Identification

Apple leafcurling midge is a fly with four life stages: eggs, larvae (or maggot), pupae and adult.

Eggs are transparent pink to pale red in colour and cigar-shaped (elliptical), approximately 5X as long as wide. They are laid irregularly in groups on the margin or upper side of unfolding leaves. Up to 500 eggs per leaf have been recorded in New Zealand, although 30-40 eggs per leaf is more typical. Although leaves are the preferred site for oviposition, in spring when fewer leaves are available, the bracts of buds and developing flowers may also be used for oviposition. Eggs take from 2-10 days to hatch depending on temperature conditions.

Larvae are red-coloured initially then becoming clear-white until the final instar when they acquire a bright orange/red colour and require approximately 20 days to reach maturity. The larvae spend most of their life within a rolled leaf and if removed from the leafroll and placed in bright sunshine will die soon afterwards. Typically a leafroll contains 20-30 larvae, but the number can vary from 1-500. Usually both margins of infested leaves are rolled towards the midvein but occasionally only one margin is rolled. The edges of two leaves may also be rolled together, especially on spur growths where leaves are clustered. Larval feeding prevents the opening leaf from unfolding, with the roll becoming tighter as the leaf growth continues.

Upon reaching maturity most larvae exit the leafroll and fall to the ground. The largest numbers of larvae escape from rolled leaves when they have been thoroughly soaked by rain. Prolonged dry conditions may prevent or delay mature larvae from exiting leaf rolls to find pupation sites. When rain occurs after a summer drought, large numbers of mature larvae may descend the trunk of infested trees to the soil beneath. A few larvae may remain inside their rolled leaf to pupate, while a larger proportion (10% or possibly higher) may crawl down the trunk of the tree and pupate under the loose bark or around pruning cuts.

Pupation takes place in a tough, white silken cocoon which is generally spun just beneath the surface of the ground, or among loose pebbles of earth. Pre-pupae have the same orange colour as mature larvae and are clearly visible inside the cocoon. The brown-coloured pupa works its way partially out from the cocoon before the adult fly emerges. During the summer generations, pupation takes 13-18 days, but for the overwintering generation, insects will remain as pre-pupae or pupae inside the pupal cocoon for much longer.

The adult apple leafcurling midge is a small fly, between 1.5 to 2.5 mm in length with wings 1.5 to 2 mm long. Female midges have a red abdomen and may be able to lay up to 200 eggs. Female apple leafcurling midge produce a sex pheromone to assist males to find them for mating. Only leaves that are about two-third's unfolded or less are attractive to midges for oviposition, although on heavily infested trees a few eggs may be laid on older leaves with existing larval infestations. Several midges may oviposit on the same leaf with individual females ovipositing on several leaves.

Host plants

Apple trees are the only host of the apple leafcurling midge, whereas the pear leafcurling midge (Dasineura pyri Kieffer) only infests pear trees. Apple leafcurling midge infestation can differ between apple cultivars, although this can vary through the season depending on the availability of shoots suitable for oviposition. No apple cultivar has been found which is free from infestation.

Damage

Apple leafcurling midge larval feeding can affect apple leaves, flowers and fruit. Flower and fruit damage occurs when high populations of apple leafcurling midge occur during flowering. Larvae developing from the eggs laid on flowers can affect developing fruitlets resulting in fruit whose skin is distorted by bumps.

Feeding on leaves prevent them from fully expanding and they may drop prematurely. Leaf and shoot damage may affect crop yields, with field trials conducted overseas showing that where insecticide applications reduced tree infestation by apple and pear leafcurling midge, an average yield increase of 10% occurred on apple and pear trees.

Some mature larvae dropping off trees fall onto fruit, where they may remain and pupate at either the stalk or calyx with the pupal cocoon firmly attached to the fruit skin. Most fruit are only contaminated by a single cocoon, although up to 40 cocoons per fruit have been found on unsprayed trees. Fruit contaminated by pupal cocoons may be rejected for export.

Distribution

Apple leafcurling midge has been recorded in apple orchards from Clyde to Auckland and is probably found wherever apple trees can be grown.

Life cycle

The numbers of generations per annum increases with latitude, with 3-5 generations recorded in Nelson, five in Havelock North and Palmerston North and 6-7 in Hamilton. At the southern edge of its distribution in New Zealand, apple leaf-curling midge probably has only two generations per annum.

Summer droughts can have a major influence on both the abundance and phenology of apple leafcurling midge. This is caused by the lack of rainfall which acts to soften leafrolls which makes it easier for mature larvae to escape, with delays of up to 10 days occurring in the absence of rainfall.

References

Allison, P.A., Meekings, J., Tomkins, A.R., and Wilson, D.J., 1995. Effects of leaf damage by apple leafcurling midge (Dasyneura mali) on photosynthesis of apple leaves. Proceedings of 48th New Zealand Plant Protection Conference: 121-124.

Harris, M.O., Foster, S.P., Agee, K., and Dhana, S., 1996. Sex pheromone communication in the apple leafcurling midge (Dasineura mali). Proceedings of 49th New Zealand Plant Protection Conference: 52-58.

Todd, D.H. 1959. The apple leaf curling midge, Dasyneura mali Kieffer, seasonal history, varietal susceptibility and parasitism 1955-58. New Zealand journal of agricultural research 2: 859-869.

A. R. TOMKINS