Pear Sawfly

Look at the top of a pear or cherry leaf. A tiny olive-green tadpole sitting in its own slime is pear sawfly, the insect people call a pear slug. Wipe it off, crush it, or hose it into the grass, because it dries out once it leaves the leaf. A Bt spray made for moth caterpillars will not kill it, and the fruit itself is not what it eats.

Pear sawfly is a small, glossy black wasp relative, Caliroa cerasi. The adult does not sting. The larva is the one you see. It coats itself in olive-green slime, sits on the upper surface, and chews the green between the veins, so the leaf turns brown and lacy. Cherry slug is the same insect on cherry. Two generations a year, and the late-summer hatch is usually the louder one.

Wipe or hose them off the top of the leaf; skip Bt

They sit on the upper surface of pear and cherry leaves, so you do not have to flip the foliage to find them. Wipe a slimy larva into a bucket of soapy water with a gloved finger or a cloth, or crush it on the leaf. A hard stream from the hose knocks them off, and they dry out on the ground because that coating that makes them look like a slug also leaves them easy to desiccate. On a small tree you can finish the job in one pass. On a tall pear, reach what you can, then skip a spray you cannot actually cover.

Bt, the Bacillus bottle sold for moth and butterfly caterpillars, does not kill this insect. It is a sawfly, related to wasps, not a moth. A soil drench does not reach larvae on the leaf, and it does not kill the pupa in the soil either. If the brown lace is already there and the larvae have dropped, a spray will not put the green back. Watch the next hatch in late summer, because the second generation is usually the one that browns a whole canopy.

They sit on the upper surface like a tiny tadpole, not a garden slug

The confirm is a wet, olive-green to almost black larva on the top of the leaf, swollen at the head end and tapered at the rear, about three-eighths to a half inch when full grown. Newly hatched and newly molted larvae are yellow until they put the slime back on. The last stage loses that coating, turns yellow-orange, and drops into the soil to pupate a few inches down. You rarely need the adult. It is a glossy black, fly-sized wasp that does not sting, and it inserts eggs as tiny tan blisters in the leaf.

Wipe the slime and you will see short true legs and several fleshy extra legs. A garden slug has no legs and no separate head capsule. Gardeners on cherry call the same insect a cherry slug. It also shows on plum, cotoneaster, hawthorn, mountain ash, quince, and serviceberry. Eggs sometimes land on peach, but the larvae do not thrive there. First larvae show in late spring or early summer. The second hatch, often August into September, is the one that can brown a tree overnight.

Brown lace on pear is not a rose slug or a Japanese beetle

A true slug is a mollusk. It has no legs, it works at night, and it leaves a slime trail across holes in soft leaves. Pear sawfly feeds in daylight on the upper surface and leaves a brown, lacy network of veins. Beer traps, boards, and slug bait will not finish it. See slugs for that animal, then come back if the larva on the pear still has a head and hidden legs.

Rose slugs are different sawflies on rose, usually yellow-green and sometimes bristly. They are not this insect, even though both get called slugs. Do not paste a rose spray calendar onto a pear. Japanese beetle is a metallic adult that chews ragged holes in daylight. See Japanese beetle. White powdery worms on dogwood are dogwood sawfly. An S-shaped group on a birch leaf edge is dusky birch sawfly. Other sawflies have their own names. This insect is the slimy larva on pear and cherry. Brown, wilted shoots that look scorched after bloom are more often fire blight, not this chew. A maggot inside the fruit is not this insect either. Pear fruit sawfly and apple maggot work the fruit, not the upper leaf.

Late skeletonizing on a healthy tree can wait

On an established pear or cherry, late-summer brown lace is usually ugly, not a dead tree. The tree has already made most of its food for the year, so one round of skeletonizing seldom kills it. Fruit size can drop that season if a lot of leaf is gone while the fruit is still filling, and a heavy second generation can cut bloom the following spring, so a young tree or a newly planted ornamental is the case to wipe this year instead of waiting.

If the larvae have already dropped and only brown lace is left, a spray does nothing useful. Water a young tree that lost most of its leaf. Watch again in June and again in August, because two generations are the usual pattern, and the second is the one that browns a canopy in a week. A 25-foot pear you cannot wet from the ground is a wait, not a ladder-and-pyrethroid job.

Soap and oil only work while the larvae are still on the leaf

Labeled insecticidal soap or horticultural oil has to wet the larva. Aim at the upper surface, because that is where this insect sits. A spray on the undersides, the way you would treat lace bugs, misses them. They are easy to kill while they are small and slimy. Once they have turned yellow-orange and gone into the soil, the bottle is too late.

Soap can burn cotoneaster, plum, and cherry foliage, so wipe those by hand, or test a few leaves and wait a day before you wet a whole shrub. Spinosad is a later contact option if you can still see larvae and you cannot wipe them, but skip it during bloom because it can hurt bees for hours after it dries. Skip the brand aisle of carbaryl, malathion, and pyrethroids as the plan. A soil drench does not reach this insect on the leaf.

Confirm the slime, then wipe or wait

1. Look at the upper surface. Olive-green tadpoles on brown, lacy pear or cherry leaves confirm pear sawfly. Hidden legs under the slime mean insect, not a garden slug.
2. If it is a night hole with a slime trail and no legs, stop. See slugs. If the larva is on a rose, it is a different sawfly. If a metallic beetle is chewing in daylight, see Japanese beetle.
3. If you can reach them, wipe or crush them into soapy water, or hose them off. Skip Bt. Do not wait for a soil drench.
4. If they are already gone and only brown lace is left, skip the spray. Water a young tree. Watch June and August for the next hatch.
5. If larvae are still present and you cannot wipe a large flush, wet the upper surface with labeled insecticidal soap or horticultural oil. Wipe cherry, plum, and cotoneaster by hand if soap has burned those leaves before. Skip bloom if you reach for spinosad.
6. Leave a healthy, established tree alone once the larvae are off. Brown late lace on a mature canopy is cosmetic. A young tree that lost most of its leaf is the one to tend.

Related reading

• Slugs — a mollusk with no legs and a night slime trail, not a sawfly on the leaf.
• Japanese beetle — a metallic adult chewing irregular holes, not a slimy larva.
• Dogwood sawfly — white powder worms on dogwood undersides, not olive slime on pear.
• Dusky birch sawfly — an S-shaped group on a birch leaf edge.
• Fire blight — scorched, wilted shoots after bloom, not brown lace from chewing.