Michigan confirmed its first detection of the elm zigzag sawfly in St. Clair County on June 23, 2026. An arborist spotted the invasive insect and submitted a report through the Midwest Invasive Species Information Network (MISIN) online reporting tool, which was then verified by USDA APHIS. The pest poses a serious defoliation threat to elm trees across the state, and residents are now being asked to watch for its distinctive leaf damage and report any sightings.
Where It Was Found and Why It Matters
The Michigan Department of Agriculture and Rural Development (MDARD) confirmed the sawfly’s presence in St. Clair County, in the state’s eastern Thumb region near the Canadian border. This location fits the pest’s broader pattern of westward spread across the eastern United States. The elm zigzag sawfly is native to East Asia and was first detected in North America in 2020. It has since moved rapidly through multiple states, with heavy defoliation observed as far south as North Carolina.
Michigan is home to several native elm species, including American elm, which has already been under pressure from Dutch elm disease for decades. Adding a leaf-stripping insect to that burden raises concerns about the long-term health and survival of remaining elm populations, particularly in urban forests and riparian corridors where elms play an important ecological role.
How to Identify the Damage
The easiest way to spot this pest is by looking at elm leaves, not the insect itself. Young larvae chew a distinctive zigzag pattern along the leaf, creating a meandering channel that starts at the leaf margin and works inward. This feeding pattern is the insect’s namesake and is unlike damage caused by any native pest on elm trees in Michigan.
As the larvae grow, their feeding becomes less precise. Older larvae consume larger sections of leaf tissue, creating big notches and a patchy appearance. In heavy infestations, multiple larvae can strip a leaf down to nothing but the midvein and a few thicker lateral veins. During a severe outbreak in North Carolina in August 2022, entire trees were defoliated.
Look on the undersides of leaves first. That’s where the zigzag channels begin. If you see the pattern on elm leaves (and only elm leaves), there’s a strong chance you’re looking at elm zigzag sawfly damage.
What the Larvae and Cocoons Look Like
Newly hatched larvae are tiny, about 1.8 mm long, and grayish-white. They’re easy to overlook at this stage. As they mature over 10 to 18 days, they turn pale green and grow to about 10 to 11 mm (just over three-eighths of an inch). Mature larvae have a black band across the head and distinctive T-shaped brown or black markings above their second and third pairs of true legs. They also have multiple pairs of fleshy prolegs along the abdomen, which helps distinguish them from beetle larvae or caterpillars at a glance.
When larvae are ready to pupate, they spin small cocoons. During summer months, these are loosely spun, net-like structures typically attached to leaves or nearby objects like fence posts and garden structures. The overwintering generation produces a different type: solid-walled cocoons found in leaf litter or soil beneath host trees. Finding either type of cocoon on or near an elm tree is a useful secondary sign of an infestation.
Why This Pest Spreads So Fast
The elm zigzag sawfly reproduces through parthenogenesis, meaning females produce viable offspring without mating. No males are known to exist. This is a significant factor in how quickly the species can establish new populations: a single female arriving in a new area can start a colony on her own.
Adult females live only one to six days, but they begin laying eggs immediately after emerging and can deposit between 7 and 49 eggs in that short window. Eggs hatch in four to eight days. Larvae feed and develop through six growth stages over 15 to 18 days, then pupate for two to three days before new adults emerge four to seven days later. A single generation, from egg to adult, takes roughly 24 to 29 days.
In temperate climates, the sawfly can produce up to four generations per year, and under ideal conditions, as many as six. Adults are active from mid-April through mid-September, and larvae can be found feeding from late May into mid-October. That long activity window, combined with multiple overlapping generations, means defoliation can escalate quickly once a population takes hold. A tree that looks fine in early June can be heavily damaged by August.
How to Report a Sighting in Michigan
Michigan’s first detection happened because a trained arborist used the MISIN online reporting tool, and MDARD is encouraging all residents to do the same. MISIN (the Midwest Invasive Species Information Network) accepts reports from the public and routes them to the appropriate agencies for verification. You can submit a report through the MISIN website or its mobile app.
When submitting a report, include clear photos of the leaf damage, particularly the zigzag feeding pattern. If you can photograph the larvae themselves, that strengthens the report. Note the location as precisely as possible and whether the host tree is an elm. Reports don’t need to be perfect to be useful. Even a photo of suspicious zigzag-patterned leaf damage can prompt a follow-up visit from a specialist.
What Can Be Done About It
Management options for the elm zigzag sawfly are still being refined in North America. On individual trees, physical removal of larvae by hand or by pruning affected branches can reduce damage when infestations are caught early and populations are small. Checking the undersides of lower elm leaves starting in late May gives you the best chance of catching larvae before they multiply through subsequent generations.
For larger infestations, insecticide treatments targeting the larval stage are an option, though timing is critical given how fast the insect cycles through generations. Because multiple overlapping generations can be present on a single tree, a one-time treatment is unlikely to be sufficient. Removing leaf litter beneath infested elms in fall can help reduce the number of overwintering cocoons that survive to produce the first spring generation.
No established biological control agents for this sawfly exist in North America yet. In its native range in Asia, natural predators and parasitoids help keep populations in check, but those organisms are absent here. This gap in natural control is a major reason the species can reach outbreak levels so quickly after arriving in a new area.