If you work with pines long enough, eventually you’ll run into these tiny troublemakers. It starts with a pine needle that looks… off. A little row of bumps, barely raised, like someone embedded a string of tiny pearls into the needle. Most folks wouldn’t give it a second glance. But for anyone growing pine bonsai—especially Scots pine—these aren’t pearls. They’re warning signs. Sawfly eggs.
These pests may not have the name recognition of spider mites or scale, but don’t let that fool you. Pine sawflies can be absolutely devastating to a pine bonsai if left unchecked. And unlike many pests that prefer weak or stressed trees, sawflies don’t care. Healthy trees, refined trees, trees in development—they’ll go after any pine they can find.
The good news is, once you know what to look for, they’re easy to spot and relatively simple to deal with. The bad news is, if you miss that first generation, they can multiply fast.
Let’s get into how to identify sawflies, what to do if you find them, and how to keep them from ruining all your hard work.
What Exactly Is a Pine Sawfly?
Despite the name, sawflies aren’t flies. They’re non-stinging wasps, part of the Hymenoptera order. The adult female resembles a short, stout wasp and uses a saw-like ovipositor to slice into pine needles and insert eggs1. She’s not flashy, but she’s efficient—and her eggs are hidden in plain sight.
You’ll usually find the eggs arranged in a single line along a pine needle, often slightly embedded beneath the needle surface. On Scots pine (Pinus sylvestris), that line looks like a zipper—smooth, evenly spaced, and disturbingly symmetrical.
These eggs hatch in 1–3 weeks, depending on species and temperature, and give rise to larvae that strongly resemble caterpillars2. These aren’t your usual lepidopteran larvae, though, and that distinction matters when it comes to treatment.
The larvae feed in groups and move together, quickly stripping needles. If left unchecked, they can defoliate entire whorls of branches in just a few days. On a bonsai, where every needle is chosen and every branch has a job, that’s a serious blow.
Species to Watch
There are dozens of sawfly species worldwide, but a few are common offenders for bonsai growers in North America:
- European pine sawfly (Neodiprion sertifer): Widespread and especially fond of Scots pine, this species lays eggs in the fall, which overwinter and hatch in early spring. Larvae are olive green with black heads and feed in dense clusters3.
- Redheaded pine sawfly (Neodiprion lecontei): More common in the southeastern U.S., larvae are larger, with reddish heads and feed later in the season. They attack a broader range of pines.
- White pine sawfly (Neodiprion pinetum): Prefers eastern white pine, but occasionally goes after Scots pine. Larvae are yellowish with dark spots.
Know your enemy’s timing: species that overwinter eggs may hatch in spring; others feed later. Timing your inspections matters.
How to Identify a Sawfly Infestation
Early detection is everything. Most infestations start small—hidden in the interior or lower branches. And in bonsai, even minor damage can set you back a year or more.
Here’s what I look for:
- Egg scars: Straight rows of small swellings or slits along a needle. It can look like the needle is stitched.
- Larvae: They look like caterpillars but curl into an “S” shape when disturbed. Groups raise their heads in unison—a dead giveaway.
- Chewed needles: Feeding usually starts at the tip, leaving only the base.
- Frass: Tiny black droppings under the feeding site or on the soil surface.
If you find one cluster, assume there are more—keep searching.
Why Sawflies Are a Bigger Deal in Bonsai
In a forest or park, a pine can lose thousands of needles and recover. In bonsai, every needle counts. If a sawfly strips a branch’s interior, it may never backbud. Timing matters too—midseason defoliation weakens energy flow, stresses summer growth, and compromises fall bud development. For trees in refinement or development, it’s a season—or more—lost.
Treatment and Eradication
Catch the eggs before they hatch and you’ve won. I remove egg‑laden needles and destroy them—not composting. Too risky.
Once larvae hatch, the clock starts ticking:
- Manual removal: The gold standard. Flick them into soapy water. They don’t survive off the tree.
- Insecticidal soap or horticultural oil: Works only on young larvae and with thorough coverage. Always test sprays on a few branches before treating the entire tree.
- Spinosad-based insecticides: Effective organic option. Works on contact or ingestion, and generally safe for beneficial insects once dry4. Common brands include Captain Jack’s and Monterey.
- Avoid Bt: Bacillus thuringiensis targets Lepidoptera caterpillars—not Hymenoptera like sawflies—so it does nothing for sawfly larvae5.
- Systemic insecticides: Products like imidacloprid are effective but used only as a last resort due to their impact on soil health and microbes.
Prevention
This is where vigilance pays off. Once you’ve seen those egg scars, you see them everywhere.
- Weekly checks: During spring and late summer, inspect every pine—while watering or doing candle work. It takes 30 seconds per tree.
- Encourage predators: Chickadees, spiders, ants, predatory wasps—they all help. Avoid broad-spectrum insecticides unless absolutely necessary.
- Clean your benches: Larvae pupate in soil and debris. If you’ve had an infestation, refresh topsoil and clean up beneath the canopy.
- Fall inspection: Some species overwinter as eggs. Check in October and again in late winter or early spring for “zippered” needles.
Long-Term Outlook
Sawflies aren’t as chronic as scale or root rot, but they’re fast and sneaky. If you’re growing Scots pine bonsai, you’re on their menu. But the good news is—they’re manageable if you stay alert and act early.
Keep your eyes on the needles. With clippers and focus, you can stop them before they strip a branch or ruin a season’s progress. It’s not rocket science—just steady vigilance and a clear understanding of their timing.
Sources
- University of Wisconsin–Madison Extension. “European Pine Sawfly.” https://hort.extension.wisc.edu/articles/european-pine-sawfly/ ↩︎
- University of Minnesota Extension. “Sawflies on Pines.” https://extension.umn.edu/yard-and-garden-insects/sawflies ↩︎
- USDA Forest Service. “European Pine Sawfly.” Forest Insect & Disease Leaflet (Neodiprion sertifer) PDF.
https://www.fs.usda.gov/foresthealth/docs/fidls/FIDL-98-EuropeanPineSawfly.pdf ↩︎ - UC IPM / University of California Statewide Integrated Pest Management. “Conifer Sawflies—Acantholyda and Neodiprion spp.” https://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/consaw.html ↩︎
- University of Illinois Extension. “Sawfly | Insects | Illinois Extension.” https://extension.illinois.edu/insects/sawfly ↩︎
