Sawflies might look like small wasps, but these flying pests are plant-eating machines—especially in their larval stage. While adult sawflies are mostly harmless, theiIf something is skeletonizing your roses or stripping your columbine and the Bacillus thuringiensis you sprayed did nothing at all, you almost certainly have sawflies. And the reason the Bt failed is the single most useful thing on this page, so let’s start there.
Sawfly larvae look exactly like caterpillars. They are not caterpillars. They are the larvae of a wasp — Hymenoptera, the order that holds bees, wasps and ants — and Bt kurstaki is specific to caterpillars. It will not touch them.
Why Bt doesn’t work, and why it isn’t your fault. Btk products — Dipel, Thuricide and the rest — are selective for the insect group named on the label, and that group is Lepidoptera: moths and butterflies. Sawflies are in a different order entirely. Three separate university extension programmes state this flatly, and it’s worth repeating because Bt is the default organic answer for anything caterpillar-shaped: Bt is not effective against sawfly larvae because they are not caterpillars.
So if Bt isn’t working on something caterpillar-shaped, your first move isn’t more Bt or stronger Bt. It’s to count its legs.
Sawfly or caterpillar? Count the prolegs
This is a ten-second test and it decides your whole treatment plan.
Both sawfly larvae and caterpillars have three pairs of hard, jointed true legs right behind the head. Those don’t help you — everybody has them. What you want are the prolegs: the soft, fleshy stubs further back along the abdomen.
| Sawfly larva | Caterpillar | |
|---|---|---|
| Pairs of abdominal prolegs | Six or more (often seven or eight) | Five or fewer |
| Tips of the prolegs | Smooth — no hooks | Ringed with tiny hooks (crochets) |
| What it becomes | A wasp-like fly that doesn’t sting | A moth or butterfly |
| Does Bt work? | No | Yes |
Count only the prolegs on the abdomen and ignore the pair right at the very back end. Entomologists at Ohio State teach it as a hand check: hold up one hand, and if the larva has more pairs of prolegs than you have fingers, it’s a sawfly.
If you can get a closer look, the hooks are the more reliable tell. Caterpillar prolegs are tipped with rings of tiny hooks that grip — it’s how a caterpillar clings so stubbornly. Sawfly prolegs are plain fleshy nubs, which is also why a jet of water knocks sawfly larvae off a plant so easily.
And if you want to confirm from the adult: sawflies are usually half an inch or less, blackish or brownish with orange, red or yellow markings, and they have no wasp waist. The abdomen is broadly joined to the thorax rather than pinched. They don’t sting. Look for them resting on leaves early in the morning.
Find your sawfly by what it’s eating
There are over 800 sawfly species in North America, but they’re specialists — so the plant it’s on narrows it down fast. The damage is nearly always one of three patterns: skeletonizing (leaf tissue eaten away between the veins, leaving a lace or window-pane effect), wholesale defoliation, or boring and galling inside stems and leaves.
| Host plant | Sawfly | What you’ll see |
|---|---|---|
| Roses | Roseslugs (rose sawflies) | Classic window-paning — leaves eaten down to a translucent skeleton between the veins. Young larvae work on leaf undersides |
| Pear, cherry | Pear sawfly, a.k.a. pear slug or cherry slug Caliroa cerasi | A serious skeletonizer. The larvae are dark, shiny and slug-like — most people don’t believe they’re insects at all |
| Raspberries, blackberries | Raspberry sawfly | Leaf feeding on caneberries |
| Caneberries, roses, salmonberry, thimbleberry | Raspberry horntail | Different problem entirely — larvae bore inside the stems, so you get shoot dieback with no visible larva |
| Pines (Scots, Mugho, Red, Jack, Swiss Mountain — nearly all pines) | European pine sawfly | Black head, green body with light and dark stripes. Groups feed on old needles first, eating the outer layer and leaving the core to dry on the branch |
| Cypress, juniper | Cypress and juniper sawflies | Foliage feeding |
| Conifers (various) | False webworms / webbing sawflies | Gregarious, and they spin silken nests packed with cast skins, frass and discarded food. This is the one sawfly group that really does make webbing |
| Rose mallow hibiscus Hibiscus moscheutos | Hibiscus sawfly Atomacera decepta | Skeletonizes, feeding in groups. Very damaging to susceptible varieties — Luna, Belle, Lord Baltimore, Lady Baltimore |
| Hollyhock | Hollyhock sawfly Neoptilia malvacearum | Light green larvae, skeletonizing leaves. Hibiscus sawfly will also feed here |
| Columbine | Columbine sawfly Pristiphora aquilegiae | Pale green larvae in groups, eating leaf tissue but not the veins. One generation a year, in spring, and it can defoliate a columbine fast |
| Currants, gooseberries | Imported currantworm | Will literally strip the foliage off a currant bush |
| Dogwood | Dogwood sawfly | Leaf feeding |
| Willow | Willow gall sawflies Pontania spp., willow leaf sawfly | Not chewing at all — larvae feed inside the tissue, raising colourful swellings and fleshy bumps on the leaves |
| Alder | Alder woolly sawfly | Leaf feeding |
Sawflies are also reported on birch, oak, poplar, azalea and mountain ash. If you find caterpillar-shaped larvae feeding in a tight group on any of these, run the proleg test before you reach for anything.
