Effect of interference and exploitative competition on associative learning by a parasitoid wasp: a mechanism for ideal free distribution?

Collective intelligence facilitates emergent resource partitioning through frequency-dependent learning

Deciding where to forage must not only account for variations in habitat quality but also where others might forage. Recent studies have suggested that when individuals remember recent foraging outcomes, negative frequency-dependent learning can allow them to avoid resources exploited by others (indirect competition). This process can drive the emergence of consistent differences in resource use (resource partitioning) at the population level. However, indirect cues of competition can be difficult for individuals to sense. Here, we propose that information pooling through collective decision-making-i.e. collective intelligence-can allow populations of group-living animals to more effectively partition resources relative to populations of solitary animals. We test this hypothesis by simulating (i) individuals preferring to forage where they were recently successful and (ii) cohesive groups that choose one resource using a majority rule. While solitary animals can partially avoid indirect competition through negative frequency-dependent learning, resource partitioning is more likely to emerge in populations of group-living animals. Populations of larger groups also better partition resources than populations of smaller groups, especially in environments with more choices. Our results give insight into the value of long- versus short-term memory, home range sizes and the evolution of specialization, optimal group sizes and territoriality. This article is part of the theme issue 'Connected interactions: enriching food web research by spatial and social interactions'.

Extrinsic Inter- and Intraspecific Competition in Parasitoid Wasps

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.

Poor adult nutrition impairs learning and memory in a parasitoid wasp

Animals have evolved cognitive abilities whose impairment can incur dramatic fitness costs. While malnutrition is known to impact brain development and cognitive functions in vertebrates, little is known in insects whose small brain appears particularly vulnerable to environmental stressors. Here, we investigated the influence of diet quality on learning and memory in the parasitoid wasp Venturia canescens. Newly emerged adults were exposed for 24 h to either honey, 20% sucrose solution, 10% sucrose solution, or water, before being conditioned in an olfactory associative learning task in which an odor was associated to a host larvae (reward). Honey fed wasps showed 3.5 times higher learning performances and 1.5 times longer memory retention than wasps fed sucrose solutions or water. Poor diets also reduced longevity and fecundity. Our results demonstrate the importance of early adult nutrition for optimal cognitive function in these parasitoid wasps that must quickly develop long-term olfactory memories for searching suitable hosts for their progeny.

Wolbachia impairs post-eclosion host preference in a parasitoid wasp

Host preference behavior can result in adaptive advantages with important consequences for the fitness of individuals. Hopkin's host-selection principle (HHSP) suggests that organisms at higher trophic levels demonstrate a preference for the host species on which they developed during their own larval stage. Although investigated in many herbivorous and predatory insects, the HHSP has, to our knowledge, never been tested in the context of insects hosting selfish endosymbiotic passengers. Here, we investigated the effect of infection with the facultative bacterial symbiont Wolbachia on post-eclosion host preference in the parasitoid wasp Trichogramma brassicae (Hymenoptera: Trichogrammatidae). We compared host preference in Wolbachia-infected individuals and uninfected adult female parasitoids after rearing them on two different Lepidopteran hosts, namely the flour moth Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) or the grain moth Sitotroga cerealella (Lepidoptera: Gelechiidae) in choice and no choice experimental design (n = 120 wasps per each choice/no choice experiments). We showed that in T. brassicae, Wolbachia affects the post-eclosion host preference of female wasps. Wolbachia-infected wasps did not show any host preference and more frequently switched hosts in the laboratory, while uninfected wasps significantly preferred to lay eggs on the host species they developed on. Additionally, Wolbachia significantly improved the emergence rate of infected wasps when reared on new hosts. Altogether, our results revealed that the wasp's infection with Wolbachia may lead to impairment of post-eclosion host preference and facilitates growing up on different host species. The impairment of host preference by Wolbachia may allow T. brassicae to shift between hosts, a behavior that might have important evolutionary consequences for the wasp and its symbiont.