All You Need to Know About the Elm Zig-Zag Sawfly

Pests and Diseases

6th May 2020 | Info

Common name: Elm Zig-Zag Sawfly 

Scientific name: Aproceros leucopoda 

Affects: The three most common Elm tree species in the UK – English Elm, Wych Elm, Field Elm 

The Elm Zig-Zag Sawfly (Aproceros leucopoda) is one of the newest pests to threaten the already significantly impacted Elm population in the UK

What are they?

The Elm Zig-Zag Sawfly is a non-native sawfly species that originated from Japan and parts of China. They first appeared in Europe in 2003 before making their way into the UK in 2017.  

How they arrived in Europe/the UK is often debated as there is no exact answer. They could have entered through imported Elm trees, or though human-aided transport via vehicles as they travelled through the continent.  

Their larvae form are minute, green caterpillars and their adults are small, black, wasp-like sawflies with white legs. 

What do they do?

These pests have a short lifecycle and can complete a full one averagely between 26-33 days.  

Adult Sawflies begin the process by laying eggs into the serrated edges of Elm leaves. These eggs then take between 4-8 days to hatch.  

Once these hatch, the larvae take up to 18 days to develop, and it is in this time that they feed on the leaves and the distinct feeding pattern can be seen. As they mature, however, their feeding pattern changes, and they begin to feed on the whole leaf. 

After they have fully developed, they then proceed to cocoon on the underside of an Elm leaf and emerge as adults (typically in 7 days).  

How can I tell if they have infested my Elm tree?

Elm Zig-Zag Sawflies have a distinct feeding pattern when they are larvae – much like a zig-zag (hence the name!). 

However, as the larvae develop this pattern changes as they instead feed on whole leaves which can make it hard to identify them.  

Typically, the Sawflies are expected to emerge in April – so keep an eye out around this time for the first signs of damage! 

What impact could they have on our Elm trees?

As Elm Zig-Zag Sawflies are relatively new pests, it is hard to tell the full extent to which they have already spread since entering the country; however, it is clear that it is only going to get worse. 

We know that in one summer alone, several generations of the sawfly can be produced. The species have no recorded males which suggests that they reproduce through parthenogenesis, hence why they can increase rapidly 

High populations have the ability to completely defoliate an elm tree on a yearly basis which over time can make trees weaker and more vulnerable to other existing pests and diseases, as well as unpredictable environmental stresses 

However, currently, in the UK, they have not yet caused high levels of defoliation, so the true impact has yet to come into fruition. 

They will also have an impact on many elm-reliant species such as the white-letter hairstreak butterfly.  

What is currently being done to prevent pests and diseases in the UK?

To prevent further spread of pests or diseases into the UK, several actions have been put into place: 

  • The UK and Ireland Sourced & Grown Assurance Scheme – Reduces the risk of importing new pests and diseases into the country. (Woodland Trust) 
  • Encouraged biosecurity at border points. (Woodland Trust) 

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