Eric Darrouzet

Phenotypic plasticity in the reproductive traits of a parasitoid.

Organisms show phenotypic plasticity--the capacity for a given genotype to express different phenotypes--in response to changes in the environment. Among the several factors that can cause phenotypic plasticity, nutritional constraints during development can affect the size of organisms and, consequently, affect most life-history traits, including reproductive traits. As their larvae are restricted by the amount of food contained in their host, parasitoids are a good model to study phenotypic plasticity related to size. The phenotypic plasticity of reproductive traits was investigated in the egg parasitoid Trichogramma euproctidis (Hymenoptera: Trichogrammatidae) by using host species of different sizes. Adult size, sperm storage organs (seminal vesicles and spermatheca), number of sperm stored and gamete size (sperm and oocyte) are all influenced by the host species; larger individuals have larger organs which contain more sperm, and both sperm and oocytes are correlated with adult size. However, while females become larger than males and mature larger oocytes in larger hosts, increase in sperm length stops after a given threshold.

Discrimination of the age of conspecific eggs by an ovipositing ectoparasitic wasp.

The recognition and avoidance of already parasitized hosts is a major issue in parasitoid behavioural ecology. A key factor affecting the fitness reward expected from superparasitism is the probability that the second or subsequent egg laid on a host will win the contest with the first‐laid egg. The present study investigated the ability of females of the solitary ecto parasitoid Anisopteromalus calandrae Howard (Hymenoptera: Pteromalidae) to (i) discriminate between unparasitized Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae) hosts and those parasitized by a conspecific, and (ii) discriminate between a host parasitized by an egg just laid (2 h) and one parasitized by an egg about to hatch (28 h). However, they did not adjust their offspring sex ratio on already parasitized hosts compared to unparasitized ones. Our results show that A. calandrae females can discriminate between parasitized and unparasitized hosts, as they lay more eggs on the latter. The probability of the second or subsequent egg laid on a host (superparasitism) winning the contest with a conspecific increases as the time between the two ovipositions decreases. Consequently, parasitoid females should lay more eggs on recently parasitized hosts than on those that have been parasitized for a long time (i.e., when the first eggs are about to hatch), and that is indeed what they were found to do. To increase their fitness in spite of the presence of already parasitized hosts, A. calandrae females have developed highly discriminative capacities regarding the parasitism status of hosts.

Sex Allocation Decision Under Superparasitism by the Parasitoid Wasp Eupelmus vuilleti

Superparasitism is a widespread phenomenon in parasitoids and may be advantageous in some circumstances. In this study, offspring sex ratio was analysed in three superparasitism situations: when the second egg was laid by a random Eupelmus vuilleti (Hymenoptera: Eupelmidae) female from a group, when an isolated female was allowed to lay two eggs on the same host (self-superparasitism) or laid one egg on a host already parasitized by a conspecific (conspecific superparasitism). Females produced a different offspring sex ratio according to the different superparasitism situations tested. These sex ratios are in line with the local mate competition theory. The results further suggest that females can discriminate between hosts parasitized by a conspecific or by themselves and adapt the sex of the eggs they lay accordingly.

Production of early diploid males by European colonies of the invasive hornet Vespa velutina nigrithorax.

The invasive yellow-legged hornet Vespa velutina nigrithorax was accidentally introduced in Europe in the early 2000s. As is the case in colonies of other wasp and hornet species, V. velutina colonies are known to produce sexuals (males and new queens) at the end of the summer. We show that early-stage colonies in French populations frequently produce males well before the usual reproductive period. The vast majority of the males produced are diploid, which is consistent with the loss of genetic diversity previously reported in introduced populations in France. Since males do not participate in colony activities, the production of early diploid males at the expense of workers is expected to hamper colony growth and, ultimately, decrease the expansion of the species in its invasive range in Europe.

4′-OH-JH-III: an additional hydroxylated juvenile hormone produced by locust corpora allata in vitro

Abstract In the present paper, we report the identification of 4′OH JH-III, an additional hydroxy-JH-III, produced in vitro by the corpora allata of the African locust Locusta migratoria. This shows that the three methyl group of JH-III linked to carbon 4, 8 and 12 can be hydroxylated generating three new putative juvenile hormones namely 4′OH-JH III, 8′-OH-JH-III and 12′-OH-JH-III. Both the role and the biosynthetic pathway of hydroxy-JH is discussed, opening new perspectives for the studies on juvenile hormones and isoprenoid compounds.

