Normal and experimentally induced changes in hormonal hemolymph titers during parental behavior of the earwig Labidura riparia

Exposure to pyriproxyfen (juvenile hormone agonist) does not alter maternal care and reproduction in the European earwig

Sublethal exposure to pesticides can alter the survival and reproduction of a wide range of non-target organisms. However, it remains unclear whether this exposure can alter behaviours that are often essential for long-term population dynamics and maintenance, such as parental care. In this study, we tested the effect of pyriproxyfen exposure (an insect growth regulator) on maternal care in the European earwig, an insect that is both used in pest control in pip-fruit orchards and considered a pest in stone fruit orchards. We exposed 424 females at doses either 10 times lower, equivalent or 10 times higher than normal application rates in French orchards. As maternal care can change over the weeks of family life, we exposed the earwig mothers at five different days before and after egg hatching. We then measured the expression of ten forms of maternal care towards eggs and juveniles, six non-caring behaviours, eggs and juvenile development, metabolic reserves in mothers at egg hatching and females' production of a terminal clutch. First, our results revealed that the three tested doses of pyriproxyfen were non-lethal and confirmed that maternal care decreased throughout both pre- and post-hatching family life. However, we did not detect any effect of pyriproxyfen on maternal care and non-care behaviours, eggs and juvenile development, quantities of lipids, proteins and glycogen in mothers at egg hatching, and on the production of a future clutch. Overall, these findings suggest that the maximal doses of pyriproxyfen authorized in French orchards is likely to have limited effects on the short- and long-term maintenance of populations of the European earwig and raises fundamental questions about the nature of the link between juvenile hormone and parental care in insects.

Sublethal exposure to deltamethrin stimulates reproduction and has limited effects on post-hatching maternal care in the European earwig

Although pesticides are typically used to limit pest population, the diversity and nature of their unintentional effects on non-target organisms remain unclear. Better understanding these effects requires to carry out risk assessments on key physiological and behavioral processes specific to beneficial insects. In this study, we addressed this question by exposing mothers of the European earwig (a beneficial insect) to two sublethal doses of deltamethrin (a common pesticide in agriculture) during family life and measured the short- and long-term effects on a series of behavioral, physiological, and reproductive traits. Somewhat surprisingly, our results first revealed that high and low doses of deltamethrin enhanced mothers' future reproduction by augmenting their likelihood to produce a second clutch, shortening the number of days until its production, and increasing the resulting number of eggs and their hatching rate. Conversely, the high dose of deltamethrin was detrimental, as it limited maternal brood defence, and reduced food consumption and expression of self-grooming. Finally, other traits were independent of deltamethrin exposure, such as three proxies of family interactions (i.e., distance to the brood, occurrence, and duration of mother-offspring contacts), mothers' walking distance, and mother weight gain during family life. Our study overall demonstrates that sublethal exposure to a pesticide such as deltamethrin can have both positive and negative effects on non-target beneficial insects. It thus emphasizes that focusing on narrow parameters can lead to misleading conclusions about the unintended impacts of pesticides in treated agro-ecosystems and call for better considering this parameters diversity in integrated pest management programs.

The genetic mechanism of selfishness and altruism in parent-offspring coadaptation

The social bond between parents and offspring is characterized by coadaptation and balance between altruistic and selfish tendencies. However, its underlying genetic mechanism remains poorly understood. Using transcriptomic screens in the subsocial European earwig, Forficula auricularia, we found the expression of more than 1600 genes associated with experimentally manipulated parenting. We identified two genes, Th and PebIII, each showing evidence of differential coexpression between treatments in mothers and their offspring. In vivo RNAi experiments confirmed direct and indirect genetic effects of Th and PebIII on behavior and fitness, including maternal food provisioning and reproduction, and offspring development and survival. The direction of the effects consistently indicated a reciprocally altruistic function for Th and a reciprocally selfish function for PebIII. Further metabolic pathway analyses suggested roles for Th-restricted endogenous dopaminergic reward, PebIII-mediated chemical communication and a link to insulin signaling, juvenile hormone, and vitellogenin in parent-offspring coadaptation and social evolution.

Chemical signals might mediate interactions between females and juveniles of Latrodectus geometricus (Araneae: Theridiidae)

Studies related to communication on spiders show that, as in other invertebrates, the interactions between conspecifics are also made through chemical signals. Therefore, in order to assess whether the composition of cuticular compounds might be involved in interactions that occur during the days after the emergence of juveniles in Latrodectus geometricus, we conducted behavioral and cuticular chemical profiles analysis of females and juveniles of different ages. The results show that females, regardless of their reproductive state, tolerate juveniles of other females with up to 40 days post-emergence and attack juveniles of 80 days post-emergence. Cuticlar chemical analysis shows that while the profile of juveniles is similar to adult's profile, they can remain in the web without being confused with threat or prey. Also, cuticular chemical profiles vary between different populations probably due to genetic and environmental differences or similarities between them. Finally, females in incubation period are able to detect the presence of eggs within any egg sac, but cannot distinguish egg sacs produced by conspecifics from the ones they had produced.

