What’s Up With The Elm Zigzag Saw Fly?
Imagine yourself walking down a residential street in mid summer. First, picture that the street is devoid of trees. The sun is hot and there is no shade to be found. You can hear a few birds chirping from bushes but otherwise the most notable thing around you is the heat from the pavement and a general lack of life. Now, imagine the street lined with elm trees. These beautiful trees are providing shade for the sidewalks, their leaves are rustling in the wind, birds flutter between them – wow, that is so much better! Though it might be something that many of us take for granted, it is impossible to ignore how important our urban greenery is for our quality of life and mental health. Trees, shrubbery, and flowers help keep us closer to nature and it is crucial that we include plants in our urban planning. From lining streets to offering a shady picnic in our parks, elm trees make up a large portion of our urban trees, but now they are in danger from an invasive insect called the elm zig zag sawfly… and we have to do something about it.
The Elm Tree
Elm trees refer to various species in the Ulmus genus. The most common in North America being American Elm (Ulmus americana). Trees from this genus are beautiful, fast growing, and sturdy trees. These factors make them a popular choice for planting in populated areas and are also what gives them a crucial role in native ecosystems. In the past, populations of these trees have suffered from Dutch Elm Disease and they are now also under threat from an invasive insect called the zig zag sawfly.
How to ID an elm tree
Knowing the names of the trees and other plants around you can be a fun way to be more connected to nature, even if it is just in your own backyard. Here are a few key factors for elm tree identification:
- Dark brown-grey bark with furrows
- Bark is flaky along furrows
- Tall with branches off of a single trunk
- Leaves are uneven at the base / have an asymmetrical stem
- Leaves are oval shaped and bright green
- Leaves have distinct veins and the edges have a saw-like appearance
What is the Elm Zigzag Sawfly?
The Elm Zigzag Sawfly (Aproceros leucopoda) is a very interesting insect that actually isn’t a fly at all. This sawfly belongs to the order hymenoptera – which is the order that contains wasps, bees, and ants! Though this is an extremely diverse group, this species is quite interesting because few organisms in the order hymenoptera have a life cycle that is so similar to that of lepidoptera (the order containing butterflies and moths).
You see, the zigzag sawfly has a four part lifecycle – egg, larvae, pupae, and adult – where the larvae eat leaves, similar to caterpillars. The weirdness doesn’t stop there. When adults emerge from their pupae they are all females! These females are immediately ready to lay eggs thanks to a weird little “trick” called parthenogenesis – the ability to lay viable eggs without mating with another individual. Until now, no males have been found so we think they might not even exist.
Once ready, the female lays her eggs on elm leaves, right along the little tips of the “saw” blades, and the larva soon emerge to eat away these leaves in their distinctive zigzag pattern. They continue eating and growing until they are ready to pupate. Their life cycle is quite short, so there can be 4 generations a year in North America! They have shorter pupations in summer and the last generation of the season pupates in a studier cocoon made to last through the winter. This in addition to the fact that they can travel up to 90 km in a year makes them a great candidate for an invasive species.
This intriguing little insect is native to Asia – namely China and Japan – and has since spread across parts of Europe and is also found in North America. This particular sawfly was first found in North America in Quebec, Canada and is spreading. It has now spread to Virginia, North Carolina, Maryland, Pennsylvania, New York, and Ohio, Vermont, and Masshucsetts
ID the elm zigzag sawfly
You are more likely to find the distinct damage of the leaves before the little insects themselves but it is good to know how to ID the insects as well. You see, the little larvae eat in a very clear zigzag pattern as they munch away on elm leaves – hence the name “zig zag sawfly”. As they get bigger the zigzag is not as clear, but there are usually multiple life stages at once on one tree, so you should be able to find the pattern! As for the insect itself, here are some key factors for zigzag sawfly identification:
- The egg is very tiny – less than 1 mm long. They are a blue green-black and found right along the saw tips of leaves.
- Larva start as a greyish white and become bright green with some black markings – including tiny black spots along their body near their breathing holes
- Pupa/the cocoons are loosely woven and light (they look like mesh or a net) and are often found on the underside of the leaves or on branches. The winter pupa are much more tightly wound and are actually found in the leaf litter where they will stay dormant until spring.
