Apple leaf curling midge

If you are seeing tightly curled, dried up leaves on your apple trees that don't seem to have leafrollers in them, look closely for several small orange maggots. These maggots are most likely the larvae of the apple leaf curling midge (ALCM), Dasineura mali (Diptera: Cecidomyiidae).*

The ALCM is an introduced species native to Europe. In recent years, apple growers in Ontario have been seeing an increased amount of damage associated with this pest in their orchards. These insects are not considered economic pests of apples and are not normally a concern in mature trees. However, high populations on young trees can result in reduced photosynthesis and may stunt the growth of terminal shoots. Larvae can feed on leaves, flowers and fruits.

There are two generations per year. The midge overwinters as a pre-pupa or mature larva. Adults emerge in swarms in late May to early June. The adult is a small, dark brown, mosquito-like fly (1.5 to 2 mm long). Mating takes place in flight, and females lay their eggs in small batches on the edges of vigorously growing new leaves, those that are just beginning to unfold in the shoot tips. A single female produces up to 200 eggs.

Young larvae are creamy white, legless maggots that become orange-red as they mature. Feeding occurs on the upper leaf surface and triggers a reaction in the leaf, causing it to roll tightly upward and inward, towards the midrib (the tightness of the roll helps to distinguish between damage caused by ALCM and leafrolling caterpillars). Infested leaves become discoloured and brittle. When larvae feed on fruitlets, the developing fruit has skins distorted by bumps. After 2-3 weeks of feeding, the larvae mature fall to the ground, and pupate under the soil surface or loose bark. Adults of the second generation emerge in late July, and their larvae are present through August.

Orius spp. and mullein bugs will feed on ALCM larvae. In Nova Scotia and New Brunswick, the European parasitoid Platygaster demades parasitizes 40-90% of the first generation larvae. However, the asynchronous development of the pest and parasitoids decreases parasitism to only 1-3% in the second generation.

It is very difficult to manage this pest using insecticides since the midges are well protected inside the curled up leaves. Growers are limited to pyrethroids for managing ALCM infestations.

Until very recently, the method for monitoring ALCM involved examining shoots for the presence of curled leaves containing larvae. However, researchers in the UK have identified the sex pheromone produced by females to attract male ALCM. Trials are underway to determine threshold numbers in traps for timing insecticide sprays. We are very interested in evaluating commercially available lures in Ontario apple orchards next year.

(NOTE: Larvae of the ALCM look very similar in size and shape to those of the aphid midge, an important predator of aphids. Both belong to the same family of insects, the cecidomyiids. IF you find orange maggots among aphid colonies, they're probably good guys).