Risk of Egg Parasitoid Attraction Depends on Anti-aphrodisiac Titre in the Large Cabbage White Butterfly Pieris brassicae

Males of a variety of insects transfer an anti-aphrodisiac pheromone to females during mating that renders them less attractive to conspecific males. In cabbage white butterflies, the transfer of an anti-aphrodisiac can result in the unwanted attraction of tiny egg parasitoid wasps of the genus Trichogramma that hitch-hike with mated female butterflies to a host plant where they parasitize the freshly laid butterfly eggs. Here, we show that the anti-aphrodisiac benzyl cyanide (BC) of the large cabbage white Pieris brassicae is depleted by frequent display of the mate-refusal posture that signals a female’s unreceptivity to mating. This depletion of BC is ecologically important because it results in a reduced risk of attracting the hitch-hiking egg parasitoid Trichogramma brassicae to mated female butterflies over time since mating. Our results indicate for the first time that a reduction in anti-aphrodisiac titre in mated females due to frequent adoption of the mate-refusal posture is beneficial to both mated females and males particularly when parasitoid pressure is high

Anti-aphrodisiac Compounds of Male Butterflies Increase the Risk of Egg Parasitoid Attack by Inducing Plant Synomone Production

During mating in many butterfly species, males transfer spermatophores that contain anti-aphrodisiacs to females that repel conspecific males. For example, males of the large cabbage white, Pieris brassicae (Lepidoptera: Pieridae), transfer the anti-aphrodisiac, benzyl cyanide (BC) to females. Accessory reproductive gland (ARG) secretion of a mated female P. brassicae that is deposited with an egg clutch contains traces of BC, inducing Brussels sprouts plants (Brassica oleracea var. gemmifera) to arrest certain Trichogramma egg parasitoids. Here, we assessed whether deposition of one egg at a time by the closely related small cabbage white, Pieris rapae, induced B. oleracea var. gemmifera to arrest Trichogramma wasps, and whether this plant synomone is triggered by substances originating from male P. rapae seminal fluid. We showed that plants induced by singly laid eggs of P. rapae arrest T. brassicae wasps three days after butterfly egg deposition. Elicitor activity was present in ARG secretion of mated female butterflies, whereas the secretion of virgin females was inactive. Pieris rapae used a mixture of methyl salicylate (MeSA) and indole as an anti-aphrodisiac. We detected traces of both anti-aphrodisiacal compounds in the ARG secretion of mated female P. rapae, whereas indole was lacking in the secretion of virgin female P. rapae. When applied onto the leaf, indole induced changes in the foliar chemistry that arrested T. brassicae wasps. This study shows that compounds of male seminal fluid incur possible fitness costs for Pieris butterflies by indirectly promoting egg parasitoid attack

Attractiveness of sown wildflower strips to flower-visiting insects depends on seed mixture and establishment success

Establishing wildflower strips has been suggested as an effective measure to promote pollination services, pest control or general insect biodiversity, but little is known about the integration of these different objectives when selecting flower seed mixtures. In ten agricultural landscapes in the Netherlands, we established a wildflower strip (0.4 – 4.9 ha) with half of each strip sown with a mixture targeting longer-tongued pollinators and the other half sown with a mixture targeting shorter-tongued pollinators and natural enemies. We determined establishment success of sown wildflowers and evaluated the attractiveness of the established flower communities to multiple functional groups of flower visitors: bumblebees (long-tongued pollinators), hoverflies (short-tongued pollinators and natural enemies), and butterflies and total flower-visitor richness (indicators of wider biodiversity values). Bumblebees clearly preferred the pollinatortargeted seed mixture and were positively associated with cover of Fabaceae and negatively with Apiaceae. Hoverflies consistently preferred the natural enemy mixture and were positively associated with Apiaceae. The other target groups displayed no clear responses to seed mixture type but instead were associated with local flower richness within strips. Across sites, responses of flower-visitors to sown mixture types did not depend on wildflower strip size, proportion of surrounding semi-natural habitat, or flower variables. However, all flower-visitors except butterflies increased with increasing cover or richness of (sown) flower species across sites. Our results suggest that, although species-rich wildflower strips may benefit several species groups, maximising different objectives involves trade-offs between functional groups that prefer short- or long-corolla flowers. Furthermore, our study suggests that sowing a wildflower mixture does not necessarily result in a vegetation with the same composition as the seed mixture as species may establish poorly or not at all. Selection of flower species for seed mixtures should therefore, in addition to insect target group, take the establishment characteristics of plant species into account