- Adults are shiny wasp-like “flies” (though they aren’t really flies) that don’t have the typical skinny waist of a wasp. They do however have typical wasp antennae. They are also small, 6-7 mm. Legs are yellow with white tarsi (the “feet”) and wings are brown.
ZigZag Sawfly and Elms
The elm zigzag sawfly relies on elm trees and when they are found outside of their native range, without any checks and balances, their population can quickly get out of control.
The thing with invasive species is they are essentially just trying to carry out their normal life cycle, but outside of their normal range. The problem here is that factors which would keep their population in check, like competitors and predators, are not always around in these new environments. With no one to slow them down, they can quickly multiply into bountiful populations, potentially causing harm to the native flora and fauna of that area.
In the case of the elm zigzag sawfly in North America, they do not have the same competition or predation that they would back home, and the elm trees here haven’t evolved the same defenses against them. These factors together make for a booming population of the insects that feed on elm leaves and leave the trees vulnerable.
What are the impacts of the elm zig zag sawfly?
Booming populations of zig zag sawfly means that elms are getting a lot of their leaves eaten. Trees have to use a lot of resources to grow leaves, and then they also need those leaves to help them create energy so that they can survive, continue to grow, and reproduce. If their leaves are under constant attack from a little army of hungry larvae then they may not be able to put enough energy towards growth and/or reproduction and could even become weak enough to become sick and die.
So far, American elms seem to be able to manage smaller populations of elm zig zag sawfly without being too impacted, as most plants are used to being munched on a bit after all. However, young trees are more vulnerable and we still do not understand how bad the problem could get if we do not do something about it. With that being said, what can be done to help?
Management
Invasive species management is very complicated and requires a lot of research to find solutions that can reduce and hopefully eradicate the invasive species without harming too many native plants and animals and without introducing any more issues.
Regulating movement of organic material
Like many modern problems, prevention of spread is the most important and effective. Researchers work to understand the ways that an invasive species spreads and then policy makers will hopefully do their best to apply this knowledge to reducing the spread of an invasive species. This is why there are strict rules about moving organic material like live plants and firewood from one area to another, whether it is a whole country or just within a small region. Checking that plants aren’t carrying potential invasive species before they are moved is a great start.
Research for control and eradication
In areas where an invasive species is already established, researchers must test various hypotheses for the control of an organism and then apply it to the field. The elm zig zag sawfly’s life cycle, mode of transport, and weaknesses to predation and/or chemicals and pesticides are being studied by researchers such as Dr. Kelly Oten, a forest entomologist, in order to control and hopefully get rid of this pest outside of its native range.
One example of a potential solution is exploring the relationship between potential predators like the assassin bug (Order hemiptera) to aid in controlling the population. This method is known as “biological control” and is implemented for other problematic invasive species such as the emerald ash borer. In parts of Europe sawfly populations have also been controlled by using different chemical applications to elm trees.
On an individual level, if you find sawflies on your elm tree you can remove them even by hand or use a tree-safe general insecticide when larvae are present. When in doubt it is best to contact your local extension or forestry office for advice.
Researchers do a lot for the control of this problem, but as resources are not always extensive and researchers are also not able to be in many places at once, the importance of citizen science comes into play. With help from the public, institutions can get more information and improve their data. Research of the zigzag saw fly for example is greatly improved through reporting of its presence by people throughout North America.
Monitoring
One of the most important parts of researching an invasive species for its control is to understand where it is currently and if it is continuing to spread. This can also help influence policy to concentrate resources on the areas that are most affected and on preventing the organism from establishing somewhere new.
Researchers do a lot of monitoring themselves but they can’t be everywhere, so this is where is is crucial to have people across the continent reporting sightings of elm zig zag sawfly if they see it.
What do I do if I find a zig zag sawfly? Reporting zig zag sawfly
If you see the classic zig zag chew patterns on leaves or see the insects themselves, take the best quality photos you can and report them to your local extension! You can also upload the pictures to iNaturalist as researchers also use this as a tool to collect data!
In Conclusion
The elm zig zag sawfly is an invasive insect that is attacking American Elms in Canada and the USA and needs to be closely monitored and controlled. The spread can be reduced by avoiding moving elm trees from areas with the sawfly to other regions and research is being done to help control and potentially eliminate them from the areas where they are established. Through research and a little citizen science we can understand more about this insect, how it lives, and how it spreads, in order to protect our beloved elm trees well into the future.