Elm zigzag sawfly (Aproceros leucopoda)

Common Names: Zigzag elm sawfly, zig-zag elm sawfly

Description:

Elm zigzag sawfly (Aproceros leucopoda) is a small, non-stinging relative of wasps from East Asia whose larvae feed only on elm trees [1][4]. The young larvae chew a distinctive zigzag path across elm leaves, and heavy infestations can strip a tree's leaves down to the veins [1][4]. MISIN tracks it because it is new to North America ? first found in 2020 ? spreads quickly, and threatens both landscape and forest elms [1][2].

Identification:

The easiest sign of this pest is not the insect but its feeding trail. From late spring through fall, young larvae eat a narrow, back-and-forth zigzag line into elm leaves, starting at the edge and working inward [1][3]. The larvae are small ? up to about 1/2 inch long ? and look like pale green caterpillars, with a black band on the head and dark T-shaped marks near the front legs; they often feed in groups along the leaf veins [1][3]. The adult is a tiny shiny-black insect under 1/3 inch long, with pale legs and smoky wings, and it does not sting or bite people or pets [1]. As the larvae grow, they chew broader patches and can erase the zigzag pattern, leaving leaves eaten down to the veins [1]. Look for fresh zigzag trails in midsummer, when feeding is heaviest.

At-a-glance cues:

1. Narrow zigzag trail eaten into elm leaves from the edge inward [1][3]
2. Pale green, caterpillar-like larvae with a black head band and T-shaped leg marks, feeding in groups [1]
3. Six or more pairs of fleshy belly legs (prolegs) ? true caterpillars have five pairs or fewer [3]
4. Found only on elm trees; heavy feeding leaves just the leaf veins behind [1][4]

Hosts:

Elm zigzag sawfly feeds only on elm trees (Ulmus species) [1][4]. Confirmed hosts include American elm, slippery elm, winged elm, Siberian elm, Chinese elm, lacebark elm, English elm, and many elm hybrids and cultivars [1][2]. It is native to East Asia ? Japan, China, the Korean Peninsula, and eastern Russia [1][4]. It reached Europe by 2003 and North America by 2020, first in Quebec, then Virginia in 2021, and has since been found in more than ten states, mostly in the Northeast and upper Midwest [1][2]. It attacks elms wherever they grow ? yards, street plantings, parks, hedgerows, and forests [1][4].

Life Cycle:

This insect spreads remarkably fast, for two reasons. First, every individual is female and lays fertile eggs without mating, so a single arriving larva can start a whole new population [1][3]. Each female lays up to about 60 eggs, one at a time, along the tiny teeth at the edges of elm leaves [3]. Second, the adults are strong fliers that spread on their own across the landscape, and people speed this up by moving infested elm nursery stock, firewood, or leaf debris ? the tiny insects can also hitch a ride on vehicles [3][4].

Eggs hatch in about a week, and the larvae feed and grow through six stages over roughly three weeks before spinning a cocoon [1][3]. In warm areas there can be four or more overlapping generations in a single year, so numbers build quickly through the summer [1][3]. The sawfly spends the winter as a tough cocoon in leaf litter or soil and emerges the next spring [1]. For control timing: the young, leaf-feeding larvae appear in spring as elm leaves expand, and that early window is when removal does the most good [1][3].

Impact and Damage:

This sawfly does not harm people or pets ? it cannot sting or bite [1]. Its damage is to elm trees. A single year of feeding rarely kills a healthy tree, but repeated heavy defoliation, year after year, weakens elms and leaves them open to other pests, diseases, and dieback; it can eventually kill young or already-stressed trees [1][2].

On yard and street elms, an outbreak can chew leaves down to the veins and leave the canopy looking brown and ragged by midsummer [1].

In natural areas, elm is an important part of floodplain forests, hedgerows, and the urban tree canopy. In Europe, where the sawfly has been established longer, outbreaks have stripped 74 to 93 percent of the leaves from infested elms, with complete defoliation in some cases [4]. Leaf loss on this scale reduces shade, food, and habitat for wildlife that rely on elms and adds further stress to elm populations already hit hard by Dutch elm disease [4][1]. Repeated outbreaks also raise the cost of caring for landscape and street elms through added monitoring and treatment [1].

Similar species:

Elm zigzag sawfly is most often confused with other leaf-feeding insects on elm rather than with a single look-alike species.

Moth and butterfly caterpillars are the most common mix-up, because the sawfly's pale green larvae look much like them. The reliable difference is the number of fleshy belly legs (prolegs): the sawfly larva has six or more pairs, while moth and butterfly caterpillars have five pairs or fewer [3].

Leaf-mining and other leaf-chewing insects can also be confused with it, because they carve winding channels or chewed patches in leaves. As the sawfly's larvae mature, they eat broad areas that erase the early zigzag pattern, so the damage starts to look like other insects' feeding; checking for several fresh zigzag trails on the same tree helps confirm it [1].

Monitoring and rapid response:

Signs to watch for:

1. Narrow zigzag trails eaten into elm leaves from late spring onward [1][3]
2. Groups of small, pale-green, caterpillar-like larvae with black head bands feeding along leaf veins [1]
3. Elm leaves chewed down to the veins, or a thinning, ragged canopy by midsummer [1]

What you can do:

1. On a small tree or low branches, pick or knock the larvae into a container of soapy water, or blast them off with a stream of water from a hose; repeat through the season as new generations appear [1][3]
2. Rake up and bag fallen leaves and litter under infested elms to remove overwintering cocoons [3]
3. Do not move elm firewood, nursery stock, or yard waste off your property, since this carries the insect to new areas [3][4]
4. For an otherwise healthy tree, tolerating light feeding is often the best choice ? elms usually releaf and recover from a single defoliation [1][2]
5. Natural area crews can monitor elm stands, flag and report new outlying detections early, and focus effort where elms are already stressed [1]

When to call a professional:

1. Large or repeatedly defoliated elms, especially specimen or street trees whose health is declining [1][2]
2. Tall trees where the canopy cannot be safely reached from the ground [1]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. NC State Extension. Elm Zigzag Sawfly (Aproceros leucopoda). content.ces.ncsu.edu
2. Minnesota Department of Agriculture. Elm Zigzag Sawfly. mda.state.mn.us
3. UMass Center for Agriculture, Food, and the Environment. Elm Zigzag Sawfly (Landscape Fact Sheet). umass.edu
4. Blank SM, Hara H, Mikulas J, Csoka G, Ciornei C, Constantineanu R, Constantineanu I, Roller L, Altenhofer E, Huflejt T, Vetek G (2010). "Aproceros leucopoda (Hymenoptera: Argidae): An East Asian pest of elms (Ulmus spp.) invading Europe." European Journal of Entomology 107(3):357?367. semanticscholar.org

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