Aphid sex pheromones to enhance parasitoid efficiency

Summary

Sector:
Cereals & Oilseeds
Project code:
PR155
Date:
01 October 1994 - 30 September 1997
Funders:
AHDB Cereals & Oilseeds.
AHDB sector cost:
£44,906 From HGCA (Project No. 0049/1/93)
Project leader:
W Powell and T Glinwood IACR-Rothamsted, Harpenden

Downloads

project_report_155

About this project

Abstract

Cultures of the major aphid pests of cereals, legumes and brassicas were established, together with cultures of seven aphid parasitoid species which attack these aphids in the UK. The parasitoids chosen included specialist species relevant to the three crop types as well as generalist species, including species that had previously been caught in field traps baited with aphid sex pheromones. These cultures were used to provide insects for laboratory studies of parasitoid responses to synthetic aphid sex pheromones.

Prior to the start of this project, only generalist parasitoids of the genus Praonhad been caught in aphid sex pheromone field traps but electrophysiological studies had shown that other, more specialised species had antennal receptors that detected the pheromones. Therefore, wind tunnel studies were done to determine whether or not these other parasitoids also responded behaviourally to the pheromones. The cereal aphid specialist Aphidius rhopalosiphi and the pea aphid specialists Aphidius ervi and Aphidius eadyi, as well as the generalists Praon volucre, Praon myzophgum and Ephedrus plagiator were all strongly attracted to synthetic aphid sex pheromones in the wind tunnel. The brassica aphid specialist Diaeretiella rapae did not fly well in the wind tunnel due to its small size but this species was strongly attracted to pheromone traps placed in oilseed rape crops during the summer. Aphid sex pheromones, therefore, appear to act as foraging cues for a wide range of aphid parasitoids including the important species associated with cereal, pea and brassica aphids.

Laboratory experiments also established that exposure of female parasitoids to synthetic aphid sex pheromones did not affect their searching behaviour on plants or their willingness to attack any suitable aphid hosts that they encountered. On the contrary, the results indicated that the sex pheromones act as searching stimulants that would potentially retain parasitoids in crops when aphids were scarce, as is usually the case at the beginning of a crop infestation.

Aphid parasitoids are also strongly attracted to volatile plant chemicals that are emitted by plants in response to aphid feeding damage. Wind tunnel studies demonstrated that the presence of aphid sex pheromones did not interfere with this response but rather increased the proportion of parasitoids responding to these plant volatile foraging cues. Therefore, the presence of aphid sex pheromone lures is likely to enhance parasitoid attraction into aphid-infested areas.

An extensive series of field experiments, using aphid populations on potted 'trap' plants, clearly demonstrated that, when deployed at the appropriate time in the late summer/autumn, the presence of pheromone lures can greatly increase rates of parasitization by both specialist and generalist parasitoid species. The number of aphids that were parasitized on trap plants in the autumn indicated that there was still enough parasitoid activity around the margins of harvested fields to generate significant overwintering populations if appropriate aphid hosts were available. It was particularly encouraging that parasitization by the specialist Aphidius species was greatly increased in the vicinity of pheromone lures, even though these parasitoids are not caught in pheromone traps. This indicates a difference in foraging behaviour compared with the generalist Praon species and the brassica specialist D. rapae, which tend to fly directly to the source of the pheromone whilst the specialist Aphidius species appear to forage intensively in the vicinity of the pheromone source without flying directly to it. This latter behaviour is a positive advantage for our proposed parasitoid manipulation strategy. In the case of the cereal aphid specialist A. rhopalosiphi, the timing of pheromone lure deployment is important because this species enters a summer diapause or resting period in August and early September.

Field plot trials in winter wheat crops in 1996 and 1997, designed to investigate the potential of aphid sex pheromone lures for attracting parasitoids back into crops in spring/early summer, gave inconclusive results. Despite poor aphid populations and consequent low parasitoid numbers, the 1996 results were encouraging in that more aphids were parasitized and parasitization was better synchronized with aphid invasion when phoromone lures were present in the centre of plots. Furthermore, the effect of the pheromone lures was evident throughout the 6 m x 6 m plots and not just around the lures. Disappointingly, these results could not be repeated in the following year. Similar field trials in legume and brassica crops had to be abandoned due to lack of insects in these crops in 1996 and 1997.

The potential for parasitoid manipulation using synthetic aphid sex pheromones was greatly supported by the results of the laboratory and field trap plant work. However, the final testing of the manipulation strategy in the field was considerably hindered by the climatic conditions prevailing during the study. The combination of the hot dry summer of 1995 followed by the late cold spring of 1996 resulted in very late and poor colonisation of crops by aphids which in turn led to low natural parasitoid numbers, preventing robust field experimentation. The conditions also hindered the establishment of experimental plots designed to test different vegetation mixes as potential field margin habitats for overwintering parasitoids. Consequently, the effectiveness of the manipulation strategy as an aphid control method remains to be tested in a season with sufficiently high aphid infestations. Based on our experimental results, this should involve the deployment of nepetalactone lures in field margins in September/early October. Field trials should be done in field margins already under development, such as wildlife strips, conservation headlands, grassland marginal strips, set aside strips to determine the potential of these different management options for providing the appropriate field margin conditions for different crop types.

×