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Feature

Fighting a Fiend with a Pheromone

Researchers are looking for specific wasp-deterring scents that will make picnicking easier and stop wasps disrupting our native ecosystems.

Dr Tony Woolhouse

Wasp spread and numbers have become of increasing concern in recent years, not just to wasp-shy humans, but also to the country's ecosystems as the insects compete with native animals for food resources. A team that comprises ecologists, entomologists and chemists is tackling the wasp problem head-on by targeting the identification of a chemical or a cocktail of them that will find a use as a long-life lure that could be used in traps or bait stations.

Scientists from Landcare Research have long been interested in control measures for both the common and German wasp species which are known to alter natural bush and forest ecosystems to the point where native species of plants and animals, especially birds, become endangered.

Bio-Control Insufficient

Much effort has been devoted to the conceptually neat search for a biological vector -- a wasp-specific disease or insect that would disrupt the breeding and subsequent proliferation of infestations of wasps. One possible candidate is a parasitic wasp (Sphecophaga vesparum), which preys upon the larvae of the common and German wasps. However, the extent of the takeover by the parasite when established at a site has been found to be limited (with only a 10% reduction predicted). Consequently, there remains an urgent need for an alternative strategy for the control of populations of these pests.

Strategies based upon the widespread distribution of poisons have long been known to offer a means of wasp population control. These, however, have never been regarded as cost-effective (in spite of the fact that it remains difficult to make such a quantitative evaluation of the nature of the problem) or even species-specific. There's also the undesirable likelihood that other non-pest species may take the toxic baits.

Concurrent with the evaluation of a biological means of control, the Landcare Research group also had an interest in honing a chemical means of control by investigating ways of enhancing the effectiveness of toxic baits with products known to be attractive to wasps. The focus of this research has been principally upon the use of proprietary food-based products. Several highly effective materials were identified, such as sardine-based cat food, fruit flavours and the like. Even these are not attractive uniquely to wasps, and show a diminished effectiveness as a result of prolonged use in the field.

As a result of an on-going interest in using pheromones in mammalian pest control, chemists from Industrial Research (myself and colleague Rod Weston) have joined forces with the Landcare Research group (including Dave Morgan and Eric Spurr) to see whether certain chemicals might be attractive to wasps. This would go some way towards overcoming problems due to the deterioration of the initially promising baited lures.

Pheromones Appealing

The use of an odorous chemical compound derived either from an attractive food product or from the insect itself (a pheromone) has an obvious appeal, as it lends itself to delivery in a more stable sustained release device. This factor is of considerable importance when considering the labour-intensive frequency with which fresh baited lures have to be distributed and redistributed when wasps occur at low density early in the season.

Pheromones are the chemical signals that insects (and other animals) synthesise in minute quantities and emit with the express purpose of communicating with others of the same species. They have been used selectively overseas and in New Zealand in devices placed strategically in orchards, for example, to monitor the increasing populations of moths prior to an insecticide spray operation.

Such an application has only come about as a result of the massive research effort made by entomologists and organic chemists who were able to identify, characterise and ultimately synthesise the molecule or molecules responsible for a range of behavioural responses. Synthetic versions of the mating pheromones emitted by females of a species are now used in traps to lure males of the same species. Were these to be used on sufficient a scale in problem areas at the appropriate time, then there is the possibility that a high degree of mating disruption could also be achieved.

The principal focus of this collaborative research effort has been to identify a potentially synthesisable attractant from any source. To date this has centred upon the identification of the chemical component(s) responsible for attracting wasps to field traps. Some food-based products, such as honeys, fish oils, fish meals, sardine cat food, horsemeat, fruit flavours, are known to be highly attractive to wasps. These have been chemically fractionated by a range of solvent extraction and odour-sampling techniques, and the various constituents of the original material re-presented to wasps to find out whether or not they are more attractive than the original material.

Using this technique should ultimately permit the identification of the molecules that are specifically responsible for attracting the wasps to the baits. The aim then would be to devise an expedient synthesis of the molecules before looking to incorporate them into slow-release devices.

Of the many fractions subjected to field bioassay, none approached the levels of potency of those derived from the honeys and the sardine cat food. In pursuit of the active components of the sardine cat food, the volatile chemicals in the odour were isolated and identified by combined gas chromatography-mass spectrometry (GC-MS).

The principal components were found to be penta- and heptadecane and an uncharacterised heptadecene in addition to a suite of n-alkanes and alkenes. Of these compounds, only pentadecane has been field-trialled as an attractant, but its effects were lack-lustre, a phenomenon that is not altogether surprising. It might well be that one or other components of this odour is in fact the attractive principle.

However, it is interesting to note that compounds such as these have been identified as pheromones of certain insects, in particular the house-fly, which is a natural prey of wasps.

In an effort to find a chemical or pheromone uniquely associated with the wasps themselves, we focused our attention upon their venom. Biologists believe that the venom attracts wasps when implanted in a potential prey.

Dissected venom sacs from each of the wasp species were steeped in solvent to extract the contents, and these were then analysed by GC-MS to reveal the presence of only a few volatile compounds in each case.

For the German wasp, the major component was the spiroketal 1 and for the common wasp, spiroketal 2. We have now synthesised large quantities -- around five grams -- of each of these putative pheromones in preparation for bioassays with wasps.

Wiring Wasps

Rather than submitting these synthetic chemicals to field assay with wasps, they will be evaluated within the laboratory. Wasp antennae will be electrically wired to a recording amplifier with measurements taken of the response to odours and chemicals that are wafted over them. The materials that elicit excitative responses from the wasp will be further evaluated in wind-tunnel experiments, where the wasps are able to choose which scent to follow, to determine which, if any, is actually attractive to them.

A more exciting project is now planned that will enable us to get an accurate handle on just which component of a mixture is responsible for attraction. In a new collaborative research effort with scientists from HortResearch, we plan to couple a gas chromatograph to the electrodes implanted in the wasps antennae and to divert the column effluent across the antennae. This technique will permit the identification of actual components of a mixture (cat food odour, for example) that are responsible for an excitatory response.

Following a parallel analysis of the material by GC-MS, the compounds can then be structurally identified in preparation for synthesis and further trialling. The venom sac, and other wasp secretions, can be examined in the same manner thereby circumventing the need to make informed (and often erroneous) decisions about which components might or might not show activity.

It is hoped that the outcome of these trials will provide an effective, long-lasting bait that will give a us a good chance of controlling wasp infestation.

Dr Tony Woolhouse is team manager of the organic chemistry team within Industrial Research Ltd.