Aphid Wing Induction and Ecological Costs of Alarm Pheromone Emission under Field Conditions

The pea aphid, Acyrthosiphon pisum Harris, (Homoptera: Aphididae) releases the volatile sesquiterpene (E)-β-farnesene (EBF) when attacked by a predator, triggering escape responses in the aphid colony. Recently, it was shown that this alarm pheromone also mediates the production of the winged dispersal morph under laboratory conditions. The present work tested the wing-inducing effect of EBF under field conditions. Aphid colonies were exposed to two treatments (control and EBF) and tested in two different environmental conditions (field and laboratory). As in previous experiments aphids produced higher proportion of winged morphs among their offspring when exposed to EBF in the laboratory but even under field conditions the proportion of winged offspring was higher after EBF application (6.84±0.98%) compared to the hexane control (1.54±0.25%). In the field, the proportion of adult aphids found on the plant at the end of the experiment was lower in the EBF treatment (58.1±5.5%) than in the control (66.9±4.6%), in contrast to the climate chamber test where the numbers of adult aphids found on the plant at the end of the experiment were, in both treatments, similar to the numbers put on the plant initially. Our results show that the role of EBF in aphid wing induction is also apparent under field conditions and they may indicate a potential cost of EBF emission. They also emphasize the importance of investigating the ecological role of induced defences under field conditions

Conserved Odorant-Binding Proteins from Aphids and Eavesdropping Predators

Background: The sesquiterpene (E)-ß-farnesene is the main component of the alarm pheromone system of various aphid species studied to date, including the English grain aphid, Sitobion avenae. Aphid natural enemies, such as the marmalade hoverfly Episyrphus balteatus and the multicolored Asian lady beetle Harmonia axyridis, eavesdrop on aphid chemical communication and utilize (E)-ß-farnesene as a kairomone to localize their immediate or offspring preys. These aphidpredator systems are important models to study how the olfactory systems of distant insect taxa process the same chemical signal. We postulated that odorant-binding proteins (OBPs), which are highly expressed in insect olfactory tissues and involved in the first step of odorant reception, have conserved regions involved in binding (E)-ß-farnesene. Methodology: We cloned OBP genes from the English grain aphid and two major predators of this aphid species. We then expressed these proteins and compare their binding affinities to the alarm pheromone/kairomone. By using a fluorescence reporter, we tested binding of (E)-ß-farnesene and other electrophysiologically and behaviorally active compounds, including a green leaf volatile attractant. Conclusion: We found that OBPs from disparate taxa of aphids and their predators are highly conserved proteins, with apparently no orthologue genes in other insect species. Properly folded, recombinant proteins from the English grain aphid, SaveOBP3, and the marmalade hoverfly, EbalOBP3, specifically bind (E)-ß-farnesene with apparent high affinity. For the firs

Social enviroment influences aphid production of alarm pheromone

In most aphid species, the volatile sesquiterpene (E)-β-farnesene (Eβf) is released as an alarm pheromone in response to predation and is also emitted continuously at low levels. Some aphid predators use Eβf as a foraging cue, suggesting that the benefits to aphids of signaling via Eβf must be weighed against the cost of increasing apparency to natural enemies. To determine whether aphids vary Eβf production in response to features of their social environment, we compared the production of Eβf by Acyrthosiphon pisum (Harris) individuals reared in isolation with that of individuals reared among conspecifics or individuals of a different aphid species, Myzus persicae. Production of Eβf by A. pisum reared in isolation was significantly lower than that of aphids reared among conspecifics or among M. persicae individuals. When we reared A. pisum individuals in isolation but exposed them to odors from an aphid colony, Eβf production was similar to that of aphids reared among conspecifics, suggesting that aphids use a volatile cue to assess their social environment and regulate their production of alarm pheromone. It is likely that this cue is Eβf itself, the only volatile compound previously found in headspace collections of A. pisum colonies. Finally, we examined the attraction of a predatory hoverfly, which uses Eβf as a foraging cue, to groups of aphids reared in isolation or among conspecifics and found that groups comprising individuals reared in isolation were significantly less attractive to the predator, suggesting that the observed variation in Eβf production may be ecologically relevant. Copyright 2009, Oxford University Press.

Potato virus Y: Control, Management and Seed Certification Programmes

The management of Potato virus Y (PVY) in potato crops poses a continual challenge due to the non-persistent mode of transmission of the virus and the propagation of seed potato tubers over several generations in the field. While PVY-resistant cultivars remain the most efficient way to protect potato crops against PVY, a vast majority of cultivars grown do not display significant resistance to PVY. Due to the short time period for PVY transmission by non-colonising aphid vectors, efficient control of PVY relies on preventing aphids landing on a crop and on adopting precautionary measures by ensuring that crops are grown in areas of low aphid and low virus pressure and limiting field generation. Prophylactic measures such as roguing and early haulm destruction limit PVY spread but are not efficient alone. Among all existing control methods, spraying potato crops with mineral oils can offer significant protection against PVY spread, but their efficacy do vary in field conditions. The combination of several control methods such as mineral oil treatments, crop borders, intercropping, straw mulching or insecticide treatments can increase protection. These emphasise the importance of controlling virus through appropriate monitoring methods and crop management enforced by seed certification schemes through the use of ‘clean’ input seed and, when possible, the segregation of seed and ware crops to minimise the risk of virus transmission. This chapter presents and discusses the most widely used techniques of control and management of PVY, their effectiveness and their mode of action. This chapter also presents the history, objectives and principles of seed potato certification schemes and their role in minimising the spread of viruses within potato crops worldwide