Stopping a runaway train (of caterpillars): how do we put the brakes on Oak Processionary Moth invasion of the railway?

“There are caterpillars on the line?” As a potential cause of expensive and politically sensitive train delay, this made my eyebrows twitch nervously, like surprised lepidopteran larvae.

“Yes, they’re a potential hazard to lineside engineering teams and the general public: the Forestry Commission says get rid”, foamed the asset engineer, hurling a pile of Statutory Plant Health Notices at me as if the antisocial grubs were on the pages and biting his fingers.

So began my expedition in summer 2015 into the wonderful, and mildly perilous, world of the Oak Processionary Moth. It’s been a fascinating journey, and struggle, in which the railway has so far played a humble but honourable part.


Hungry, hairy and hazardous
Oak Processionary Moth (Thaumetopoea processionea) is a significant pest in continental Europe and a notable defoliator of oak trees. The caterpillars live gregariously, and are named after a particular behavioural trait - moving in large processions, head to tail, to feed at the crowns of host oaks.

OPM caterpillars live gregariously ?Mike O’Connor


In Europe, very dense populations, comprising up to 100,000 individuals, can congregate on a single oak and the larvae are able to defoliate many trees over large areas. During outbreaks, the caterpillars also pose a particular threat to human and domestic animal health as a result of the huge quantity of urticating hairs that the more mature individuals release into the environment. These hairs can disperse up to 500m and cause skin reactions and potentially serious respiratory problems because of the constituent protein after which the species is named, thaumetopoein .

Oak defoliation and skin reaction caused by OPM hairs ?Henry Kuppen/ Forestry Commission


Oversexed, overeating and over here
Oak Processionary Moth (OPM) is not a native species to Britain. It was accidently introduced to two areas of west London in 2006, on oak trees imported from Europe which bore a quantity of undetected eggs. From these sites, it has reproduced rapidly and become established in a number of core areas.

During winter months, female moths lay several hundred eggs in batches. Eggs hatch from March onwards and the larvae pass through six stages (instars) during their development, becoming progressively larger from one stage to the next.

The public health potential in London boroughs, as heavily urbanised and populated areas, prompted an urgent control response with aim to eradicate OPM. However, extensive survey work by the Forestry Commission demonstrated that OPM instead greatly increased and expanded its range - from two known sites and 700 nests in 2007, to an area of infestation of several hundred square kilometres and thousands of nests in 2016 (the number of nests found peaked in 2010 at over more than 10,000).

Annual spread of OPM from 2006 (yellow ring) to 2015 (dark purple) ?Forestry Commission


This rate of spread to oak woodlands outside of London remains a significant public health concern because OPM will become impossible to eradicate if it accesses this green network and secures footholds in other highly populated areas.

To enforce action, statutory provisions were invoked by the Forestry Commission under the Plant Health (Forestry) Order 2005, allowing landowners to be issued with Statutory Plant Health Notices (SPHNs) where OPM was found. SPHNs put landowners under an obligation to take control measures on infested trees, and provided the Commission with enforcement powers (e.g. legal action) if the mandated action was not carried out.


Where are we now?
Following sharp increases in detected numbers of OPM between 2006 and 2011, a three-year ￡4million enhanced programme of control was established by the Forestry Commission in 2013. This programme concluded in late 2016 and the results are in: we appear to have won some battles but ultimately are losing a war.

Even with focussed effort, it has not proven possible to exterminate OPM although its overall abundance and rate of spread in main outbreak areas has been blunted significantly. However, small new outbreaks outside of known zones have appeared in 2016, one as far north as Watford Junction railway station.

2016 OPM occurrence - pheromone trapping results ?Forestry Commission


The consensus of expert opinion is that it will not now be possible to eradicate OPM, at least with currently available resources and methodology, and the focus has instead switched to containment and slowing the rate of spread.

Pheromone trapping (chemical lures to attract male Oak Processionary Moths) hints that the green infrastructure of woods and parks, and also of linear infrastructure, play a part in the spread of the moth. Trapping efforts have revealed small occurrences of moth all along the motorway corridor from Maidenhead to Reading to Pangbourne, the latter a large outbreak site and the most western to date.

Several outbreaks in South London, Kent, Sussex, Wessex, Middlesex and Hertfordshire are associated with trees found along the railway corridor.


Rail repository risk?
Moths and butterflies benefit and are attracted to the shelter and food resources that are found along railway land. Sun-warmed earthworks hosting pools of nectar plants are favoured habitat, and often support multi-species lepidopteran communities.

In the case of OPM, it is a very fussy eater. Principally oak, oak and more oak. This favoured breakfast, lunch and dinner occurs in abundance by the side of the rail network and, furthermore, the distribution of trees is in a pattern that appears to be selected for by OPM. Along the railway, trees tend to occur in single stands or isolated patches, and OPM prefers oak trees that have some separating space between neighbouring trees - the moth is less inclined to choose oak trees in contiguous blocks of woodland.