The developmental genetics and physiology of honeybee societies

Eusocial animal societies, as diverse as those found in the ants, bees, wasps, shrimp and naked mole-rats, are structured around one or few reproductive females. The remaining females are helpers called 'workers' that are mostly sterile. A paradigm in studies of eusociality is that worker sterility is a key to societal functions because advanced sociality cannot be achieved when there is conflict over reproduction. Yet, traits such as sensory responsiveness, foraging and hoarding behaviour that change between female reproductive life stages also vary between workers. This variation is central to worker division of labour, a complex social trait believed to be instrumental for the ecological success of animal societies. Thus, we took a step back from established views on worker sterility and societal functions, and hypothesized that division of labour can be better understood if adaptive variation in worker behaviour is seen as emerging from pre-existing mechanisms associated with female reproduction. In exploring this reproductive ground plan hypothesis (RGPH) in honeybee workers, we established that variation in foraging division of labour correlates with ovary size and is affected by expression changes in vitellogenin, an egg yolk protein precursor. Here, we explain and reconcile the RGPH with data on honeybee sensory sensitivity, genomic mapping, transcript and endocrine profiling, and link our discussion with Ihle et al. (2010, this issue, pp. xx-xx). The findings bring together mechanistic and evolutionary explanations of honeybee worker behaviour. This essay suggests that a broader view on worker reproductive traits can increase the understanding of animal social behaviour.

The curious case of aging plasticity in honey bees

As in all advanced insect societies, colony-organization in honey bees emerges through a structured division of labor between essentially sterile helpers called workers. Worker bees are sisters that conduct all social tasks except for egg-laying, for example nursing brood and foraging for food. Curiously, aging progresses slowly in workers that engage in nursing and even slower when bees postpone nursing during unfavorable periods. We, therefore, seek to understand how senescence can emerge as a function of social task performance. The alternative utilization of a common yolk precursor protein (vitellogenin) in nursing and somatic maintenance can link behavior and aging plasticity in worker bees. Beneficial effects of vitellogenin may also be mediated by inhibitory action on juvenile hormone and insulin-like signaling.

The making of a social insect: developmental architectures of social design

We marvel at the social complexity of insects, marked by anatomically and behaviorally distinguishable castes, division of labor and specialization--but how do such systems evolve? Insect societies are composed of individuals, each undergoing its own developmental process and each containing its own genetic information and experiencing its own developmental and experiential environment. Yet societies appear to function as if the colonies themselves are individuals with novel "social genes" and novel social developmental processes. We propose an alternative hypothesis. The origins of complex social behavior, from which insect societies emerge, are derived from ancestral developmental programs. These programs originated in ancient solitary insects and required little evolutionary remodeling. We present evidence from behavioral assays, selective breeding, genetic mapping, functional genomics and endocrinology, and comparative anatomy and physiology. These insights explain how complex social behavior can evolve from heterochronic changes in reproductive signaling systems that govern ubiquitous and ancient relationships between behavior and ovarian development.

Reproductive ground plan may mediate colony-level selection effects on individual foraging behavior in honey bees

The colony-level phenotype of an insect society emerges from interactions between large numbers of individuals that may differ considerably in their morphology, physiology, and behavior. The proximate and ultimate mechanisms that allow this complex integrated system to form are not fully known, and understanding the evolution of social life strategies is a major topic in systems biology. In solitary insects, behavior, sensory tuning, and reproductive physiology are linked. These associations are controlled in part by pleiotropic networks that organize the sequential expression of phases in the reproductive cycle. Here we explore whether similar associations give rise to different behavioral phenotypes in a eusocial worker caste. We document that the pleiotropic genetic network that controls foraging behavior in functionally sterile honey bee workers (Apis mellifera) has a reproductive component. Associations between behavior, physiology, and sensory tuning in workers with different foraging strategies indicate that the underlying genetic architectures were designed to control a reproductive cycle. Genetic circuits that make up the regulatory "ground plan" of a reproductive strategy may provide powerful building blocks for social life. We suggest that exploitation of this ground plan plays a fundamental role in the evolution of social insect societies.

Plasticity in juvenile hormone in male burying beetles during breeding: physiological consequences of the loss of a mate

Burying beetles, Nicrophorus orbicollis, have facultative biparental care. They bury and prepare small vertebrate carcasses that provide food for their young. Here we establish the juvenile hormone (JH) profiles of paired females, paired males and single males and investigate some of the environmental and social factors that may affect these profiles. Before larvae hatch JH profiles of paired males and females were similar. However, after larvae hatch and during brood care, JH titers of females were very high and those of single males were significantly higher than those of paired males. We tested the hypothesis that higher JH was a response to the need for increased parental care by manipulating brood size. Although JH titers of single males caring for small versus large broods were not significantly different, when comparing JH titers and larval growth (a measure of parental effort), a significant positive correlation emerged. In contrast, we found that food quality had no effect on JH levels suggesting that increased feeding by males and females after carcass discovery cannot explain the elevation of JH. The regulation of JH in male burying beetles appears thus to be dependent on the presence of a mate and on critical stimuli from young.

Social stimuli affect juvenile hormone during breeding in biparental burying beetles (Silphidae: Nicrophorus)

Extended biparental care is rare in insects but provides an excellent opportunity to investigate the interaction between the endocrine system and the physical and social environment in the regulation of this behavior. Burying beetles (Nicrophorus spp.) have facultative biparental care and depend on locating a small vertebrate carcass that they bury and prepare as food for their young. Commonly, both male and female Nicrophorus orbicollis remain in the burial chamber after eggs hatch to feed and guard the larvae. In both sexes, juvenile hormone (JH) rises rapidly in response to the discovery and assessment of the carcass; it returns to near baseline in 24 h; then in females it reaches very high titers at the onset of maternal care. In this paper, we investigate some social (presence of a mate, mating history, larval age) and environmental (carcass size) factors that may affect this endocrine profile. For females, neither the presence of a mate nor mating status (i.e., virginity) affected the initial rise of JH. However, the absence of a mate significantly depressed the JH rise in males. Eighty-seven percent of the single males buried the carcass like paired males but 87% also released pheromones to attract a mate. JH hemolymph titers in females whose broods were replaced every 24 h with newly hatch larvae were significantly higher than those of females rearing aging broods. Lastly, even though larger carcasses took longer to bury and prepare and oviposition was delayed, neither JH titers nor speed of ovarian development was affected by carcass size.