Tree Sawflies: Pine Sawflies, Rose Sawflies, and How to Stop Defoliation

You walk out one morning in late April and a row of “caterpillars” has chewed half the needles off your mugo pine. You grab the BT spray that worked on last year’s tent caterpillars, soak the branch, and wait. Three days later the bugs are still eating. They aren’t caterpillars at all.

What sawflies are (and why they’re not caterpillars)

Sawflies are in order Hymenoptera, the same group as wasps and bees. The adults look like stingless wasps. Caterpillars are in order Lepidoptera, the larval stage of moths and butterflies. Different order, different gut chemistry, different treatment.

This matters because Bacillus thuringiensis kurstaki, sold as BT or BTK, only kills lepidopteran caterpillars. The bacterial toxin binds to receptors in the caterpillar midgut, and sawfly larvae don’t have those receptors. You can drench a sawfly cluster in BT and nothing happens. For a full pest workup that covers when each treatment applies, see the tree pest guide.

The diagnostic you can do with your own eyes comes down to prolegs, the fleshy stub legs on the abdomen behind the six true legs. Sawfly larvae have six or more pairs of prolegs and no crochets (no tiny hooks on the bottom). Caterpillars have five or fewer pairs, with crochets. Count them. If you see seven or eight pairs of stub legs, you’re looking at a sawfly.

The other tell is behavior. Sawfly larvae feed in tight clusters on one needle bundle or one leaf, side by side, like a small green congregation. Nudge them and the whole group rears up in S-shaped defensive curls. Tent caterpillars and most lepidopteran larvae spread out or build webbing. For the actual caterpillars where BT does work, see tree caterpillars.

Adult sawflies look nothing like the larvae and most homeowners never see them. They’re small, dark, stout-bodied wasps without a wasp waist. They don’t sting. The female has a saw-like ovipositor (the source of the common name) that she uses to slit open a needle or leaf and insert eggs. By the time you see damage, the adults are long gone and the larvae are the ones doing all the chewing.

Pine sawflies, the worst of the bunch

Three species cause most of the pine sawfly damage in landscape plantings. European pine sawfly (Neodiprion sertifer) is the one most homeowners run into. It feeds in spring, usually April through May, on mugo pine, Scots pine, Austrian pine, and red pine. Redheaded pine sawfly (Neodiprion lecontei) shows up later, July through September, and prefers young jack pine, red pine, and other hard pines. Swaine jack pine sawfly (Neodiprion swainei) hits jack pine specifically in the upper Midwest and Canada.

The damage pattern on pines is distinctive. The larvae cluster on a single needle bundle or whorl and eat the previous year’s needles down to brown stubs, often leaving the current year’s growth alone. That’s why a mugo pine hit by European pine sawfly looks half stripped from the inside out, with green fuzz on the branch tips and bare wood behind. The tree doesn’t usually die from one season of feeding because the new candles survive, but two or three years in a row will take it.

The S-curl is most obvious on pine sawflies. Brush a cluster with your finger and the whole row of larvae arches back at the same time. It’s a defensive display meant to startle birds. It works on birds. It does not work on a homeowner with a bucket of soapy water.

Penn State Extension and University of Wisconsin-Madison Extension both keep up-to-date factsheets on pine sawfly identification and timing. See Penn State Extension and University of Wisconsin-Madison Extension for state-specific monitoring guidance. If you also see needle browning, cankers, or oozing resin, run through the pine tree diseases guide because sawfly damage often shows up on trees already stressed by something else.

Rose and azalea sawflies

Rose sawflies don’t eat needles, they eat leaves, and the damage looks completely different. Bristly roseslug (Cladius difformis), European roseslug (Endelomyia aethiops), and curled rose sawfly (Allantus cinctus) all attack garden roses. The larvae scrape the top layer off the leaf surface, leaving translucent papery patches called window panes. From a few feet away the rose leaves look bleached or skeletonized.