Impact of mating status on egg-laying and superparasitism behaviour in a parasitoid wasp

Most parasitoid female wasps can distinguish between unparasitized and parasitized hosts and use this information to optimize their progeny and sex allocation. In this study, we explored the impact of mating on oviposition behaviour (parasitism and self‐ and conspecific superparasitism) on both unparasitized and already parasitized hosts in the solitary parasitoid wasp Eupelmus vuilleti (Crw.) (Hymenoptera: Eupelmidae). Virgin and mated females had the same oviposition behaviour and laid eggs preferentially on unparasitized hosts. The sex ratio (as the proportion of females) of eggs laid by mated females in parasitism and conspecific superparasitism was 0.67 ± 0.04 and 0.57 ± 0.09, respectively. Likewise, females laid more eggs in conspecific superparasitism than self‐superparasitism under our experimental conditions. These experiments demonstrate that E. vuilleti females can (i) discriminate between unparasitized and parasitized hosts and adapt the number of eggs they lay accordingly, and (ii) probably discriminate self from conspecific superparasitized hosts. Finally, mating does not appear to influence the host discrimination capacity, the ovarian function, or the oviposition behaviour.

Parasitoids Modify Their Oviposition Behavior According to the Sexual Origin of Conspecific Cuticular Hydrocarbon Traces

Hydrocarbons play a crucial role in insect behavior in general and in sexual recognition in particular. Parasitoids often modify their oviposition behavior according to hydrocarbons left by conspecifics on the reproductive patch, such as oviposition markers left by females after oviposition, or cuticular hydrocarbon (CHC) traces left by individuals by walking or rubbing. This study determined whether Eupelmus vuilleti females are able to distinguish CHCs left by male or female conspecifics on seeds. The results show that the cuticular profile of E. vuilleti differs according to its gender, and that females are able to detect the sexual origin of these CHCs. Moreover, they adjust their oviposition behavior according to the nature of these traces. Although females lay fewer eggs on hosts when confronted with female CHCs, they lay more daughters when confronted with male CHCs, thus changing the sex ratio.

Endocrine control of cuticular hydrocarbon profiles during worker-to-soldier differentiation in the termite Reticulitermes flavipes

The social organization of termites, unlike that of other social insects, is characterized by a highly plastic caste system. With the exception of the alates, all other individuals in a colony remain at an immature stage of development. Workers in particular remain developmentally flexible; they can switch castes to become soldiers or neotenics. Juvenile hormone (JH) is known to play a key role in turning workers into soldiers. In this study, we analyzed differences in cuticular hydrocarbon (CHC) profiles among castes, paying particular attention to the transition of workers to soldiers, in the subterranean termite species Reticulitermes flavipes. CHCs have a fundamental function in social insects as they serve as cues in inter- and intraspecific recognition. We showed that (1) the CHC profiles of the different castes (workers, soldiers, nymphs and neotenics) are different and (2) when workers were experimentally exposed to a JH analog and thus induced to become soldiers, their CHC profiles were modified before and after the worker-presoldier molt and before and after the presoldier-soldier molt.

Ontogenic potentialities of the worker caste in two sympatric subterranean termites in France

SUMMARY In termites, the capacity of workers to differentiate into neotenic reproductives is an important characteristic that deserves particular attention. To gain insight into the differentiation pathway, the potentialities of workers and the endocrinal changes during the formation of neotenics were compared in two sympatric termites, Reticulitermes flavipes and Reticulitermes grassei. After 1 year of development, 100% of R. flavipes worker groups produced neotenics against only 63% of R. grassei groups. The average production of female neotenics was significantly higher in R. flavipes worker groups compared with R. grassei groups and R. flavipes produced a greater proportion of female neotenics. Moreover, R. flavipes produced more offspring, not only because there were more females, but also because R. flavipes females were more productive. Moreover, the offspring produced by R. flavipes grew faster than the offspring of R. grassei. Both ecdysteroid and juvenile hormone (JH) titers varied significantly during the development of neotenics. The two species showed similar ecdysteroid titer variation patterns. However, the JH titer variation patterns strongly differed: in R. grassei, the concentration of JH increased in maturing neotenics then dropped in mature neotenics, whereas in R. flavipes, the level of JH was significantly higher than in R. grassei and remained constantly high in mature neotenics. Overall, these results suggest that these two species differ strongly in many life‐history traits as well as in the physiological control of their caste differentiation system. Possible origins and mechanisms of such interspecific variations are discussed, as well as their evolutionary and ecological consequences.

A scientific note about a parasitoid that can parasitize the yellow-legged hornet, Vespa velutina nigrithorax, in Europe

The yellow-legged hornet, Vespa velutina nigrithorax, was accidentally introduced from China to southwestern France in 2004 (Haxaire et al. 2006). The species is currently colonizing France and neighboring countries (Spain, Portugal, Belgium, and Italy) (Darrouzet and Gevar 2012; Monceau et al. 2013). V. velutina is a pest in France because it preys upon domestic honeybees, Apis mellifera. Beekeeping operations are directly affected by this predation, with some beekeepers reporting colony losses (Darrouzet and Gevar 2012; Monceau et al. 2013).While many factors may play a role in the successful establishment of an invasive species, research has focused on the effects of parasites (Prenter et al. 2004). It has previously been suggested that invasive species are subject to less pressure from parasites than are sympatric native species because invasive species leave their parasites behind when they invade new habitats (enemy release hypothesis). They can thus allocate more resources to gr ...