Host of the month - Elm

‘Host of the month’ is a series of Blogs and PDF’s that highlight a tree host and their associated priority pests and diseases that are best seen and recorded in that month. For June we’re looking at Elm (Ulmus species) and Elm zigzag sawfly (Aproceros leucopoda).

Mature elm trees were once a prominent feature in the British landscape but in 1920 that all started to change with the arrival of the first wave of Dutch elm disease (Ophiostoma ulmi). However, it was the arrival of a more aggressive species, Ophiostoma novo-ulmi, in the 1960’s that sounded the death knell for mature elms. By the 1980’s all but a few isolated mature elms were gone. The various species are still here though, as small trees or in hedgerows, but few make it past 20 years old before they’re hit by the disease.

The number of elm species present in the UK is a contentious issue that is still being resolved, and whatever the number they are not that easy to tell apart. However, they all share some characteristics which make it easy to tell an elm from other trees. Leaf bases are usually oblique, the blade of the leaf coming lower down one side of the leaf stalk than the other. The leaves themselves range from oval to ovate, can be either rough or smooth in texture, and usually have double-toothed margins. The simple and inconspicuous flowers emerge before the leaves and soon form prominent bunches of fruits consisting of a thin papery wing that surrounds the seed, many of which are not viable.

Priority pest – Elm zigzag sawfly

Elm zigzag sawfly is a non-native pest originating in eastern Asia which was first seen in the UK in 2017; and it is not known how it arrived here. Damage to elms elsewhere is variable, in some parts of Europe Elm zigzag sawfly has caused severe or even complete defoliation (74-100%), but as yet there are not reports of any trees having been killed by it. However, there are concerns regarding competition with other invertebrates such as caterpillars of white-letter hairstreak butterflies which also feed on the foliage.

Identification

Elm zigzag sawfly is named after the zigzag feeding trace, which is the most obvious sign that the pest is present. The young larvae chew this characteristic trail between the lateral veins of elm leaves, though as they grow the zigzag pattern is lost, leaving only the leaf veins remaining. Once mature the larvae weave a filigree cocoon on the underside of the leaves, emerging as adults after around 7 days to begin the cycle anew. The empty cocoons can remain in place for some time and are another good sign that this pest is present.

For more information check the Observatree resource pages for or the Host of the Month for June. You can also test your knowledge with the Host of the month Quiz.

Reporting

June is an ideal time to seek out oak trees and see if you can identify any of the signs and symptoms of Elm zigzag sawfly. This sawfly is a priority pest so please report possible sightings via TreeAlert. Healthy tree data is equally important so please do report those too.