Oak processionary moth: luring the pest with pheromones

CUSTOM-DESIGNED traps loaded with insect pheromone lures are proving a useful tool for pest monitoring of oak processionary moth (OPM). Pheromone traps are routinely deployed in late summer/early autumn to detect and quantify the presence of adult male moths. Adult moths are on the wing in late July through to September, after emerging from the pupa stage and exiting the OPM nest.

The pheromone lure contains a mixture of synthetic chemicals that exactly matches and mimics the natural sex pheromone emitted by the adult female OPM to attract a mate for breeding. The constituent chemicals are intrinsically volatile and emitted as a plume into the atmosphere. The Forestry Commission (FC) has been researching and using pheromone traps as a pest-monitoring tool for OPM for around 10 years. The Funnel trap has proved to be more effective and efficient than the Delta trap.

With respect to OPM status, the United Kingdom is divided into three zones: core, control and protected. The core zone, covering the west side of London and the northernmost part of Surrey, does not legally require stakeholders to control OPM. It is surrounded by a control zone, which functions as a buffer where eradication remains the aim, to prevent or minimise outward spread of OPM from the core. By far the largest part of the UK is in the protected zone, where plant health authorities are required to prevent incursions by OPM or, if they do occur, take action to eradicate them.

Deployment of traps within the control zone can be an effective tool in identifying direction and dimension of pest movement from core areas of infestation, in this case London and some immediate areas of adjoining Home Counties.

However, adult male moths are strong flyers with the capacity to travel up to 20 km from nests where they emerged from the pupal stage. As such, the trapping and capture of adult male OPM in pheromone traps some distance from known OPM-affected areas does not necessarily mean there is a breeding population in the local area. Just one or two moths may have flown into the area from a good distance away. However, increasing numbers of adult male OPM secured in a single trap or a cluster of traps points to a higher probability they have come from a local breeding population.

A bucket-type plastic funnel trap.

If adult male OPM is found, then all oak trees within a 200 m radius of the trap are inspected for nests as soon as possible, with a second follow-up inspection strongly advised, as well as visual surveys for egg masses during winter. English oak trees may hold onto a substantial proportion of leaves well into December. Thus, later inspections, which are increasingly unobscured by foliage, should prove more fruitful if nests and egg masses are present. Inspections are resumed in spring, this time for hatching larvae feeding on buds and emerging leaves.

Pheromone traps have been successfully deployed for a good number of years to monitor the spread of OPM, but this year’s operation appears to have gone pear-shaped because of faulty pheromone lures supplied for use in the traps. A few months ago, the FC discovered that pheromone lures deployed in late summer were defective. It is unclear whether the defect was chemical in nature and due to non-specificity (mismatch) with the natural female pheromone of OPM or physical and related to vapour pressure and therefore volatility of the formulation.

READ MORE: Potential effects of OPM insecticide spraying on native insects and birds

Be that as it may, this could not have happened at a worse time, because in 2019 plant health authorities intercepted at least 70 OPM outbreaks across the protected zone. This had resulted from large consignments of oak tree planting imported from Europe and subsequently dispatched for planting around the UK in autumn/winter 2018/2019. It would appear the oak tree planting material was not checked sufficiently rigorously (if at all) for OPM infestation, either in the European country of export or upon arrival in the UK. Interceptions were made throughout the summer and into autumn 2019 with affected trees and OPM insect pests destroyed at all sites.  

That all outbreaks may not have been intercepted must be of concern. However, provided all documentation relating to the importation of oak tree planting material and its dispatch for use around the UK was intact, a simple trace forward exercise should have identified all at-risk plantings. Perhaps more worrying is that some outbreaks were discovered too late in the day (late July onwards), when adult moths had already emerged from the pupa stage, exited the nest and dispersed to mate and oviposit egg masses that would hatch in spring 2020. Deployment of pheromone traps in late summer/autumn 2019 would have assisted in this respect.

However, it would appear that some infestations may have been missed or intercepted too late, because the FC says it has responded to OPM at five sites within the protected zone (Sussex, Oxfordshire, Bedfordshire and Cheshire) during June and July 2020. Trees in the affected areas were sprayed with insecticide and local surveillance enhanced, but pheromone trapping in late summer/autumn 2020 would have obviously provided an added safeguard. And with reduced arb and forestry activity on the ground during late March, April and May at least, due to restrictions on operational activity caused by COVID-19, then it is reasonable to assume that there has been under-reporting of OPM in 2020.

Delta trap.

The FC says it sourced new lures to bait the traps, but has asked all professional and amateur entomologists to be vigilant and familiarise themselves with OPM. It has expressed particular interest in suspected OPM findings in the following areas where OPM was intercepted in summer/autumn 2019. The majority of these areas are in the protected zone, although some are in the control zone (according to the FC’s map of OPM zones, Hertfordshire, Middlesex and Surrey are entirely within the control zone). The areas are:

• England: Cheshire, Cambridgeshire, County Durham, Devon, Dorset, Essex, Gloucestershire, Greater Manchester, Hampshire, Hertfordshire, Kent, Lancashire, Leicestershire, Lincolnshire, London, Merseyside, Middlesex, Nottinghamshire, Oxfordshire, Southampton, Staffordshire, Suffolk, Surrey, Sussex, Warwickshire, West Midlands, Wiltshire and Yorkshire

• Scotland: Angus, Edinburgh, Fife, Glasgow and Inverness

• Wales: Glamorgan and Flintshire.

So far the OPM pheromone has only been used as a pest-monitoring tool in the UK. However, pheromone technology has been in widespread use against scores of insect pests in agriculture and horticulture for at least 50 years, and used for pest control as well as pest monitoring. The pheromone methodology used for insect pest control includes lure-and-kill, mass trapping and mating disruption techniques. Such applications of pheromones to control insect pests may well become part of a sustainable pest management programme for OPM in the future, when the collateral ecological fallout and damage caused by ever-widening and intensifying insecticide spraying is understood and accepted.

Author’s note:

There are two main types of pheromone-loaded trap used for securing adult male OPM.

Delta traps: comprising a waxed-cardboard or corrugated, white plastic sheet folded lengthways into a prism or tent-like shape and suspended from a tree branch by wire or string. A sticky cardboard insert acts as a floor to capture and retain moths attracted to the pheromone lure which is placed in a small plastic receptacle inside the trap.

Funnel traps: more costly, but also more resilient and longer lasting. Available in various designs including the most common ‘bucket trap’ comprising a plastic ‘bucket’ with a lid, under which the pheromone lure is located. Male moths attracted by the pheromone lure fall down through a funnel and into the body of the trap.

Both Delta and Funnel traps are available in a variety of colours and colour combinations. Choice is ultimately down to what (if any) is most attractive to the insect species in question and environmental considerations such discreetness in urban situations.

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