Aphid and Plant Volatiles Induce Oviposition in an Aphidophagous Hoverfly

Episyrphus balteatus DeGeer (Diptera, Syrphidae) is an abundant and efficient aphid-specific predator. We tested the electroantennographic (EAG) response of this syrphid fly to the common aphid alarm pheromone, (E)-β-farnesene (EβF), and to several plant volatiles, including terpenoids (mono- and sesquiterpenes) and green leaf volatiles (C6 and C9 alcohols and aldehydes). Monoterpenes evoked significant EAG responses, whereas sesquiterpenes were inactive, except for the aphid alarm pheromone (EβF). The most pronounced antennal responses were elicited by six and nine carbon green leaf alcohols and aldehydes [i.e., (Z)-3-hexenol, (E)-2-hexenol, (E)-2-hexenal, and hexanal]. To investigate the behavioral activity of some of these EAG-active compounds, E. balteatus females were exposed to R-(+)-limonene (monoterpene), (Z)-3-hexenol (green leaf alcohol), and EβF (sesquiterpene, common aphid alarm pheromone). A single E. balteatus gravid female was exposed for 10 min to an aphid-free Vicia faba plant that was co-located with a semiochemical dispenser. Without additional semiochemical, hoverfly females were not attracted to this plant, and no oviposition was observed. The monoterpene R-(+)-limonene did not affect the females’ foraging behavior, whereas (Z)-3-hexenol and EβF increased the time of flight and acceptance of the host plant. Moreover, these two chemicals induced oviposition on aphid-free plants, suggesting that selection of the oviposition site by predatory hoverflies relies on the perception of a volatile blend composed of prey pheromone and typical plant green leaf volatiles

Transgenerational effects and the cost of ant tending in aphids

In mutualistic interactions, partners obtain a net benefit, but there may also be costs associated with the provision of benefits for a partner. The question of whether aphids suffer such costs when attended by ants has been raised in previous work. Transgenerational effects, where offspring phenotypes are adjusted based on maternal influences, could be important in the mutualistic interaction between aphids and ants, in particular because aphids have telescoping generations where two offspring generations can be present in a mature aphid. We investigated the immediate and transgenerational influence of ant tending on aphid life history and reproduction by observing the interaction between the facultative myrmecophile Aphis fabae and the ant Lasius niger over 13 aphid generations in the laboratory. We found that the effect of ant tending changes dynamically over successive aphid generations after the start of tending. Initially, total aphid colony weight, aphid adult weight and aphid embryo size decreased compared with untended aphids, consistent with a cost of ant association, but these differences disappeared within four generations of interaction. We conclude that transgenerational effects are important in the aphid–ant interactions and that the costs for aphids of being tended by ants can vary over generations

Aphid parasitoid responses to semiochemicals - Genetic, conditioned or learnt?

Parasitoid foraging behaviour is known to be influenced by interactions of genetic, physiological, environmental and experiential factors. Although the role of genetics, learning and conditioning in determining responses to foraging cues has been studied in lepidopteran parasitoids, aphid parasitoids have been less intensively researched. Using the tritrophic system, Vicia faba - Acyrthosiphon pisum - Aphidius ervi, evidence for the role of genetics and learning in parasitoid foraging is presented, and the difficulty of differentiating between genetic responses and those conditioned during parasitoid development is discussed. Aphidius ervi responds to aphid sex pheromones both in the field and in the laboratory. Since laboratory reared individuals have never experienced sexual aphids, the response must be genetic as it cannot have been conditioned during development. An example of a response conditioned during development is the variable response of A. rhopalosiphi to different wheat cultivars depending upon host feeding. Aphid parasitoids also are adept at learning as shown by their responses to plant-derived cues which are learnt as Conditioned Stimuli (CS). Host products such as honeydew, as well as the host itself, can act as the Unconditioned Stimulus (US) in the learning process. Aphidius ervi offers a good model for investigating the role of these factors in parasitoid foraging behaviour. Finally, the value of such research for biological control programmes involving aphid parasitoids is discussed