Window-paning is the giveaway. Aphid damage is curled and puckered. Japanese beetle damage is full holes through the leaf. Window panes mean a sawfly larva sat on the top of that leaf and grazed off everything but the bottom epidermis. Cornell University covers rose sawfly identification at Cornell.

Azalea sawfly (Amauronematus azaleae) does the same window-pane scraping to azalea and rhododendron leaves. The larvae are pale green with a darker stripe and they blend in with the leaf, so people often see the damage before they see the bugs. Flip a few leaves and look on the undersides.

Rose sawfly feeding usually starts in May once the new flush of foliage is out. By June you’ll see the worst of it. A second generation can hit in July or August in warmer zones, so don’t put the sprayer away after the first round. Bristly roseslug, the species you’ll see most often in suburban gardens, has up to six generations a year in southern zones, which is why some rose growers in zones 8 and 9 deal with it from spring through fall.

Other sawflies worth knowing

Pear sawfly, also called pear slug (Caliroa cerasi), attacks pear, cherry, and ornamental plum. The larva is shiny black and slug-shaped because it covers itself in a layer of slime. It window-panes the leaves the same way rose sawflies do.

Elm sawfly (Cimbex americana) is the giant of the group, sometimes two inches long when fully grown. It hits elm, willow, birch, and maple, but rarely in numbers that defoliate a healthy tree.

Dogwood sawfly (Macremphytus tarquinius) skeletonizes dogwood leaves in midsummer. The larvae go through a chalky white powdered stage before turning yellow and black striped, which throws people off because the same insect looks like two different bugs at different ages.

Damage signs, what to look for

Sawfly damage shows up in clumps, not evenly across the tree. One branch of a mugo pine will be stripped to bare brown wood while the branch six inches over looks untouched. That gregarious feeding pattern is the first clue.

Below the feeding site you’ll find frass, the dark green or black pellets sawfly larvae drop as they eat. On a paved driveway under an infested pine it looks like coarse pepper. On lawn it disappears.

The defensive S-curl is the confirming tell. If you walk up to a cluster of green or pale larvae on a needle bundle or rose leaf, give the branch a light tap. If the whole row arches their back ends up in unison, you have sawflies. Caterpillars don’t do that.

Why BT doesn’t work and what does

BT, the bacterial spray sold as Thuricide, Dipel, Monterey BT, and a dozen other brands, only kills caterpillars in the order Lepidoptera. Sawflies are in Hymenoptera. The toxin doesn’t bind to their gut receptors. Spraying BT on a sawfly cluster is the same as spraying water. Save your money.

Here’s what actually works on sawfly larvae.

Physical removal. Sawfly larvae don’t run. They sit in tight clusters and chew. You can pick them off with a gloved hand, drop them into a jar of soapy water, and be done in ten minutes on a small mugo pine. A strong jet from a garden hose blasts them off the branch and most won’t crawl back. This is the most underrated method on the list because it’s free and it works the same day.

Insecticidal soap and horticultural oil. Both work on contact by breaking down the larva’s cuticle. Spray when larvae are small, ideally first or second instar, and reapply every five to seven days while feeding continues. Coverage matters. If you don’t hit the underside of the needle bundle or leaf, you’ll miss them. Bonide neem oil concentrate handles sawfly larvae if you spray when they’re young and you cover the cluster thoroughly. Older larvae are tougher and may need a second application.

Spinosad. This one trips people up because spinosad sits next to BT on the organic shelf at the garden store and gets lumped in with it. They’re different. BT is a bacterial protein. Spinosad is a fermentation-derived spinosyn compound that affects the insect nervous system. Spinosad kills sawflies. BT does not. Spinosad is also rough on bees when wet, so spray in the evening after pollinators have stopped foraging and let it dry overnight.

Systemic insecticides for high-value specimens. Acephate (Orthene) and imidacloprid are taken up by the tree and kill larvae as they feed. Reserve these for mature pines or rhododendrons you can’t afford to lose. They also kill pollinators that visit the tree, so don’t use them on anything in flower.

