Cherry slug / pear slug (Caliroa cerasi): how to identify it and when to spray

Cherry slug (also called pear slug or cherry sawfly) is the larva of Caliroa cerasi, a small sawfly wasp. It is a worldwide pest of deciduous fruit, including in Australia. The larvae eat the soft leaf tissue between the veins on cherry, pear, plum, quince, apple and a few other trees. Heavy skeletonising cuts photosynthesis and can pull sugar and quality out of developing fruit.

This page is for commercial Australian growers and anyone searching cherry slug, pear slug, cherry sawfly or Caliroa cerasi. It covers identification, life cycle, spray timing, and how to use Py-Bo (80 g/L pyrethrins + 320 g/L piperonyl butoxide, EC) as a contact spray. Always read the current APVMA-approved label. The label is the law.

Identification and symptoms

You may never notice the adult. It is a mostly black flying sawfly about 10 mm long, without the skinny waist or sting of typical wasps. Females have a saw-toothed ovipositor and insert eggs into leaf tissue alongside a vein. An adult lives about two weeks and lays around 50 eggs. Adults have mandibles; they are not a feeding pest.

Larvae are the problem:

They emerge from inside the leaf and feed on soft tissue between veins (including fine veins).
They produce a slime coat that darkens to shining black as they age.
Eight larval stages (instars) over about 3–4 weeks; mature larvae about 15 mm long.
Streamlined slug shape, no snail-like antennae.
Underneath: three pairs of true legs and five pairs of prolegs; they undulate along the leaf, then drop to the soil to pupate.

Damage: skeletonised leaves. If enough canopy is chewed, there is not enough photosynthesis to fill cherries (or pears, plums, apples; quince is the same pest). Fruit quality suffers even when fruit is not eaten.

Life cycle (what you are spraying, and what you are not)

Stage	Where	Rough timing	Hit by a foliar contact spray?
Adult sawfly	In the canopy, laying	~2 weeks	Only if droplets hit the insect
Egg	Inserted in leaf tissue	Hatch ~1 week	No — eggs are inside the leaf
Larva (slug)	On the leaf, feeding	8 instars, ~3–4 weeks	Yes — if coverage contacts them, including undersides
Pupa	Soil, 30–50 mm deep	~2 weeks in season; overwinters in the pupal case if heading into winter	No

In summer, while host trees still have leaves, Caliroa cerasi can complete 2–3 generations. Late-season pupae stay dormant in soil until spring.

Birds and beneficial wasps are not a reliable commercial control of this pest. Lime, wood ash and poultry scratching after 7–8 mm pupae at 30–50 mm are backyard ideas, not orchard practice.

When to spray

Spray when larvae are on the foliage and you can see feeding or slugs. Contact insecticides only kill what they hit. Eggs in the leaf and pupae in the soil keep the cycle going.

A spray that kills feeding larvae does not kill eggs already in the leaf. Those eggs can still take up to about two weeks to hatch.
Larvae then feed 3–4 weeks before dropping to pupate.
Pupation is another couple of weeks. From a hatching egg to a new egg-laying female is on the order of two months.
Pupae already in the soil can emerge as adults and start laying the next day after you spray.
Plan a follow-up about two weeks after the first spray, when the next hatch is visible.

Bees: If bloom and leaf attack overlap, apply natural pyrethrum after dark, when bees are in the hive. Residues continue to repel bees into the next morning until UV degrades the pyrethrins (often around mid-morning). Do not treat as bee-safe in daylight on open flowers.

How to spray Py-Bo

Product: Py-Bo Natural Pyrethrum Insecticidal Concentrate
Actives: 80 g/L pyrethrins + 320 g/L piperonyl butoxide (PBO), emulsifiable concentrate
Mode: contact only. Knockdown of insects the spray actually hits. Not systemic. Not residual in the plant. Pyrethrins break down rapidly in sunlight.
Rate: 1 mL Py-Bo per 1 L water, or 100 mL per 100 L water.
Withholding period: 1 day on labelled uses. Confirm crop listing and WHP on the current label.

Mix 1 mL/L (100 mL/100 L) in clean water.
Apply so droplets contact larvae, including undersides of leaves.
Prefer dusk / after dark when bees are present.
Coverage is the job. If live larvae remain after the spray has dried, they were not contacted.
Follow up ~2 weeks later for the next hatch.

Py-Bo is APVMA-approved for listed uses. Use only on crops and situations named on the label. This page does not authorise off-label use.

What Py-Bo will not do

Will not kill eggs inside the leaf.
Will not kill pupae 30–50 mm in the soil.
Will not give a season-long residual.
Will not replace coverage.

FAQ

Is cherry slug a true slug?

No. It is the larva of a sawfly wasp, Caliroa cerasi. The slug look comes from a dark slime coat over a streamlined larva with three pairs of true legs and five pairs of prolegs.

How does it hurt fruit if it only eats leaves?

Skeletonised leaves cannot photosynthesise properly, so the tree has less sugar to fill cherries, pears, plums or apples. Fruit size and quality drop even though the fruit is never chewed.

What rate of Py-Bo do I use, and what is the withholding period?

1 mL per 1 L of water (100 mL per 100 L). The withholding period is 1 day on labelled uses — always confirm the crop and WHP on the current APVMA-approved label.

Will one spray fix it for the season?

No. Py-Bo is a contact knockdown with no residual. Eggs inside the leaf and pupae in the soil are not affected, so plan a follow-up about two weeks later for the next hatch.

What about bees?

Spray after dark when bees are in the hive. Residues repel bees into the next morning until UV degrades the pyrethrins. Do not treat it as bee-safe in daylight on open flowers.

Quick reference

Common names: cherry slug, pear slug, cherry sawfly, pear sawfly
Scientific name: Caliroa cerasi
Hosts: cherry, pear, plum, quince, apple and some other deciduous trees
Symptoms: skeletonised leaves; soft tissue eaten between the veins; shining black slug-like larvae on leaves
Instars: 8 larval stages over about 3–4 weeks; mature larvae about 15 mm
Eggs: inserted into leaf tissue beside a vein; about 50 per female; hatch in about a week
Pupation: in soil 30–50 mm deep; pupae 7–8 mm; about 2 weeks in season, or overwintering
Generations: 2–3 per summer while leaves are on the trees
Spray: Py-Bo (80 g/L pyrethrins + 320 g/L PBO) at 1 mL/L (100 mL/100 L); contact only; 1 day WHP; apply after dark where bees are active; repeat about 2 weeks later