Pine processionary moth: tackling the tree pest in Brittany

THOUSANDS of visitors choose to holiday on Brittany’s magnificent Côte d’Émeraude each year. Some hike trails that edge the sheer cliffs of its heathery headlands, others prefer to take their leisure on unspoilt beaches fringed by an aquamarine sea. But whichever activity each prefers, they can hardly fail to notice the pine species that add character to the area.

Even a cursory glance at a tree can sometimes reveal oval-shaped, silken ‘tents’ hanging from the upper branches. What’s more, it seems that the presence of these requires the tree’s trunk to be fitted with an encircling metal band from which dangles a clear plastic bag filled with soil and bearing a warning. People have good reason to question what they see; for the tents are nests spun to hold the over-wintering young of the pine processionary moth (Thaumetopoea pityocampa).

Signs explain the purpose of the tents and advise that they should not be interfered with.

Passing through various stages of larval development, the caterpillars emerge from their tents at night to feed voraciously on the needles in the host tree’s crown. They are known to have very complex navigational systems and by following these they are enabled to return to their own tent where they rest and digest during the daylight hours. In mountainous areas the caterpillars have been noted as remaining active in darkness even when the thermometer plunges to well below zero. Nests are usually located to take maximum advantage of any winter sunshine.

In the early spring the caterpillars gather together before, led by a female, progressing nose to tail down the trunk of the tree in a lengthy procession. At this stage they are no longer interested in feeding but are desperately driven to reach the ground, burrow into soft soil and pupate into the moths which emerge in late summer.

Pine processionary moths live for only one day, during which they must reproduce. The female lays her eggs in a host tree that is preferably a pine. After mid-winter, the hours of daylight lengthen, and groups of larvae spin white, silken nests or ‘tents’ in the tree’s branches. Each is sited to take advantage of winter sunshine. The caterpillars remain in the nests until spring, only emerging at nightfall to feed on the tree’s needles.

In France attempts are being made to reduce the amount of chemical intervention in the environment. Insecticidal sprays are no longer acceptable for dealing with this highly destructive forest pest. The biological control using Bacillus thuringiensis is still available, but as it is only effective on the eggs and the first and second stages of the caterpillar’s development, its use is limited to the early autumn months.

Encouraging natural predators is also on the agenda. However, those that can handle the well-defended caterpillars are few in number. Even so, the great tit (Parus major) and to a certain extent the coal tit (P. ater) can, in the right circumstances, make great inroads into the populations. In recent years, another innovative attempt to disrupt the life cycle of the pest has been devised by French foresters. The aforementioned strange apparatus observed on the trunks of the pines is now finding widespread use on both amenity and forest trees.

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The fully encircling trough-like collar only temporarily interrupts the line of caterpillars progressing towards the ground. Their leader may circle the bole for a while but will soon start to track downwards again; passing through a tube that leads from the collar into the sealed bag hanging below. When this takes place, the whole marching army will follow. The deception is completed by persuading individual larvae to search for their own patch in what they believe to be the natural forest floor. Here they will settle down to pupate with the expectation of emerging as moths some months later. They will not mature; suitably protected operatives remove the bags in early summer after which they are destroyed by incineration.

Cap Fréhel’s pinewood, which lies on a popular long-distance hiking trail, is suffering from the predations of defoliating pine processionary caterpillars. The efficiency of the collar trap system remains a matter of discussion. A naturally occurring insecticide derived from a soil-dwelling bacterium can be effective on the early larval stages. Once developed into the hairy caterpillars, the few birds that can tolerate the irritants they produce provide the only natural control.

Cap Fréhel’s attractions are well documented. Visitors of all nationalities arrive by the coachload to walk the clifftop trail from the promontory’s lighthouse to the 14th-century Fort la Latte perched at its opposite point. If the day is clear enough, it is possible to discern the distant outline of Jersey. This section of route GR34 passes through a windswept expanse of protected heathland before arriving at a small pinewood established by the Fort la Latte estate. As they approach the trees, sharp-eyed walkers will no doubt notice the tents that hang from the ends of some branches.

