The weevil dead - tackling the forestry scourge

Wood Focus magazine
30 Dec 2011
Hylobius abeitis (the large pine weevil)

When winter seeps cold and damp, large pine weevils seek shelter. They choose the stumps and roots of recently felled conifer trees, as they are ideal for hibernating and babymaking. Each spring, the weevils emerge from their nooks in great numbers. In just one hectare, there can be as many as 150,000 biting into sapling bark.

According to Finlay McAllister of Forestry Commission Wales, in economic terms, the large pine weevil (Hylobius abietis) is the most significant insect pest in British forestry. If they are unprotected, an average of 50% of new tree plantings will die in the first few years of establishment. However, proliferations of Hylobius numbers and ineffectual chemical protection have left scientists stumped – until now.

A Forestry Commission Wales team, along with partners from Swansea University and Aberystwyth University in the UK, and the National University of Ireland in Maynooth, claims to have found a solution to the Hylobius problem in recently felled or replanted areas using a cocktail of microscopic nematode worms and fungi.

The Integrated Management of Forest Pests Addressing Climate Trends (IMPACT) team uses Steinernema carpocapsae and Heterorhabditis bacteriophora nematodes and the Metarhizium anisopliae fungus to attack and kill Hylobius larvae, pupae and young adults.

Hugh Evans, Head of Forest Research in Wales, explains that the nematodes ‘attack a wide range of invertebrates and deliver a bacterium that affects the host insect and enables the nematode to multiply inside the affected insect’. He adds, ‘the target insect is infected by the fungus. The fungus grows and produces a mycelium (fungal mat) that produces green spores.’ This cocktail is applied to the insect’s breeding grounds using a water-based suspension of spores or nematodes at a volume of 0.5 litres per stump.

He says, ‘Both the nematodes and fungi have long-established track records of safe use in a wide range of agricultural, horticultural and woodland situations. They have no impact on vertebrates, amphibians, reptiles and fish and are an ideal replacement for chemical pesticides.’

At present, synthetic pyrethroid pesticides, such as cypermethrin and alphacypermethrin, are applied to newly planted trees as a deterrent to feeding. However, Evans notes, ‘Because they kill only relatively small numbers of adult weevils, the chemicals do not significantly impact Hylobius populations, unlike the biological control methods we are developing’.

‘The advantage of this natural method of control is that we are actually reducing the population pressure from the weevil numbers at a whole forest scale,’ he claims. ‘This will eventually reduce the likelihood of damage to new plantings of trees to very low levels so that we can move away from the reliance on use of seedlings treated with chemical insecticides. This has obvious environmental benefits, as well as moving away from control methods that rely on non-renewable resources.’

That said, the trials have had their shortcomings. At present, the solution requires machinery that can carry a large amount of water, which makes it difficult to work on boggy ground and steep terrain.

According to the team, trials at Cwm Berwyn suggest that the cocktail of natural agents can reduce the Hylobius population by about 40%, and the method is being taken to full trials in Welsh and Irish forests. They are also working on pests that could become increasingly problematic in future climates. These include trials of fungi and nematodes against rootfeeding insects (wireworms, cutworms, chafers and craneflies) and leaf-feeding insects (for example horse chestnut leafminer moth).