unprotected eggs offered 33 days later. Our data provide the first demonstration of such long-term avoidance of chemically protected lepidopteran prey by an invertebrate. The discovery of a mechanism promoting learned avoidance on the part of ant predators has important ramifications to egg-laying strategies of female Utetheisa, both in terms of the dispersion of eggs, and the extent to which eggs are provisioned with alkaloid.

Variation in social and sexual behaviour in four species of aposematic seed bugs (Hemiptera: Lygaeidae): the role of toxic and non-toxic food.

Understanding variation in social behaviour both within and among species continues to be a challenge. Evolutionary or ecological theory typically predicts the optimal behaviour for an animal under a given set of circumstances, yet the real world presents much greater variation in behaviour than predicted. This variation is apparent in many social and sexual interactions, including mate choice, and has led to a renewed focus on individual variation in behaviour. Here we explore within and among species variation in social behaviour in four species of aposematic seed bug (Lygaeidae: Hemiptera). These species are Müllerian mimics, with characteristic warning colouration advertising their chemical toxicity. We examine the role of diet in generating variation in two key behaviours: social aggregation of nymphs and mate choice. We test how behaviour varies with exposure to either milkweed (a source of defensive compounds) or sunflower (that provides no defence). We show that although the four species vary in their food preferences, and diet influences their life-history (as highlighted by body size), social aggregation and mate choice is relatively unaffected by diet. We discuss our findings in terms of the evolution of aposematism, the importance of automimicry, and the role of diet in generating behavioural variation.