First Report of the Ash Sawfly, Tomostethus nigritus, Established on Fraxinus excelsior in the Republic of Ireland

SIMPLE SUMMARY: Ash sawfly, Tomostethus nigritus, larvae feed on the leaves of Fraxinus excelsior. In the last 20 years, outbreaks of these insects have become more common, and they cause severe defoliation of the tree crown. This pest is native to Europe, and it was recorded for the first time on the island of Ireland in Belfast, Northern Ireland (UK) in 2016. This article is the first report of T. nigritus in the Republic of Ireland (IE). Damaged leaves and larvae were observed on ash trees in Co. Kildare in summer 2021. The larvae were collected and then identified using molecular analyses. Similar damaged leaves were observed on trees in Co. Dublin, which showed mild defoliation of the crown. However, severe defoliation of ash trees has also been recorded in Co. Meath in 2021. No control treatments are available against the insects, and little is known about the cause of the outbreaks. Severe outbreaks by T. nigritus could further affect the delicate situation that the Irish ash tree population is currently facing caused by another severe antagonist of Fraxinus, the ash dieback pathogen Hymenoscyphus fraxineus. ABSTRACT: This is the first report of the ash sawfly, Tomostethus nigritus, in the Republic of Ireland. We observed defoliated leaves of Fraxinus excelsior L. and T. nigritus larvae at a forestry plantation in Co. Kildare. Morphological observation of the larvae and DNA analysis using mitochondrial COI barcoding confirmed the identification of this pest of ash.