Pheromone traps are a useful method of surveying for the presence of adult male insects, including OPM. Traps work by containing a lure made up of a synthetic chemical cocktail which mimicks the sexual pheromone produced by the female to attract a mate for breeding. They also incorporate a means of capturing specimens which have been attracted to them.
Deploying traps, especially in the area outside the Control Zone, is an effective tool for identifying potential outward spread of the pest, enabling prompt action to be taken to combat it.
Detection of adult males, which are strong flyers and can travel several kilometres from the nest from which they emerged, does not necessarily mean that the infestation has spread and a new breeding population is being established.
By contrast, the female is not a strong flyer, and is rarely found more than about 500 metres from her original nest. However, carrying out visual surveys for egg masses over the following winter, and for larvae the next spring, is recommended in areas where males are trapped. All oak trees within 200 metres of where OPM is trapped should be inspected as soon as possible in case there are nests present. A second inspection later in the year is also considered wise.
The Forestry Commission manages a pheromone trapping programme, throughout the Control Zones and in the surrounding 10 kilometres or so. However, if you want to put traps up in your own trees there is nothing to prevent you from doing so.
Two main types of trap are in use:
- the Delta trap, which is made from waxed cardboard or corrugated white plastic sheet folded length-ways into a triangular shape and suspended from a tree branch by wire or string. A sticky cardboard insert is slid into the trap to capture and retain the moths which have been attracted to the lure, which is placed in a small plastic receptacle inside the trap; and
- plastic funnel traps, which are more robust and therefore last longer, but are more expensive. There are many designs, but one of the most common consists of a plastic 'bucket' with a lid, under which the lure is placed. Male moths attracted to the lure fall down through a funnel into the body of the trap. They are available in yellow, clear or green, but the green is preferred in urban situations because they are more discreet.
Although both types of trap will catch OPM, field trials show that the funnel trap catches six times as many adult males as delta traps, and are therefore likely to be more effective at detecting low numbers of moths.
Both types of trap are best hung about 10 to 15 metres high in the canopy of the tree. This can be done either by using a cherry picker, or by climbing the tree, or by throwing a weighted line over a convenient branch, which can then be tied off and used to raise or lower the trap when it is inspected. Traps placed lower, about 5 metres off the ground, have caught very few moths, and are considered ineffective for OPM.
OPM seems to prefer oak trees growing in open, sunny locations, and traps placed in these situations will have a better chance of capturing any males in the area.
Most of the moths trapped will be OPM. These have a wingspan of 30-32mm and grey fore-wings suffused with lighter and darker grey markings.
Correctly identifying trapped moths is important, because sometimes other species are trapped, and they can look very similar to OPM. Expert help might therefore be required to correctly identify the species.