Ash Sawfly - Defoliation of ash trees in NI

Date published: 13 June 2023

Area of Expertise:

AFBI, supported by DAERA, has an active surveillance programme for threats to plant health. Through this surveillance AFBI has recently received several reports from the public where massive numbers of caterpillars have been seen damaging ash trees, particularly along the Lagan Valley area.

Adult Ash sawfly

The ash trees in some cases were completely devoid of leaves (defoliated), with a considerable number of the insect larvae descending the trunk of the trees towards the surrounding soil.

Ash sawfly caterpillars on leaf – Ash sawfly caterpillars are voracious eaters and can strip leaves and trees bare
Ash sawfly caterpillars on leaf – Ash sawfly caterpillars are voracious eaters and can strip leaves and trees bare
These are the larval stage of Tomostethus nigritus, or ash sawfly, which until 2016 had never been observed in Northern Ireland. T nigritus is a Euro-Siberian species distributed throughout Europe, but considered to be more frequently damaging in eastern rather than western Europe.

T. nigritus has been responsible for the complete defoliation of large ash plantations during severe outbreaks with up to 800 ha being affected between 1999 and 2000 in the Czech Republic. Other countries such as Norway and Croatia have seen defoliation of ash trees only in urban areas.

Sawfly caterpillars on ash trunk – At this time of year, ash sawfly caterpillars migrate down the trunk from the branches to pupate in the soil or to attack a new tree
Sawfly caterpillars on ash trunk – At this time of year, ash sawfly caterpillars migrate down the trunk from the branches to pupate in the soil or to attack a new tree
While the damage looks severe, it is rarely fatal for the tree. Nonetheless, continual defoliation over successive years may have an impact on the tree’s vigour. Studies in Croatia suggest that factors such as autumn or spring flooding, mild winters, and the absence of predators or parasites may affect the occurrence of these outbreaks.

What is to be done about them?

Severe defoliation of ash – Ash sawflies can completely strip a tree bare of leaves. The leaves will grow back but consistent defoliation each year may stunt the tree’s growth
Severe defoliation of ash – Ash sawflies can completely strip a tree bare of leaves. The leaves will grow back but consistent defoliation each year may stunt the tree’s growth
Chemical control at this stage is inadvisable and the trees will recover their leaves throughout the summer. Pesticide application may also have a negative impact on potential predators and parasites, which may have a controlling impact on this pest long-term. Alternatively, the larvae may be collected before they burrow into the soil and pupate, which will reduce the population of the subsequent generation and limit the risk of recurrence. While these larvae do not have urticating hairs, like some lepidopteran species, care should be taken not to touch the larvae, which can be destroyed humanely by placing in sealed polythene bags and retained overnight in a freezer before disposal with household waste. AFBI will continue to monitor the situation and provide further recommendations should this problem become more widespread.

Notes to editors: 

AFBI’s Vision is “Scientific excellence delivering impactful and sustainable outcomes for society, economy and the natural environment”.

AFBI’s Purpose is “To deliver trusted, independent research, statutory and surveillance science and expert advice that addresses local and global challenges, informs government policy and industry decision making, and underpins a sustainable agri-food industry and the natural and marine environments”.

AFBI’s core areas:

  • Leading improvements in the agri-food industry.
  • Protecting animal, plant and human health.
  • Enhancing the natural and marine environment.

Share this page

Latest news

More news …