Protecting native ash trees from pest that can strip them bare

Traps have been laid as part of a research project to protect native ash trees from an invasive species of insect that can strip a tree bare.

The ash sawfly larvae feeds on tree leaves between May and June.

Scientists will spend the next three years looking at the best, environmentally-friendly, ways to deal with the pest.

They said that although a healthy ash tree can usually recover, their growth will be affected and if they are already suffering from ash dieback, a fungal disease, an infestation can be fatal.

Dr Florentine Spaans from the Agri-Food and Biosciences Institute (AFBI) said the larvae is a "defoliating pest", meaning it strips leaves from trees.

The insect is thought to have arrived in Northern Ireland in 2016 from mainland Europe - and is harmless to humans.

"The adults actually appear in late April, maybe early May, and come out of the ground and lay their eggs on the leaves," Spaans explained.

As the larvae develop and eat the leaves, "they then drop to the ground and they try to climb back up if they need to eat more".

On the ground, the larvae pupate, or develop into a pupa, before becoming adult sawflies.

That's were the traps come in.

It is hoped they will stop the larvae making their way back up tree trunks and falling again to pupate - ending the fly's life cycle.

Spaans said the insects "can completely strip the leaves from the tree every year".

"So this actually has an impact on the tree because it needs to re-flush its leaves and it causes it to expend a lot of energy, which leaves it more susceptible to other pests and diseases.

"If this happens every year, the growth of the tree is slowed down and it can have a potentially serious impact in the long term."

Ash trees have also been impacted as a result of the ash dieback disease, which is believed to have arrived on imported saplings and was first detected in November 2012.

A sawfly infestation can be fatal to a tree suffering from ash dieback.

Some traps have already been placed on trees in the Belvoir forest area of Belfast.

Dr Ryan Weir, from the AFBI, said that a banded sticky trap is a "nice yellow colour" that "helps to attract the adult sawflies in during their flight".

"They'll be stuck here to control their population.

"We're looking at novel control methods for the ash sawfly but the project as a whole is looking at the leaves, the volatile compounds released from the leaves of ash trees, and it's all being tied into a much bigger narrative about some of the regulated pests on the EU lists."

Weir said that this type of trap "really primarily affects next year's population of flies".

"These guys won't fall into the soil now and they won't pupate so that's a big chunk of the population that won't be emerging as flies next year."

Other traps such as a ground cover traps uses mesh that surrounds the trunk and then expands out into the foliage, trapping the sawflies as they emerge from the soil next year.

After arriving in 2016, the invasive insects have spread throughout the greater Belfast area and Lisburn.

An earlier AFBI study, carried out between 2019 and 2021, found that 90% of trees surveyed along the Lagan Towpath in Belfast showed signs of ash sawfly damage.

In Twinbrook in west Belfast the ash trees are no different.

Resident Michael McCrory said the sawflies are a "bit of a nuisance".

"People are getting them caught in their clothes and in their hair and they're being walked into the house.

"I have noticed a big change this year the starlings and the wood pigeons are starting to get a taste for them and they are taking quite a few but there's so many of them it's just not enough."

"Last year there was kids running about screaming and yelling because they didn't know what they were. They thought they were going to be bitten but they're totally harmless," McCrory said.

Belfast City Council said it is aware of ash sawfly activity.

"At present, the council does not operate a targeted treatment programme for ash sawfly and is not actively treating infestations.

"However, we continue to monitor the issue and follow experts' advice as part of our broader tree management responsibilities."

It is not all doom and gloom for the ash tree however.

Spaans said: "There are projects underway that look at breeding resistant varieties of ash trees.

"The good news is that they're self-seeding, so they have a lot of genetic variation naturally, so I think they'll be able to cope with some of these challenges in the long term."

Weir added that through a partnership in the Republic of Ireland, which is working on breeding resistant ash trees to the ash dieback disease, AFBI are offering the new breeds to the sawfly in lab conditions.

"A bit of a choice experiment to see if the sawfly will also eat those trees that are resistant to the dieback."