A useful hibiscus note nobody mentions: hibiscus sawfly attacks Hibiscus moscheutos — the hardy perennial rose mallows — and it does not feed on Hibiscus syriacus (Rose of Sharon) or Hibiscus rosa-sinensis (the tropical florist hibiscus). If something is chewing one of those, it isn’t this.
Before you treat: do you actually need to?
This is the part most pest advice skips, and it will save you more time than any spray.
Sawfly larvae are heavily predated. Parasitic wasps, predaceous beetles, birds, small mammals, spiders, and naturally occurring fungi and viruses keep most populations low without any help from you. A sawfly outbreak in a diverse garden is frequently over before you’ve finished deciding what to do about it.
Washington State University’s threshold is a good one to adopt: management is usually unwarranted unless 20% or more of the foliage is threatened. Twenty percent is more damage than you’d think — a rose with some window-paned leaves in July is a rose that will be fine.
Two situations do justify acting quickly:
- Young conifers. Most conifers rarely re-leaf, so repeated defoliation can kill them outright. European pine sawfly starts on old needles, which buys you time, but once those are gone the larvae move onto new growth — and that’s when it becomes serious.
- Herbaceous perennials that can be stripped in days. Columbine and hibiscus sawflies work in groups and move fast.
Organic control, in the order worth trying
- Knock them off with water. Start here, because sawfly prolegs have no hooks — they can’t grip and they can’t climb back up. A firm jet from the hose removes a colony in seconds and costs nothing. Go easy on soft new growth and delicate perennials, and do it in the morning so foliage dries.
- Handpick. Check leaf undersides morning or evening, where the young larvae feed. Wear gloves or use a soft brush and drop them into soapy water. Because most species feed in tight groups, one pass takes out most of the population — this is far more effective on sawflies than on scattered pests.
- Clip out what’s badly hit. If the infestation is confined to a few leaves or one stem, cut it off and bin it rather than treating the whole plant. Don’t compost infested material.
- Insecticidal soap, horticultural oil or neem. Openly feeding sawfly larvae are soft-bodied and easy to kill — almost anything contact-based works on them, so there’s no reason to escalate beyond these. Apply early morning or late afternoon to avoid leaf burn, cover leaf undersides where they actually are, and reapply after rain. Timing matters more than product: these work well on young larvae and poorly on large ones. Catch the colony early and soap is plenty.
- Spinosad, if you need more. Effective against sawflies, and the right next step up. But read this first.
Spinosad and bees. Spinosad stays toxic to bees for several hours after it dries, so do not apply it to plants that are flowering. That restriction bites hard on this particular pest list — roses, hibiscus, hollyhock and columbine are all things you grow for the flowers, and sawflies turn up right when they’re blooming. In practice this means water, handpicking and soap do most of the work on flowering plants, and spinosad is for conifers and for shrubs between bloom cycles.
And skip the broad-spectrum persistent insecticides entirely — carbaryl, malathion, pyrethroids. They kill the parasitic wasps and predatory beetles that were controlling your sawflies for free, which is how a one-season problem becomes an annual one.
Breaking the cycle for next year
Most sawflies overwinter in one of two places, and knowing which one tells you what to do in autumn.
- As eggs inserted into plant tissue. The female’s saw-like ovipositor — which is where the name comes from — cuts a slit in a leaf, needle or stem and she lays inside it. European pine sawfly leaves distinctive crescent-shaped slits in conifer needles, so you can find next spring’s problem on the tree in winter and prune it out.