 Oak tree lined embankments - good habitat for OPM ?Mike O’Connor


The risk from a control perspective is that railway land, being fenced off, not publicly accessible and so relatively undisturbed, can act as a both refuge and dispersal corridor for the moth. Significant investment in control and eradication of OPM by a landowner can be undermined by the adjacent railway acting as a harbour for the caterpillar.

The concern from a railway environmental and safety perspective is that there is a growing repository of risk - hazards to the health and wellbeing of railway neighbours, the rail workforce, and to lineside oak tree populations.


Defoliated, dying, dead?
The risk to a lineside oak is that OPM can put additional significant stress on the tree. While there have been no reported cases to date of oak tree dieback in Britain caused by OPM, the added stress potentially makes affected oaks more susceptible to secondary pests or pathogens.

Dead, diseased or dying oak trees pose high risk to the operational railway. Where whole, or parts, of unhealthy trees fall on to the railway this can result in serious safety incidents and operational disruption.

Unhealthy trees can easily disrupt rail operations ?Mike O’Connor


OPM on the railway - where and how many?
Unfortunately, identifying OPM infestations and nests on the railway is not straightforward. The caterpillars blend in remarkably well with the surface of an oak, the egg plaques are small and dapple into gnarled oak bark, and the nests also can be unobtrusive.

 OPM nest circled – very difficult to see 20m up in dense canopy and poor light ?Mike O’Connor


Logistically, simply gaining access to railway land can be a challenge. It cannot yet be proven scientifically but certainly appears that OPM deliberately choose the tallest trees, on the largest and steepest embankment, with completely unsuitable access, along the most complex and busiest junctions on the rail network (which therefore cannot be descended upon by potential pest controllers for safety reasons)!

Weather conditions, challenging physical environments and dense oak canopies all reduce the probability of OPM being detected by the average trackside worker. Even an experienced ecologist with high-powered binoculars can easily miss signs of OPM...

Two dozen professionals failed to spot all signs of OPM in this oak ?Mike O’Connor


So, at present, we simply do not know the full distribution and numbers of OPM infested trees on the railway network.


Refining railway response
But where we have detected the presence of OPM, there are two approaches to control. The first involves spraying oak trees with insecticide to kill young OPM caterpillars. This method has delivered some promising results. It is most effective when two dosages are delivered one week apart and this timing has achieved 90%+ reduction in OPM nest occurrence. In a key site on the South East rail network, after one season of pesticide application on a patch we observed a substantial decline in the numbers of live OPM nests the following year.

The challenge for this approach is that more than one visit to the same area on the railway within a short space of time is not always possible due to access constraints. In addition, Forestry Commission research indicates that size of tree is a limiting factor when it comes to pesticide effectiveness. The 90% reduction figure applies to trees up to 20m tall. For trees above 20m tall, the effectiveness of pesticide application declines dramatically - beyond 30m in height it is virtually ineffective as it is not possible to apply adequate doses to the tree canopy from the ground (methods reach up to 10m) or from mobile works platforms (up to 20m).

 OPM pesticide spraying – expensive and ineffective on large oak Both pics ?Forestry Commission


Some trees on the railway are very large, or are growing on steep earthworks or other difficult to reach locations. Consideration also needs to be given to the multitude of homes, schools and businesses adjacent to the railway. Pesticide spraying to control OPM is then often impractical and costly. A further disbenefit is the impact on non-target biodiversity - pesticides such as deltamethrin are highly toxic to a wide range of other invertebrates and aquatic life.

Access to railway oaks often involves intricate elevated platform work ?Mike O’Connor/Nash


Safety first, incineration second
The alternative control method is to manually remove nests later on the year. Railway operations in the South East have focussed on this approach and have developed a working method more suitable for the rail environment:

Text & pics ?Mike O’Connor/ Nash


While more environmentally-friendly and yielding quantifiable results, manual removal is also costly and a labour intensive process. So the effectiveness of this method will be evaluated in the spring with resurveys for OPM along sections of line treated.


How to stop a runaway train (of caterpillars)? Apply slow, steady deceleration.
It will be very difficult to eradicate OPM entirely from where it occurs on the railway network. However, the objective is to put the brakes on its utilisation of the green lineside infrastructure. The forward plan includes:

• Upgraded lineside identification and survey programme - resurveys of trees that were known to be infested last year, supplemented by wider surveys of potential connecting corridors. An objective for 2017 is also to deploy pheromone traps along select sections of line.
• Improved survey techniques, recording tools and analysis - trialling of new techniques to pick out OPM in trees; use of a new biodiversity recording app; GIS mapping and analysis of results.
• Targeted pesticide spray control - a directed programme of pesticide control, factoring in lineside constraints.
• Systematic manual nest removal control - informed by survey findings and applying good practice methodology developed for the railway environment.
• Enhanced communications plan and liaison with adjacent landholders and stakeholders.

Track workers will use new reporting app to log OPM occurrence ?Mike O’Connor


The runaway OPM caterpillar train is not likely to be halted soon but the railway community is working to slow its momentum. By introducing new process and working collaboratively with stakeholders, we aim to play our part in preventing this hazardous caterpillar going completely off the rails.