Weevil threatens our forests

Concern: The pine weeval has long been a threat to replanted forests across Ireland. Foresters are now concerned that a potential EU proposal to ban cypermethrin could heighten the problem. Photo: Roger Jones.

William Merrivale

April 07 2014 06:04 PM

One of the greatest threats to replanted forests in this country is the pine weevil. Traditionally it has been treated with the active ingredient cypermethrin, but if this chemical is banned by EU regulators, how will forestry owners control this pest?

This is an increasingly worrying question for foresters.

The large pine weevil is widespread in mature forests but there they feed in the canopy without causing any significant damage. The problems occur following replanting of clear-felled sites.

Here, the adults are attracted to the site by the smell of freshly cut timber where the females lay their eggs in the stumps of recently cut trees and the immature weevils develop under the bark.

Stumps should be inspected regularly for evidence of larvae, which are creamy-white with brown 'heads'. The damage occurs when the emerging adults feed on the bark of the young forest transplants.

While pine is the preferred food source, weevils feed on a variety of both conifers and broadleaves.

Young transplants can withstand a small amount of damage, but extensive weevil feeding can ultimately cause needle loss, impaired growth and death where the young tree is ring-barked.

There have been recorded instances of 100pc failure in the absence of proper control measures.

The standard control method is to dip the transplants in a cypermethrin solution prior to planting. A second application by knapsack sprayer following planting may also be necessary.

But the chemical is not cheap, and its application may be outlawed by Brussels.

For some years now the search has been on to develop alternative, biological means of control.

Various natural enemies prey on both immature and adult pine weevils. The most encouraging research has focused on insect killing nematodes. These microscopic worms actively seek out and invade potential "host" insects, killing the insect and reproducing inside it.

They feed on the dead insect and move on to find other hosts when food supply is exhausted.

Nematodes are considered to be ideal biological control agents. They pose no risk to fish, birds and mammals. They are not harmful to humans, they are fast acting, they can be commercially produced and can be applied using conventional spraying equipment.

However, biological control agents can have unintended and, occasionally, very harmful, consequences so great care must be taken at the research stage to eliminate this risk.

A team at NUI Maynooth led by Dr Christine Griffin has been conducting research into nematodes including their potential to be used as an environmentally sound alternative to chemical pesticides.

The team is currently collaborating with Coillte to investigate whether nematode worms can be applied as an effective means to control pine weevil.

Encouraging Results

There have been some encouraging results to date, but further trials are needed to determine which species of nematode work best. It is hoped to commence a breeding programme once these trials are completed.

Researchers in the NUI Maynooth team have also discovered evidence suggesting that the tendency of these simple animals to fight to the death is determined by the conditions the animal experiences during its early development.

This finding could be a significant breakthrough in the study of animal behaviour as well as well as influencing the development of biological pest controls.

Experiments indicate that the difference in fighting patterns between the generations of nematode worms is due to a developmental switch during their juvenile stage.

The research found that the tendency to fight to the death among male worms is unique to those that have experienced arrested development because of crowding at the juvenile stage.

"Conditions during early stage development have a profound effect on the expression of fighting in adults. Nematodes may be simple animals, but many of the biochemical and neural processes that take place in them have parallels in higher animals, including humans," said Dr Griffin.

"Fighting, let alone fatal fighting, has not previously been described in any other nematode and fighting to the death is relatively rare amongst animals. Studying their behaviour helps us develop better pest control strategies – as well as providing us with a fascinating insight into the fundamental behaviour of animals and the external conditions that influence their fighting instincts."

In nature, the worms use symbiotic bacteria to kill insects and then reproduce inside the insect cadavers.

Several generations can pass through a large insect and, eventually, tens or even hundreds of thousands of nematodes emerge from the spent cadaver in soil and wriggle away in search of fresh hosts.

These are the kind of conditions where we might expect fatal fighting to evolve.

Killing is a feature of the first colonists that enter the insect, but not of the subsequent generations that develop inside the insect.

There are simply too many competitors (many of them close relatives) for these worms to make fighting a viable survival strategy.

Dr Griffin is cautiously optimistic that the NUI Maynooth research will yield results. Let's hope that we will soon be ordering nematodes when tackling our pine weevil problem.

Meanwhile, the Irish Timber Growers Association (ITGA) is holding a field trip on Friday, April 11 to the Clonad woodlands outside Tullamore, Co Offaly.

Topics include the planning and management of timber sales. The harvesting and sale of windblow plus the potential for harvesting biomass will be discussed.

To find the Clonad woodlands, follow the R421 from Tullamore towards Kinnity and about 1.5kms beyond Tullamore Golf Club follow the ITGA signs.

William Merivale is national secretary of PEFC Ireland and a forestry consultant based in Cork