Signs placed in prominent positions explain the purpose of the tents and advise that they should not be interfered with. Protected by thousands of hairs containing the protein thaumetopoein which causes severe irritation and allergic reactions, the caterpillars of the pine processionary moth pose a serious health risk to both humans and animals. It is pointed out that the collars and bags attached to a number of the trees are an accepted mechanical means of controlling the pest and that they too should not be touched. The significance of the great tit as a biological means of reducing caterpillar numbers is also mentioned.

The pine processionary moth has spread steadily northward from more southern climes and is now present in pines established on Brittany’s clifftops. But will this poor flyer eventually succeed in crossing the English Channel and infesting southern England’s pine forests? The Forestry Commission reports that solitary vagrant adult moths were recorded in 1966, 2013 and 2017. The latter two are thought to have been blown across the Channel into south-coast locations. The origin of the 1966 moth, which was found in Berkshire, is unknown.

The pine processionary moth – which is native to North Africa, areas of the Middle East and the southern Mediterranean region – has spread northwards through Europe and breeding colonies are now well established even in the Paris area. In many cases, it is thought, the movement of plants in a growing medium infested with pupae may well have assisted the seemingly inexorable northward progress of the pest.

Some authorities note that the limited flying range of the female adult moth should have kept the spread of the pine processionary moth under control, but its presence in Brittany suggests that such theories may be wide of the mark. Both amenity and timber-producing pines are very commonly planted along the French Atlantic coast, but the speed of the moth’s arrival on the shores of the English Channel has taken foresters by surprise.

Saint-Cast-le-Guildo’s magnificent properties adorn the slopes that rise from its Grande Plage. The gardens of these are often shaded by pines of considerable stature. Owners have not resorted to bizarre methods of ridding their conifers of the pine processionary caterpillar, instead fitting environmentally acceptable traps and nest boxes. Bird feeders containing peanuts, sunflower hearts and seeds are sometimes used to encourage great tits and coal tits to take up residence in the autumn.

The UK has seen one outbreak of caterpillars that were introduced with imported plant material (as it happens, in Scotland). It is also reported that adult moths have sometimes been found as vagrants in the southern counties of England. It is not inconceivable that strong southerly winds could assist a few of the weak flyers to make landfall on the British side of the Channel. If they manage to make the crossing from Normandy’s Cherbourg Peninsula or Brittany’s rocky shoreline, prevailing winds could possibly enable them to live out their short lives in the extensive pine forests of the Dorset heaths.

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It is, perhaps, hardly advisable to rely upon previous advice issued by some authorities that the British climate would prove unsuitable for the survival of the pest, as over a relatively short period of time, it has spread from the hot southern shores of Europe to the cool coast of northern France. The prospect of climate change is unlikely to make southern Britain less accommodating for the pest, but it may well be the case that the caterpillars are adapting to life in more northerly latitudes.

Maritime pine (Pinus pinaster) – which is one of the species said by the FC to be particularly susceptible to attack by the pine processionary moth – has traditionally been a major constituent of Wareham Forest in Dorset. The heathland is notable for the flora and fauna that take advantage of the unusually mild climate; similar to that found on Brittany’s coastal promontory, Cap Fréhel.

French arborists and foresters are agreed that the efficiency of different methods of control is dependent on the setting. Stands of isolated amenity pines that are protected by mechanical collar traps can lead to a dramatic reduction in the numbers of the pest – or even its eradication in local areas. Within the woodland the system is regarded as less effective. Now deprived of chemical controls, the French are reliant on the soil-borne bacterium Bacillus thuringiensis and members of the tit family to reduce the numbers. The provision of nesting boxes is a major factor in encouraging natural predators such as the great tit, and the birds certainly will not hesitate to use the repulsive caterpillars as a food source. Taking one is a far cry from grabbing an easy meal, however. Up on Cap Fréhel, the birds in the pine plantation of Fort la Latte were going to great lengths to ensure that their prey was well and truly stunned before it was ingested.

Were Thaumetopoea pityocampa to be discovered in numbers amongst the pines of, for example, Dorset’s Wareham Forest, the use of chemical insecticides (at this moment in time) is still an option in the British forester’s armoury. Spraying the pine woodlands lying between the villages of the Isle of Purbeck or on the islands of Poole Harbour would pose far more problems. Should the tell-tale silken tents appear in the branches of Bournemouth’s stately and emblematic pines, the fitting of stem collars and sealed collecting bags would probably offer the most effective and acceptable solution.

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