Q What is apple sawfly?
A It is actually a kind of fly (Hoplocampa testudinea) but the adult looks like a flying ant and its larvae resemble caterpillars.
Caption: A corky scar on the fruit's surface shows the effects of a brief sawfly attack
Q How do I recognise this pest?
A The adult is black with a red/yellow underside and legs, and it has one pair of wings. It is not very striking, lives for less than two weeks and is only about 6mm long. It flies only in warm, sunny weather, around blossom time. The damage it causes is easy to spot.
Q How do I recognise the damage?
A Mature fruit with corky scars on the surface show the effects of a brief sawfly larva attack. More severe attacks result in the fruits dropping early and, if the apple is cut open, will often reveal a cream-coloured grub about 15mm long. The grubs pupate in the ground during July. This pest bores into fruit, causing it to fall early, though it rarely affects a significant proportion of the crop. It’s not usually a serious problem
Q What could I mistake it for?
A The caterpillars of codling moth and fruitlet-mining tortrix moth look similar to apple sawfly larvae. However, codling moth caterpillars attack in late summer after the apple sawfly larvae. They also produce a little dry frass (droppings) around the entry hole, rather than the mass of wet frass inside the fruit that's associated with apple sawfly larvae.
Fruitlet-mining tortrix moth caterpillars occur in early summer at the same time as those of the apple sawfly, but they feed in the centre of a cluster of apples and are covered in a web. Their tunnels wander around the surface of the fruitlet instead of going into the core, and they make puncture marks rather than corky scars. Both caterpillars and sawflies have true legs and sucker-like legs, but caterpillars have a gap between the two.
Q Can you tell me more about apple sawfly?
A Each female lays about 30 eggs singly in the opening flowers. Sap seeps out of the wound made by the female when laying her eggs. It turns red/brown and can be a good early warning of an attack. A larva hatches after two weeks and burrows into the fruitlet, usually at the eye, aiming for the pips on which it feeds. Once under the skin of the fruitlet it tunnels in the surface layer before it burrows into the core. If it does not reach the core it still causes superficial damage which results in the characteristic curved scars. It tunnels out an area of the fruit, filling it with wet brown droppings or frass. Larvae may move on and attack two or three apples, and sometimes more than one grub is found in a single fruit. Often the fruitlets attacked later have masses of frass at the eye where the entry hole was made. After two weeks' feeding, the larvae leave the fruit to overwinter in the soil.
Q How serious a problem is it?
A Infestations rarely affect a significant proportion of the crop. In fact, when the fruit-set is heavy, the thinning effect of the sawfly can be helpful. The surviving apples grow bigger and there is less chance of branches breaking. Most mature fruit with surface-scarring will be otherwise undamaged and remain good to eat. Such fruit can be stored, provided the scar is well-healed. This damage can be more common in certain years and places, so could need control.
Q How do I control apple sawfly?
A Where small trees are affected, pick over fruitlets from early June, looking for tunnels under the skin. Remove affected fruitlets by the third week in June before the mature larvae leave. With large trees, collect up fruitlets that fall early and consign to the dustbin. There is no cure, but spraying with Westland Plant Rescue Fruit and Vegetable Bug Killer can help reduce the severity of the attack.
Q Are any varieties more susceptible?
A 'Charles Ross', 'Discovery', 'Early Victoria', 'Ellison's Orange', 'James Grieve' and 'Worcester Pearmain' are more susceptible than average. Cooking apples are less susceptible, with the exception of 'Early Victoria' and 'Edward VII'.