Pheromone traps are a useful method of surveying for the presence of adult male insects, including those of oak processionary moth (OPM).

The traps work by containing a lure comprising a synthetic chemical cocktail which mimics the sexual pheromone emitted by the female to attract a mate for breeding.

Deploying traps, especially in the area outside the Buffer Zone and known OPM-affected areas, can be an effective tool for identifying possible outward spread of the pest, enabling prompt action to be taken to combat it. However, pheromone trapping cannot be an effective means of population control.

Adult males are strong flyers and can travel up to 20 kilometres (12 miles) from the nests from which they emerge. Therefore capturing OPM males some distance from known OPM-affected areas does not necessarily mean that the pest has spread to the area of the trap: they might just be moths which flew into the area from elsewhere. However, the greater the number of adult males caught in a single trap or cluster of traps, the greater is the likelihood that they indicate a local breeding population.

By contrast, females are not strong flyers, and are rarely found more than about 500 metres from their original nests.

All oak trees within 200 metres of where OPM is trapped should be inspected for nests as soon as possible after males are found in traps. A second inspection later in the year is also advised – surveying in winter has the benefit of the view of the trees being unobscured by leaves.

Visual surveys for egg masses during the following winter, and for caterpillars (larvae) the next spring, are also recommended in areas where males are trapped. See Section 5 – Surveying trees, and timing of treatment.

The Forestry Commission manages a pheromone trapping programme throughout the Buffer Zone and in the surrounding 10 kilometres (6 miles) or so. This is done from mid-July to early September, when the adult moths are emerging from pupation.

It welcomes offers of trapping sites where it can erect traps, especially outside the known affected areas, and landowners are welcome to undertake their own pheromone trapping. The Forestry Commission welcomes reports of results from private trapping programmes, which can add value to its own.

Contact opm@forestrycommission.gov.uk if you can help in one of these ways. The Forestry Commission can also provide details of trap suppliers.

Types of trap

Two main types of trap are in use.

1. Delta traps. These comprise a 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 captures and retains the moths which are attracted to the lure, which is placed in a small plastic receptacle inside the trap.
2. Plastic funnel traps. These are more expensive, but are more robust and last longer. 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. Funnel traps are available in yellow, clear or green: green traps are preferred in urban situations because they are more discreet.

Although both types of trap will catch OPM, funnel traps catch about six times as many adult males as delta traps, and are therefore likely to be more effective at detecting moths where numbers are low.

Both types of trap are best hung about 10 to 15 metres (35 to 50 feet) high in the canopy of the tree. This can be done either by using a mobile platform (‘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 lower and re-erect the trap when it is inspected. Traps placed lower than about 10 metres catch very few moths and are considered ineffective for OPM monitoring.

OPM prefers oak trees in open, sunny locations, so traps placed in these situations will have a better chance of capturing any males in the area.

Identification

Some other species will enter the traps, but most moths trapped will be OPM. They have a wingspan of 30-32 mm (1.25 inches) and grey fore-wings suffused with light and dark grey markings.

Correctly identifying trapped moths is important, because a number of other species look very similar to OPM. Expert help might therefore be required to correctly identify the species. Contact the Forestry Commission for this help.

Our guide to Monitoring oak processionary moth with pheromone traps has more-detailed guidance on this work.

Next – Other work on oak trees

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Additional resources

• Main OPM resources page