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I’m a moth lover but London’s ‘toxic caterpillars’ worry even me

Moths are the insect we truly love to hate. The press report almost annually on the looming threat of clothes moths. I have previously written in defence of diamondback moths, a migratory pest of cabbage crops, and highlighted the quirks of biology that drove the spectacle of thousands of Silver Y moths gatecrashing the Euro 2016 final without tickets. I am absolutely unapologetic about my love for these diverse and intriguing cousins of the much better-loved butterflies. Moths get a bad press thanks to a few species which negatively affect our lives (this also applies to other insects, such as wasps), but most are harmless (or beneficial), fascinating, and often even beautiful.

And so we come to recent news reports of a plague of toxic caterpillars descending on London. The caterpillars in question are those of the oak processionary moth (Thaumetopoea processionea, or just OPM) – just about the only species for which I struggle to summon up much sympathy.

So what’s the issue?

Delicately sketched in pencil, the adult oak processionary moth. Gyorgy Csoka, Hungary Forest Research Institute,, CC BY-SA

It’s not to say that this is an unattractive moth. The grey-black colour scheme of the adults, active in late summer, lends them the look of having been delicately sketched in pencil. You are much more likely to encounter the caterpillars, which are covered in very long, white hairs. Colonies of OPM caterpillars form white silk nests on oak trees and can be spotted moving about in remarkable nose-to-tail processions. Other moth species form similar nests in the UK, including occasionally on oak: if such a nest is found outside London (especially in Essex or Cambridgeshire), it is more likely to be the Brown-tail moth.

Keep an eye out for OPM caterpillars moving in crocodile-formation between oak trees. OlegD / shutterstock

Unlike the Brown-tail, however, the oak processionary moth is not native to the UK. It was first recorded in Britain in 1983, but the species established properly in around 2006, when it’s believed some eggs arrived on imported oak trees. This isn’t in itself a reason to dislike OPM, as conservationists (including myself) can sometimes be hypocritical about non-native species: for instance, we are vocally concerned about the arrival of the horse-chestnut leaf-miner moth because it harms horse chestnut trees – even though the trees themselves are non-native.

However, OPM is also a potentially a public health problem. Each of the caterpillar’s hairs contains a toxin called thaumetopoein. Touching an OPM caterpillar directly could bring you out in a rash – in fact, as a general rule it’s always best to avoid hairy caterpillars unless you know what you’re dealing with. The hairs of OPM caterpillars can also break off and drift on the air and, if there are sufficiently high densities of caterpillars, these hairs can cause rashes and respiratory problems even to bystanders.

Besides affecting people, OPM can also impact the oak trees on which it feeds. A particularly severe infestation could strip a tree completely bare of its leaves, though few cases of this taking place in the UK have been reported.

Tackling the problem

Most people agree that something needs to be done, although the NGO Butterfly Conservation argues that, rather than tackling the moth wherever it appears, control efforts should focus on areas where the threat to human health is high or large numbers of trees are at risk of death. Nevertheless, controlling OPM outbreaks is difficult. The nests are constructed in the crowns of oak trees when they are in full leaf, and even if they can be reached, removing them manually requires full protective equipment to ward off the toxic hairs. For that reason the preferred approach is currently to tackle nests remotely, spraying trees with insecticide when the moth is most vulnerable – as a young caterpillar, between April and June.

There is currently no insecticide that is specific to OPM, so a bacterium known as “Bt” is used. Unfortunately, Bt is toxic not just to OPM, but to the caterpillars of all moths and butterflies. The financial cost of these control efforts is astronomical – estimated at around £1.2m per year in 2016-17.

The uncounted, and incalculable, cost to the oak woodland ecosystem could be greater still. The loss of much of the insect biodiversity from our woodlands would be tragic in itself but is likely to have further implications for the bats, birds and other wildlife that rely on these insects for food during their breeding seasons – a study of an OPM control programme in woods near Pangbourne, Berkshire, suggested that blue and great tits were breeding less after spraying took place. It’s these losses that have put me off OPM.

The attractive green silver-lines moth inhabits oak woodland, and is among the species likely to lose out from attempts to control OPM. Hildesvini

Looking to the future

But let’s not panic – there are plenty of reasons to feel hopeful about the future of Britain’s oak woodlands. It’s true that the moth has been recorded “across vast regions of the south-east”, but that mostly only refers to the highly-dispersive males. To spread the outbreak requires the egg-laying females to travel, and they don’t fly nearly as far. This means that, for now, the toxic caterpillars are mainly confined to London. The outbreak has crossed the M25 ringroad in just a few places, and is still only expanding at a slow pace.

Encouragingly, some of the more isolated sections of the outbreak also appear to be coming under control. New outbreaks in Watford, Barnet, and Pangbourne all appear to have been successfully removed. An outbreak at Bethlem Hospital, Croydon, estimated to contain 4,000 nests in 2012, was confined to just four trees by 2016. Vigilance is key, and this year the Forestry Commission is once again asking the public to report any potential sightings of OPM through its Tree Alert scheme.

Finally, we may have some unexpected allies on our side. In its native southern and central Europe, OPM is not especially problematic because it rarely reaches sufficiently large population densities. That’s partly because its numbers are kept in check by its natural enemies – parasitoids. This is a catch-all term for various insects with a rather gruesome life-cycle: eggs are laid inside caterpillars and other insects, before the larvae eat their victim from inside out (killing it in the process) and emerge as a fully-formed fly or wasp ready to seek out new prey.

Often, when an insect expands its range by artificial means (as OPM did, entering the UK on imported trees), it can take some time for its parasitoids to catch up, and in this lag period the insect may do particularly well. However, a recent study found nearly half of the OPM caterpillars sampled from the Croydon outbreak in 2014 were infested by one such natural enemy, the Carcelia iliaca tachinid fly. This suggests the oak processionary moth may be reaching the end of its lag period in the UK, and as the flies attack more caterpillars, this could help the control efforts. My enemy’s enemy is truly my friend, and in this case, perhaps it is a tiny fly.