Pearslug (pear sawfly)
Pearslugs are the larvae of the pear sawfly, Caliroa cerasi, a small shiny black non-stinging wasp relative. The larvae look like tiny slugs because of a slimy olive-green to black coating. They graze the upper surface of pear, cherry and plum leaves down to the fine veins, and the grazed patches turn brown. There are usually two generations a year, and the second is usually the bigger one. UC IPM says pearslug is often kept in check by natural enemies. Wash larvae off with a strong jet from the hose, or pick them off on small trees; UC IPM says the larvae are easy to kill with soap, oil, neem or spinosad if the spray covers them thoroughly. Bt does NOT work. The caterpillar Bt kills only the larvae of moths and butterflies, and sawflies are not caterpillars.
Caliroa cerasi larvae on the upper surface of a leaf, with grazed brown patches. Photo: pending.
Identification
Newly hatched larvae are white with a yellowish-brown head. Soon after they start feeding they cover themselves in a slimy olive-green coat that makes them look nearly black, and mature larvae are yellow (UC IPM). WSU adds that they are yellow just after each moult and yellow-orange just before they pupate. They are slug-shaped and club-shaped, wider at the head end (RHS), and grow to about 3/8–1/2 inch (10–13 mm); UC IPM gives both figures, the RHS about 10 mm, and Minnesota 12 mm. The adult is a shiny black sawfly about 1/5 inch (4–6 mm) long with clear wings, and is rarely noticed.
The larva is an insect, not a mollusc: it has three pairs of true legs behind the head and fleshy prolegs under the abdomen, although the slime makes them hard to see. Sawfly larvae that feed in the open have six or more pairs of prolegs; caterpillars have five or fewer. That is why Bt, which kills caterpillars, does nothing to pearslugs.
Easily confused with
- Rose slug (rose sawfly)Also a pestA related slug sawfly, but it lives only on roses and is pale, translucent green; the pearslug is on pear, cherry or plum and coated in olive-green to black slime.
- True slugsAlso a pestSlugs are legless molluscs that feed mostly at night and leave silvery slime trails; pearslugs have legs and sit exposed on top of tree leaves in daylight.
- CaterpillarsAlso a pestCount the fleshy prolegs under the abdomen: caterpillars have five pairs or fewer, sawfly larvae six or more. It matters because Bt kills caterpillars but not pearslugs.
Damage symptoms
The larvae do not chew holes through the leaf. They eat the upper surface, removing all the tissue except the fine network of veins, a pattern called skeletonising or windowpane feeding. The patches look lacy at first, then turn brown or grey, and heavily grazed leaves may shrivel and drop. UC IPM warns that feeding by both generations can reduce fruit size, and that severe defoliation by the second generation can reduce bloom the following spring. The RHS, for British gardens, calls the damage usually insignificant.
- Upper leaf surface grazed away between the veins, leaving a lacy skeleton
- Grazed patches drying brown to grey
- Small, shiny, slug-like dark larvae sitting on top of the leaves in daylight
- Shrivelled leaves and defoliation on heavily attacked trees, often worst in late summer
- Smaller fruit, and less bloom the next spring, after severe defoliation (UC IPM)
Lifecycle & degree days
The pear sawfly spends the winter as a mature larva (prepupa) in a cell or cocoon in the topsoil, and pupates in spring. Adults lay eggs on the leaves, larvae feed for about a month, then drop to the soil to pupate, and a second generation follows. UC IPM gives two generations a year in California, the second usually larger and faster-developing than the first, and advises checking for eggs from April. Minnesota puts egg laying in late June and July, with the second generation feeding from early August. The RHS gives two or sometimes three generations in Britain between June and October, the July–August generation being the most noticeable.
- Generations / yr
- 2 (UC IPM, Minnesota); 2–3 (RHS, UK)
- Larval feeding
- About 1 month (Minnesota)
- Second generation
- Early August in Minnesota; usually the larger one (UC IPM)
- Overwinters
- Prepupa in a cell in the topsoil
Host crops
Cherry and pear are the main hosts, and plum is fed on too (WSU, UC IPM). Minnesota, UC IPM and the RHS add ornamental relatives in the rose family: hawthorn, cotoneaster, mountain ash (sorbus), quince and flowering quince (chaenomeles). The rose slug on roses is a different sawfly that does not feed on fruit trees.
Monitoring & action thresholds
WSU HortSense advises spraying only when larval damage approaches 25% defoliation. UC IPM sets no numeric threshold and bases decisions on monitoring, and the RHS advises tolerating it in gardens. UC IPM says feeding does not seriously threaten otherwise healthy plants, but WSU warns that severe infestations can weaken trees, and UC IPM that heavy feeding can cost fruit size and next year's bloom.
From April, check the shoot tips in the top of the canopy, where UC IPM says the adults prefer to lay. Keep looking through summer and after harvest, since the second generation is usually the bigger one. Minnesota Extension advises looking in mid and late summer.
Biological control
Nothing to buy. UC IPM says pearslug is often under effective biological control, and WSU says natural enemies control it fairly effectively. For sawflies in general UC IPM names insectivorous birds, parasitic wasps, predaceous beetles, small mammals and fungal and viral diseases; the RHS adds birds and ground beetles. UC IPM notes pearslug has become a problem in orchards that dropped broad-spectrum sprays before their natural enemies had built back up. The one product people reach for is Bt, and it does not work: UC IPM and Minnesota say it does not kill sawfly larvae, because they are not caterpillars.
Cultural controls
On backyard trees, water and fingers are usually enough. UC IPM recommends a hose spray because it dislodges the larvae without harming their natural enemies, and advises spot-treating a small outbreak before it spreads. If you spray, UC IPM notes the larvae are easy to kill with almost any insecticide that covers them, so use the gentle ones and avoid broad-spectrum products that kill the pearslug's enemies.
- Wash larvae off the leaves with a strong jet of water from the hose
- Pick larvae off by hand on small trees
- Spot-treat the branches that carry most of them before they spread
- If needed, spray the larvae with insecticidal soap, horticultural oil or neem, wetting the upper leaf surfaces thoroughly
- Keep broad-spectrum sprays (carbamates, organophosphates, pyrethroids) off the tree to protect natural enemies
Chemical, last resort
- Insecticidal soapREI 12 h
- Horticultural oil (narrow-range)REI 12 h
- Neem (azadirachtin)REI 4 h
- SpinosadREI 4 h
- Methoxyfenozide (commercial orchards)REI 4 h
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UC IPM Pest Management Guidelines: Pear, Pear Sawfly (Pearslug)
UC IPM Pest Management Guidelines: Pear, Relative Toxicities to Natural Enemies
UC IPM Home and Landscape: Pear Sawfly; Sawflies; Snails and Slugs
UMN Extension, Sawflies (J. Hahn & J. Lloyd)
WSU HortSense: Cherry: Pear slug (cherry slug)
RHS Advice: Pear and cherry slugworm
Page status
Drafted from the sources below, then checked claim by claim against those sources on 24 September 2026.
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