- As prepupae in leaf litter and the top layer of soil beneath the host plant. For these, raking up and removing the litter under affected plants in late autumn is the cultural control that works.
Then through the season: inspect susceptible plants weekly in spring and early summer, which is when nearly all of this happens. Check leaf undersides, since that’s where young larvae start. Most species have one generation a year, though some run several — so finding roseslugs in May doesn’t mean you’re done for the summer.
Things that get recommended but won’t carry the job
- Bt. Covered above, and worth repeating because it is the single most common wasted effort on this pest.
- Companion planting. Garlic, chives, tansy, yarrow, lavender and mint all get recommended for repelling sawflies, and there’s no solid evidence that any of them does. Where companion planting genuinely helps here is indirect: flowering plants that feed adult parasitic wasps support the natural enemies doing the real work. So plant yarrow — just plant it to feed the predators, not to repel the sawflies.
- Mulching to suppress overwintering larvae. Adding mulch doesn’t reliably stop prepupae emerging. Removing the existing litter in autumn is the version that works.
- Waiting to see if it gets worse, on a young conifer. This is the one case where the wait-and-see advice above doesn’t apply. Conifers don’t re-leaf.
Related pest guides
- Caterpillars — the ones Bt does work on
- Weevils, beetles, aphids and whiteflies
- Beneficial nematodes and birds in the garden
- Growing berries — raspberries, currants and gooseberries all have their own sawfly
- All organic pest management guides
Common questions
Why didn’t Bt kill my sawflies?
Because Bt kurstaki only affects caterpillars — moth and butterfly larvae. Sawflies are wasp larvae, a completely different insect order, and Bt products are selective for the group named on the label. It isn’t a dose problem and a second application won’t help.
How do I tell a sawfly larva from a caterpillar?
Count the fleshy prolegs along the abdomen, ignoring the pair at the very back. Six or more pairs means sawfly; five or fewer means caterpillar. If you can see closely, caterpillar prolegs are tipped with rings of tiny hooks and sawfly prolegs are smooth.
Do sawflies sting?
No. The “saw” is an egg-laying organ for cutting slits in plant tissue, not a stinger. The adults are harmless to you, and so are the larvae.
My rose leaves look like lace. Is that sawflies?
Very likely roseslugs. They eat the tissue between the veins and leave a translucent window-pane effect, usually starting on leaf undersides. A firm spray of water is the first thing to try — and since your roses are probably in bloom, it’s also the thing to try instead of spinosad.
Should I treat at all, or will it sort itself out?
Often it sorts itself out. Natural enemies keep most sawfly populations low, and the usual threshold is that treatment isn’t warranted until 20% or more of the foliage is threatened. The exceptions are young conifers, which can be killed by repeated defoliation because they rarely re-leaf, and fast-stripping perennials like columbine and hibiscus.
What’s eating my pine needles in groups and rearing up when I touch the branch?
European pine sawfly. The larvae have black heads and green bodies with light and dark stripes, and they feed in colonies. They start on the old needles, which gives you time, but handpick or hose them off before they move onto the new growth.
Something is skeletonizing my hibiscus but not my Rose of Sharon. Why?
Because hibiscus sawfly feeds on hardy rose mallow hibiscus and not on Rose of Sharon or tropical hibiscus. The susceptible varieties include Luna, Belle, Lord Baltimore and Lady Baltimore.
My raspberry canes are dying back but I can’t find any larvae.
Look at raspberry horntail rather than raspberry sawfly. Its larvae bore inside the stems, so there’s nothing on the outside to find. Cut out and destroy the affected canes — sprays won’t reach it.
Can I spray spinosad on my flowering roses?
Don’t. Spinosad remains toxic to bees for several hours after drying and shouldn’t be applied to flowering plants. Use water, handpicking or insecticidal soap on anything in bloom.
Sources
University of Illinois Extension. Sawfly. — European pine sawfly life cycle, hosts and damage.
Ohio State University Extension, Buckeye Yard & Garden onLine. Is it a Sawfly Larva or a Caterpillar? — the proleg-counting method.
UC Statewide IPM Program. Sawflies. — species and hosts, adult identification, treatment options, spinosad and bee precautions.
Washington State University Puyallup. Sawflies (PLS-64). — proleg hooks, species list, and the 20% treatment threshold.
University of Connecticut IPM. Sawflies on herbaceous perennials. — columbine, hollyhock and hibiscus sawflies.