Attractiveness of sown wildflower strips to flower-visiting insects depends on seed mixture and establishment success

Establishing wildflower strips has been suggested as an effective measure to promote pollination services, pest control or general insect biodiversity, but little is known about the integration of these different objectives when selecting flower seed mixtures. In ten agricultural landscapes in the Netherlands, we established a wildflower strip (0.4 – 4.9 ha) with half of each strip sown with a mixture targeting longer-tongued pollinators and the other half sown with a mixture targeting shorter-tongued pollinators and natural enemies. We determined establishment success of sown wildflowers and evaluated the attractiveness of the established flower communities to multiple functional groups of flower visitors: bumblebees (long-tongued pollinators), hoverflies (short-tongued pollinators and natural enemies), and butterflies and total flower-visitor richness (indicators of wider biodiversity values). Bumblebees clearly preferred the pollinator-targeted seed mixture and were positively associated with cover of Fabaceae and negatively with Apiaceae. Hoverflies consistently preferred the natural enemy mixture and were positively associated with Apiaceae. The other target groups displayed no clear responses to seed mixture type but instead were associated with local flower richness within strips. Across sites, responses of flower-visitors to sown mixture types did not depend on wildflower strip size, proportion of surrounding semi-natural habitat, or flower variables. However, all flower-visitors except butterflies increased with increasing established cover or richness of (sown) flower species across sites. Our results suggest that, although species-rich wildflower strips may benefit several species groups, maximising different objectives involves trade-offs between functional groups that prefer short- or long-corolla flowers. Furthermore, our study suggests that sowing a wildflower mixture does not necessarily result in a vegetation with the same composition as the seed mixture as species may establish poorly or not at all. Selection of flower species for seed mixtures should therefore, in addition to insect target group, take the establishment characteristics of plant species into account

Differential effects of brain size on memory performance in parasitic wasps

Small animals usually have relatively larger brains than large animals. This allometric brain–body size scaling is described by Haller's rule. However, one of the smallest known insects, Trichogramma evanescens, a parasitic wasp, shows brain isometry, leading to similar relative brain sizes in small and large conspecifics. The somewhat larger Nasonia vitripennis parasitic wasp displays diphasic brain–body size scaling with isometry in small individuals and allometry in large individuals. These two species may have undersized brains for small wasps, with reduced cognitive abilities. Here, we induced intraspecific body size variation in genetically identical T. evanescens and N. vitripennis and examined cognitive trade-offs of brain scaling. We compared visual and olfactory memory retention between small and large conspecifics. Results showed that diphasic brain scaling affected memory retention levels in N. vitripennis, whereas isometric brain scaling did not affect memory retention in T. evanescens. The two species may experience different evolutionary pressures that have shaped the cognitive consequences of isometric brain–body size scaling. A possible trade-off of brain isometry in T. evanescens could be present in brain properties other than memory performance. In contrast, it may be more adaptive for N. vitripennis to invest in other aspects of brain performance, at the cost of memory retention

Differential effects of brain size on memory performance in parasitic wasps

Small animals usually have relatively larger brains than large animals. This allometric brain–body size scaling is described by Haller's rule. However, one of the smallest known insects, Trichogramma evanescens, a parasitic wasp, shows brain isometry, leading to similar relative brain sizes in small and large conspecifics. The somewhat larger Nasonia vitripennis parasitic wasp displays diphasic brain–body size scaling with isometry in small individuals and allometry in large individuals. These two species may have undersized brains for small wasps, with reduced cognitive abilities. Here, we induced intraspecific body size variation in genetically identical T. evanescens and N. vitripennis and examined cognitive trade-offs of brain scaling. We compared visual and olfactory memory retention between small and large conspecifics. Results showed that diphasic brain scaling affected memory retention levels in N. vitripennis, whereas isometric brain scaling did not affect memory retention in T. evanescens. The two species may experience different evolutionary pressures that have shaped the cognitive consequences of isometric brain–body size scaling. A possible trade-off of brain isometry in T. evanescens could be present in brain properties other than memory performance. In contrast, it may be more adaptive for N. vitripennis to invest in other aspects of brain performance, at the cost of memory retention