Beneficial wasps and predatory insects. Native parasitoid wasps lay eggs inside sawfly larvae. Ground beetles, ants, and birds also eat them. If you spray a broad-spectrum insecticide every time you see a bug, you wipe out the parasitoids and the sawfly populations rebound harder the next year. The healthier your garden ecosystem is, the less work the sawflies create for you. For pollinator-friendly yard practices that support those beneficial insects, see mklibrary.com on garden biodiversity.

Prevention

Healthy pines get hit less hard than stressed pines. Drought, compacted soil, and root damage from construction all push pines toward attractive-to-sawfly status. Water deeply once a month in summer if you’re in a dry zone 7 through 9, and mulch the root zone with two to three inches of bark.

In European pine sawfly territory, which is most of the Northeast and Midwest, walk your mugo pines and Scots pines in early April and inspect the previous year’s needles. The egg pockets, small brown slits cut into the needle by the female sawfly the previous fall, are visible if you look for them. Knock down any small larval clusters you find before they spread to the rest of the tree.

If you have room, plant a mix of conifer species rather than a row of identical mugo pines. Sawflies host-specialize. A single infested mugo in a mixed planting is a contained problem. A row of twelve identical mugos turns into a buffet. The same logic applies to spring scouting in general, which is covered in spring tree care tips.

Fall cleanup helps too. Many sawfly species overwinter as prepupae in cocoons in the leaf litter or topsoil beneath the host plant. Rake up fallen rose leaves in November, dispose of them in the trash rather than the compost, and you’ll lower the population that emerges to lay eggs the next spring. The same goes for pine needle litter directly under infested mugo pines.

When to call an arborist

For a tall mature pine with heavy infestation across the whole crown, especially in a region where European pine sawfly or redheaded pine sawfly has established populations, you need someone with a sprayer that can reach 40 feet up. That’s an arborist call, expect $200 to $500 depending on tree size and access.

For a mugo pine in your front yard, a few rose bushes, or an azalea by the porch, this is a hand-pick and soap-spray job. You can handle it in an afternoon.

FAQ

Why doesn’t BT work on sawflies? BT is Bacillus thuringiensis, a bacterium that produces a toxin specific to the gut receptors of caterpillars in the order Lepidoptera. Sawflies are in the order Hymenoptera (wasps and bees) and don’t have those receptors. The bacterial toxin passes through them without effect.

How do I tell sawflies from caterpillars? Count the prolegs, the fleshy stub legs on the abdomen behind the six front legs. Sawfly larvae have six or more pairs. Caterpillars have five or fewer. Sawflies also cluster tightly on one leaf or needle bundle and rear up in S-curls when disturbed. Caterpillars typically don’t.

What kills sawfly larvae? Insecticidal soap, horticultural oil, neem oil, and spinosad all work on contact. Physical removal by hand or with a strong water jet works the same day. For mature specimens, systemic insecticides like acephate or imidacloprid kill larvae as they feed. BT does not work.

Are sawflies caterpillars? No. Sawflies are larvae of stingless wasps in the order Hymenoptera. They look like caterpillars because both are leaf-eating larvae with chewing mouthparts, but they’re in a completely different insect order with different biology. The visible difference is the number of prolegs.

Will sawflies kill my pine tree? One season of European pine sawfly feeding usually won’t kill a mature mugo or Scots pine because the larvae prefer the prior year’s needles and leave the current candles alone. Two or three consecutive years of heavy feeding can kill a tree, especially a young one. Redheaded pine sawfly is more damaging because it eats both old and new needles.

References

Penn State Extension, University of Wisconsin-Madison Extension, University of Minnesota Extension (extension.umn.edu), and Cornell University all maintain current sawfly identification and management resources. State extension factsheets are the most reliable starting point for timing and treatment specific to your region.