Nature
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The elm zigzag sawfly is not a fly, but it is a problem. Since first being found in North America back in 2020, the species has now spread into 15 US states and four Canadian provinces, chomping its signature zigzag pattern into the continent’s elm tree leaves. And the weirdest part? They’re doing it all without a male in sight.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.“Six years into its North American invasion, the elm zigzag sawfly (Aproceros leucopoda) is making waves – or, more appropriately, zigzags – across the continent,” wrote Kelly Oten, an entomologist at North Carolina State University and lead author of a recent paper on the insect’s alarming spread.
And while the feeding pattern of their larvae might look cute, “don’t let the cute factor distract you,” she cautioned in an accompanying article for Entomology Today. “As they grow, they can strip foliage down to the veins, defoliating entire trees when populations are high enough. Repeated heavy feeding over several years can weaken trees, causing branch dieback and reduced growth, likely worsened when combined with other stressors like drought or disease.”
Unfortunately, their spread seems so far to be unstoppable. The bugs – technically a type of wasp – have worked out two huge advantages to help in their takeover of North America. First, they’ve branched out (ha) from elms, adding the Japanese zelkova tree to their menus – a finding which, according to Véronique Martel, a forest entomologist with Natural Resources Canada, is surprising.
“It is rare that insects can switch hosts,” Martel, who wasn’t involved in Oten’s study but reported the first case of an elm zigzag sawfly in North America back in 2020, told Science News. With a new source of food – particularly one which, at least this year, leafed earlier in the season than elms – the sawflies “can make a lot of generations within a summer,” she said.
But it’s the second talent that makes the sawflies truly unstoppable: they can reproduce on their own – no males required.
That’s right: Canada and the US are basically at war with an all-female insect clone army. It’s called “thelytokous parthenogenesis” – Greek for like, “female-birth virgin-creation”, and basically exactly what that makes it sound like: females lay eggs, which remain unfertilized and hatch only other females. So far, not a single male has been found.
It’s interesting and cool, but it’s also a big problem. “[It] means even a single insect can start an entirely new infestation,” wrote Oten, whose team documented sawfly cocoons attached to trucks and building materials like wood during their project. While the spread of the bugs was to a certain extent inevitable – adults can actively disperse 45 to 90 kilometers per year, the team points out – it seems humans are unwittingly helping the little critters move much further and faster than they would normally be able to.
And, with their preferred food native to most of the eastern US and Canada, these little ladies are likely to set up shop wherever they end up. “I do think we will have many more reports in additional counties and likely more states,” Oten told Science News. “Right now, it’s North Carolina to Canada. That’s pretty wide.”
So, what can be done? Well, for now, not much. While Oten is investigating which pesticides are best for curtailing the sawflies – the results should be published within months, she said – the current advice is just to stay alert for cocoons and leafy zigzags.
“Keep your eyes open for that telltale zigzag pattern on elm or zelkova leaves, especially in spring and early summer, and report sightings to your local extension office or state forestry agency,” Oten advised. “The elm zigzag sawfly may have style, but this is one trend we don’t want catching on.”
The study is published in the Journal of Integrated Pest Management.
