A parasitoid's dilemma between food and host resources: the role of volatiles from nectar-providing marigolds and host-infested plants attracting Aphidius platensis.

The use of nectar-producing companion plants in crops is a well-known strategy of conserving natural enemies in biological control. However, the role of floral volatiles in attracting parasitoids and effects on host location via herbivore-induced plant volatiles is poorly known. Here, we examined the role of floral volatiles from marigold (Tagetes erecta), alone or in combination with volatiles from sweet pepper plant (Capsicum annuum), in recruiting Aphidius platensis, an important parasitoid of the green peach aphid Myzus persicae. We also investigated whether marigold floral volatiles are more attractive to the parasitoid than those emitted by sweet pepper plants infested by M. persicae. Olfactometry assays indicated that floral volatiles attracted A. platensis to the marigold plant and are more attractive than sweet pepper plant volatiles. However, volatiles emitted by aphid-infested sweet pepper were as attractive to the parasitoid as those of uninfested or aphid-infested blooming marigold. The composition of volatile blends released by uninfested and aphid-infested plants differed between both blooming marigold and sweet pepper, but the parasitoid did not discriminate aphid-infested from uninfested blooming marigold. Volatile released from blooming marigold and sweet pepper shared several compounds, but that of blooming marigold contained larger amounts of fatty-acid derivatives and a different composition of terpenes. We discuss the potential implications of the aphid parasitoid attraction in a diversified crop management strategy.

Silicon Supplementation of Maize Impacts Fall Armyworm Colonization and Increases Predator Attraction.

Supplementation with Silicon (Si) is well-known for increasing resistance of grasses to insect herbivores. Although the exact underlying mechanism remains unknown, Si accumulation interacts with the jasmonic acid-signalling pathway, which modulates herbivore-induced plant defences. We examined whether Si supplementation alters direct and induced indirect defences in maize plants in ways that deter the initial infestation by the fall armyworm Spodoptera frugiperda (JE Smith). We assessed the herbivore's oviposition preference, neonate and third-instar larval performance as well as the recruitment of a predator of young larvae, the flower bug Orius insidiosus (Say), by herbivore-induced plant volatiles (HIPVs). In choice tests, S. frugiperda deposited about two times more eggs on -Si than on +Si maize. The mortality of neonate S. frugiperda larvae was about sixfold higher in +Si compared to -Si plants, even though they consumed similar leaf area on both treatments. Although there were no mortality differences, Si supplementation also impacted third-instar larvae that gained about twofold less weight than those fed on -Si maize. In olfactometer assays, O. insidiosus was not attracted to volatiles of uninfested maize plants with or without Si supplementation, but it was attracted to those emitted by fall armyworm-infested plants, irrespective of whether plants received Si supplementation. However, when the flower bug could choose between the volatiles released from -Si and +Si fall armyworm-infested plants, it preferentially oriented to +Si fall armyworm-infested plant. Our results show that Si supplementation in maize may deter fall armyworm colonization because of greater direct defences and attractiveness of HIPVs to the flower bug.

Role of methyl salicylate on oviposition deterrence in Arabidopsis thaliana.

Plants attacked by herbivores have evolved different strategies that fend off their enemies. Insect eggs deposited on leaves have been shown to inhibit further oviposition through visual or chemical cues. In some plant species, the volatile methyl salicylate (MeSA) repels gravid insects but whether it plays the same role in the model species Arabidopsis thaliana is currently unknown. Here we showed that Pieris brassicae butterflies laid fewer eggs on Arabidopsis plants that were next to a MeSA dispenser or on plants with constitutively high MeSA emission than on control plants. Surprisingly, the MeSA biosynthesis mutant bsmt1-1 treated with egg extract was still repellent to butterflies when compared to untreated bsmt1-1. Moreover, the expression of BSMT1 was not enhanced by egg extract treatment but was induced by herbivory. Altogether, these results provide evidence that the deterring activity of eggs on gravid butterflies is independent of MeSA emission in Arabidopsis, and that MeSA might rather serve as a deterrent in plants challenged by feeding larvae.

Weather forecasting by insects: modified sexual behaviour in response to atmospheric pressure changes.

Prevailing abiotic conditions may positively or negatively impact insects at both the individual and population levels. For example while moderate rainfall and wind velocity may provide conditions that favour development, as well as movement within and between habitats, high winds and heavy rains can significantly decrease life expectancy. There is some evidence that insects adjust their behaviours associated with flight, mating and foraging in response to changes in barometric pressure. We studied changes in different mating behaviours of three taxonomically unrelated insects, the curcurbit beetle, Diabrotica speciosa (Coleoptera), the true armyworm moth, Pseudaletia unipuncta (Lepidoptera) and the potato aphid, Macrosiphum euphorbiae (Hemiptera), when subjected to natural or experimentally manipulated changes in atmospheric pressure. In response to decreasing barometric pressure, male beetles exhibited decreased locomotory activity in a Y-tube olfactometer with female pheromone extracts. However, when placed in close proximity to females, they exhibited reduced courtship sequences and the precopulatory period. Under the same situations, females of the true armyworm and the potato aphid exhibited significantly reduced calling behaviour. Neither the movement of male beetles nor the calling of armyworm females differed between stable and increasing atmospheric pressure conditions. However, in the case of the armyworm there was a significant decrease in the incidence of mating under rising atmospheric conditions, suggesting an effect on male behaviour. When atmospheric pressure rose, very few M. euphorbiae oviparae called. This was similar to the situation observed under decreasing conditions, and consequently very little mating was observed in this species except under stable conditions. All species exhibited behavioural modifications, but there were interspecific differences related to size-related flight ability and the diel periodicity of mating activity. We postulate that the observed behavioral modifications, especially under decreasing barometric pressure would reduce the probability of injury or death under adverse weather conditions.

The impact of coffee and pasture agriculture on predatory and omnivorous leaf-litter ants.

Ants are known to function as reliable biological indicators for habitat impact assessment. They play a wide range of ecological roles depending on their feeding and nesting habits. By clustering ants in guilds, it is possible both to assess how agriculture and forest fragmentation can disturb ant communities and to predict the ecological impacts due to losses of a specific guild. This study aimed at determining the impact of non-shaded coffee and pasture agriculture on predatory and omnivorous guilds of leaf-litter ants of Atlantic Forest fragments in Minas Gerais, Brazil. Both coffee and pasture agriculture influenced leaf-litter ant community, although coffee was more disruptive than pasture. Coffee agriculture not only disturbed the diversity of predatory ants, but also negatively affected the number of predatory and omnivorous ants when compared to forest fragments. In contrast, pasture agriculture only disrupted the abundance of predatory ants. Fragment edges skirting crops were negatively affected in terms of leaf-litter ant abundance, but not diversity. Cluster analysis showed that forest fragments were similar irrespective of the cultivation, but the borders were similar to the crop. The study assessed agriculture impact by surveying ant guilds, and revealed that the predatory guild is more susceptible than omnivorous ants.