The optimal residence time of a forager exploiting a resource patch in the absence of competitors has been much studied since the development of the marginal value theorem. However, several foragers are frequently observed exploiting the same patch simultaneously, and patch residence time has been surprisingly little studied in such competitive situations. The few theoretical models developed on this topic predict that foragers should engage in a war of attrition and stay in the patch longer than when foraging alone. We tested this prediction in Pachycrepoideus vindemmiae (Hymenoptera: Pteromalidae), a solitary parasitoid species in which females are known to defend the hosts they are exploiting via intraspecific fighting. By measuring the effect of direct (i.e., presence of conspecifics) and indirect (i.e., presence of already-parasitized hosts) competition on patch exploitation strategies, we revealed an apparent polymorphism of strategies. Indeed, in competitive situations, some members of the population tended to retreat almost immediately from the patch whereas other members tended to remain, further exploit the patch, and be more involved in interactions with conspecifics. The proportion of “retreaters” strongly depended on competition intensity and prior experience of competition. The forager's physiological state (age) also affects patch exploitation strategy and the intensity of interactions between competing females. Our results highlight the necessity for further theoretical studies that consider cases where contests between foragers are costly and take into account prior experience of competition and the forager's physiological state.

Chemical communication is very important in herbivorous insects, with many species being important agricultural pests. They often use olfactory cues to find their host plants at a distance and evaluate their suitability upon contact with non-volatile cues. Responses to such cues are modulated through interactions between various stimuli of biotic and abiotic origin. In addition, the response to the same stimulus can vary as a function of, for example, previous experience, age, mating state, sex, and morph. Here we summarize recent advances in the understanding of plant localization and recognition in herbivorous insects with a focus on the interplay between long- and short-range signals in a complex environment. We then describe recent findings illustrating different types of plasticity in insect plant choice behavior and the underlying neuronal mechanisms at different levels of the chemosensory pathway. In the context of strong efforts to replace synthetic insecticides with alternative pest control methods, understanding combined effects between long- and close-range chemical cues in herbivore–plant interactions and their complex environment in host choice are crucial to develop effective plant protection methods. Furthermore, plasticity of behavioral and neuronal responses to chemical cues needs to be taken into account to develop effective sustainable pest insect control through behavioral manipulation.

In this paper, we compare the host selection behaviours of two parasitoids, Aphidius rhopalosiphi and Aphidius picipes, in order to analyse whether behavioural adaptations to the defensive behaviour of their host (the grain aphid Sitobion avenae) could, in part, be responsible for the simultaneous presence of both species in cereal fields. The oviposition behaviour of A. picipes differed from that of A. rhopalosiphi by including a number of ‘fluttering wings’ sequences followed by immobility. It resulted in a 44 times longer host-handling time for A. picipes than for A. rhopalosiphi. Hosts attacked by A. picipes exhibited fewer defensive behaviours than hosts attacked by A. rhopalosiphi. A. picipes and A. rhopalosiphi rejected respectively 0% and 53% of unparasitized hosts presenting cornicle secretions, one of the defensive means of aphids. Furthermore, A. picipes females rejected 100% of the hosts that were already parasitized, whereas A. rhopalosiphi was previously described to reject only 20 to 40% of such hosts. Such differences could be explained by the way the two species deal with the aphid defensive behaviour. Field analyses, showed that A. rhopalosiphi was already present in wheat fields in early April whereas A. picipes appeared later and only achieved a low level of parasitism. However, when both species were present simultaneously, they shared the same resource.

Volatiles resulting from plant–herbivore interactions play an important role in the behavioral decisions of phytophagous, predatory, and parasitoid insects and could be used for managing pest insects. However, to date and after about 40 years of research, documented studies on applications in the field remain extremely scarce.Delia radicumL. (Diptera: Anthomyiidae), the cabbage root fly, is a major pest of brassicaceous crops for which classical control strategies are currently lacking. Our previous studies showed that dimethyl disulfide (DMDS), a compound emitted by roots heavily infested byD. radicumlarvae, was attractive for the fly’s main natural enemies and could lead to a reduction of 60% in number of eggs laid on treated plants in the field. As a follow‐up of this work, we conducted another field study to select additional volatiles that could be used in a push–pull approach. Several synthetic herbivore‐induced plant volatiles, selected on the basis of their potential action on the behavior of both the fly and its natural enemies, were placed in odor dispensers in experimental broccoli plots and their influence on oviposition byD. radicumand egg predation by ground‐dwelling predators was assessed. Our results confirmed the role of DMDS in reducingD. radicumegg numbers on broccoli plants and revealed that (Z)‐3‐hexenyl acetate, a green leaf volatile released by recently damaged plants, strongly stimulated fly oviposition. Also, two of the compounds tested slightly modified predation activity of ground‐dwelling predators: acetophenone decreased the proportion of predated patches, whereas methyl salicylate increased it. This study is a first step in designing a push–pull strategy to control the cabbage root fly.

In solitary parasitoid species, superparasitism incurs a high cost because only one individual can emerge per host. While avoiding already-parasitize

Facultative intracellular symbionts like the α-proteobacteria Wolbachia influence their insect host phenotype but little is known about how much they affect their host microbiota. Here, we quantified the impact of Wolbachia infection on the bacterial community of the cabbage root fly Delia radicum by comparing the microbiota of Wolbachia-free and infected adult flies of both sexes. We used high-throughput DNA sequencing (Illumina MiSeq, 16S rRNA, V5-V7 region) and performed a community and a network analysis. In both sexes, Wolbachia infection significantly decreased the diversity of D. radicum bacterial communities and modified their structure and composition by reducing abundance in some taxa but increasing it in others. Infection by Wolbachia was negatively correlated to 8 bacteria genera (Erwinia was the most impacted), and positively correlated to Providencia and Serratia. We suggest that Wolbachia might antagonize Erwinia for being entomopathogenic (and potentially intracellular), but would favor Providencia and Serratia because they might protect the host against chemical plant defenses. Although they might seem prisoners in a cell, endocellular symbionts can impact the whole microbiota of their host, hence its extended phenotype, which provides them with a way to interact with the outside world.