Sawfly damage ranges from small chewed spots on leaves to complete defoliation of branches, depending on the species and how large the infestation has grown. The larvae are the culprits. They look like small caterpillars and feed voraciously on leaf tissue, often leaving behind skeletonized or lacy-looking foliage that’s hard to miss. If you’ve noticed leaves on your roses, pines, birches, or fruit trees that look like something ate everything but the veins, sawflies are a likely explanation.
What Sawfly Damage Looks Like
Sawfly larvae feed in predictable patterns that change as the larvae grow. Young larvae often start by scraping the surface of a leaf from the underside, creating translucent “windowpane” patches where only a thin layer of tissue remains. As the larvae get bigger, they chew all the way through, leaving holes or consuming everything except the major veins. This gives heavily damaged leaves a lacy, skeletal appearance.
On roses, the progression is especially clear. Early feeding skeletonizes leaves from below, and later feeding removes entire leaflets down to the largest veins. Hibiscus sawfly larvae produce the same lacy pattern on hibiscus, hollyhock, and mallow. On pines, the damage looks different: larvae consume needles in clusters, and heavy infestations can strip individual branches bare. Large infestations of redheaded pine sawfly can defoliate and kill small pines entirely.
Birch leafminer sawflies work differently still. Their larvae tunnel between the upper and lower surfaces of birch leaves, creating brown, blister-like mines rather than the chewed edges you’d see from external feeders. The result is papery brown patches across the leaf surface.
Plants Most Commonly Affected
Sawflies are a large group of insects, and different species target different plants. Here are the most common pairings:
- Roses: Three separate sawfly species attack roses. The roseslug feeds only on roses, while the bristly roseslug (which can produce two to six generations per year) and the curled roseslug also target them heavily.
- Pines: European pine sawfly is especially common on mugo, Scots, red, and jack pine. White pine sawfly targets eastern white pine.
- Birch: Birch leafminer attacks European, white, gray, and paper birches. The dusky birch sawfly also feeds exclusively on birch.
- Fruit trees: Pear sawfly (sometimes called pear slug) feeds on pear, apple, plum, cherry, cotoneaster, and hawthorn.
- Oaks: Scarlet oak sawfly is most common on northern red oak and pin oak.
- Other ornamentals: Dogwood sawfly feeds on ornamental and native dogwoods. Mountain ash sawfly is the most common insect pest on mountain ash. Columbine sawfly targets columbines. Elm sawfly prefers elm and willow but sometimes attacks maple, cottonwood, poplar, and birch.
How to Tell Sawfly Larvae From Caterpillars
This distinction matters because many common pesticides that target caterpillars (like Bt, or Bacillus thuringiensis) do not work on sawfly larvae. Sawflies are actually related to wasps, not moths or butterflies, so their larvae respond to different treatments.
The easiest way to tell them apart is to count the fleshy, leg-like stubs (called prolegs) along the abdomen. True caterpillars have five or fewer pairs of prolegs on their abdominal segments. Sawfly larvae have six or more pairs. If you flip a larva over and count more than five pairs of those stubby legs along the belly, you’re dealing with a sawfly. Sawfly larvae also tend to feed in groups, curling their bodies into an S-shape when disturbed.
When Damage Is Most Likely
Female sawflies use a saw-shaped egg-laying organ to cut into leaf tissue and deposit eggs inside. Eggs typically hatch in about five days, and the larvae feed for roughly two weeks before dropping to the ground to pupate. Depending on the species, adults emerge from mid-spring through the end of summer. Some species, like the bristly roseslug, produce multiple generations per year, meaning you can see fresh waves of damage from spring well into fall.
Pine sawfly damage tends to be most visible in spring when the first generation of larvae hatches and feeds on older needles. Rose and deciduous tree damage often peaks in late spring or early summer, though second and third generations can cause repeated rounds of feeding throughout the growing season.
Damage to Crops and Yield Loss
For home gardeners, sawfly damage is mostly cosmetic on established trees and shrubs. A healthy mature tree can tolerate significant defoliation and leaf out again the following year. Repeated years of heavy defoliation, however, weaken plants and can eventually kill smaller specimens, particularly young pines.
In agriculture, the economic stakes are higher. Wheat stem sawfly is one of the most damaging agricultural pests in the northern Great Plains. Unlike leaf-feeding sawflies, its larvae bore into wheat stems, weaken them from the inside, and cause the stems to fall over (lodge) before harvest. Montana State University estimates that farms in heavily infested areas lose an average of 13.5% of winter wheat yield and 11.5% of spring wheat yield. Infested hollow-stemmed wheat varieties can yield 15 to 25% less than uninfested plants.
Managing Sawfly Damage in Gardens
For ornamental plants, the simplest approach is handpicking. Sawfly larvae feed in clusters, so you can often remove dozens at once by picking them off or knocking them into a bucket of soapy water. This works well for roses, columbine, and small trees where the larvae are within reach. Check the undersides of leaves, since many species start feeding there.
A strong spray of water from a garden hose can dislodge larvae from shrubs and low tree branches. Knocked-off larvae rarely climb back up to resume feeding. For larger infestations, insecticidal soap and spinosad (a naturally derived insecticide) are both effective against sawfly larvae. Products combining these two active ingredients specifically list sawfly larvae, including pear and rose slugs, as target pests. Apply thoroughly to all plant surfaces, including leaf undersides, and spray in the early morning, evening, or on overcast days to avoid plant stress and leaf burn. Avoid spraying delicate plants like sweet peas, nasturtiums, or ferns.
Remember that Bt products, which are popular organic caterpillar treatments, do not kill sawfly larvae. Because sawflies are related to wasps rather than moths, Bt is ineffective against them. Spinosad-based products are the better organic option.
Managing Sawfly Damage in Wheat
Wheat stem sawfly requires a different set of strategies since the larvae feed inside the stem where sprays can’t reach them. The most effective approach is planting solid-stemmed wheat varieties. Sawflies can still lay eggs in solid-stemmed wheat, but far fewer larvae survive, and the stems rarely lodge. Variety selection involves tradeoffs: the traditionally best-performing solid-stem variety, Warhorse, matures later and yields significantly less (42 bushels per acre in one trial versus 63 for conventional varieties). Newer options offer better yield while still resisting lodging.
Crop rotation helps, particularly when new wheat fields are planted away from the previous year’s infested stubble. Sawflies tend to infest fields most heavily when they’re adjacent to last year’s wheat residue. Tillage before sawfly emergence in May can also reduce populations by burying the cocoons where larvae overwinter inside crop stubble. Plowing or discing is more effective than minimal tillage, since the goal is to get stubble fully underground. Adjusting planting dates to avoid the peak egg-laying window shows some promise, though this strategy hasn’t been well quantified yet.