Apple sawfly larvae damage apple fruits at the fruitlet stage in late spring to early summer, usually causing affected fruits to drop off in June. This is not to be confused with maggoty apples in late summer which are caused by another pest called codling moth.
What is apple sawfly?
Adult apple sawflies are small insects with blackish brown heads and thorax and brown abdomens. The caterpillar-like larvae initially tunnel beneath the skin of the developing apples, causing a scarring and then further damage.
Symptoms
Signs of an apple sawfly problem include:
- In late May and June, damaged fruitlets have obvious maggot holes with the larva’s blackish-brown excrement pellets spilling out
- Affected fruitlets drop off the tree as part of the June drop (when apple trees shed excess fruits)
- Fruitlets that suffered only initial feeding damage by a larva will stay on the tree and develop as fruits. These fruits are misshapen and have a long ribbon-like scar about 4mm wide, often starting at the eye end of the fruit and extending around the circumference
Internal damage caused by apple sawfly larvae.
Control
Non-chemical control
Pick off damaged fruitlets when they are seen to prevent the larvae moving to other fruitlets or going into the soil to pupate.
Chemical control
If damage has been heavy in previous years, spray within seven days of petal fall with deltamethrin (Bayer Provado Ultimate Fruit & Vegetable Bug Killer) or lambda-cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer) to control the newly hatched larvae.
Note that, in years when there has been a heavy fruit set, a bit of fruit thinning caused by apple sawfly can be beneficial.
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Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Biology
Adult apple sawflies are active in late April-May and can be seen visiting the open blossom. They are 4-5 mm long with blackish brown heads and thorax and brown abdomens.
Eggs are laid on the base of the flowers. After petal fall, the eggs hatch and the caterpillar-like larvae start feeding. Initially the larvae tunnel beneath the skin of the developing fruitlets. Later they bore into the core of the fruitlets, and this feeding damage causes them to drop off in early summer. If the larva dies before reaching the core, the fruit survives and develops but will have a ribbon-like scar on the skin where the early feeding took place.
When fully fed, the larva is about 10mm long and has a brown head and white body. Each larva can damage 3-5 fruitlets before it completes its feeding and goes into the soil, where it overwinters as a non-feeding larva and pupates in the following spring.