Pheromone research

Most pheromones consist of two or more chemicals which need to be blended at precise proportions to be biologically active. Other compounds identified from the female effluvia or sex gland are related to the pheromone components but show no apparent biological effect.

To determine the sex pheromone in a moth species we follow this procedure;

• First we extracts the glands in the female which we think can produce the pheromone.
  
• Then we expose the male to the extracted odour and check how he reacts.
  
• We expose the male antenna (photos of male antennae) to the extracted odours and if we get a good reaction we continue to the next step.
  
• We inject a female extract into a gas chromatograph, a machine that has the ability to fractionate the injected sample into separate chemicals, which comes out of the machine in second or minute intervals, and are exposed directly to the antenna.
  The insect antenna is still much more sensitive than the most advanced man made machine. By measuring the current in the antenna (EAG) while exposing it to the compounds coming out from the GC we can determine which compounds are biological active.
  These compounds can then be identified using another machine, a gas chromatograph coupled to a mass selective detector (GC-MS). Photos of a gas chromatograph and an antenna being tested
  When you measure over the whole antenna (EAG) you get the signal from 10 000 up to 100 000 nerve cells simultaneously. Many of these cells reacts to food odours, wind, humidity etc. By probing a thin wolfram electrode into the antenna or cutting the tip of a single hair on the antenna we can measure a single receptor. This is called "single cell recording". By testing with synthetic compounds you suspect being pheromone candidates (often short chain acetates, alcohols or aldehydes) you can determine which are active in this insect species.
  
• When we think we have found the pheromone components we make a synthetic blends with different proportions and strength of the components. This is first tested in a wind tunnel; a rubber septum with a pheromone blend is put in the end of the wind tunnel. Then a male is released in the other end, downwind. If the blend is correct the male smells the bait, flies towards it upwind and tries to copulate with the bait ! Photos of a male turnip moth being attracted to a synthetic female in a wind tunnel.

Practical use of pheromones

Generally, however, what is called "mating disruption" is more effective: Synthetic pheromone is released from numerous sources placed in the field; the males are then unable to locate the females and the number of matings and resulting offsprings is reduced. Mating disruption has been particularly successful in the pink bollworm, Pectinophora gossypiella and the Oriental fruit moth, Grapholita molesta . In European vineyards, the European grape moth, Eupoecilia ambiguella , and the grapevine moth, Lobesia botrana , are successfully controlled with pheromone.

But the most common use of pheromones are still for prognosis. By using a few traps on a field the farmer can determine when the population reach the level when it is time to spray insecticides, or, if the levels never reach critical levels, no spraying is necessary.

Text: Peter Valeur & Erling V. Jirle, 1998.
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