Apple leafcurling midge - Life cycle

The number of generations of apple leafcurling midge completed each year decreases from north to south in New Zealand. There are 6-7 generations annually in the north of New Zealand. Populations in the north reach particularly high levels in the wetter regions (e.g. the Waikato), whereas dry weather and the cessation of terminal shoot growth often prevents population build up during mid- and late-summer on the east coast (e.g. Hawkes Bay). Only the first two generations in spring are discrete, and there is considerable overlap in the later generations. This overlapping of later generations also occurs in Palmerston North, Nelson and Marlborough in central New Zealand, where there are usually 5 generations per year. Further south in Canterbury and Otago, the 4 generations are more discrete, although this varies from season to season and the timing of generations is influenced by temperature, drought conditions, and rainfall episodes. Summer droughts can have a major influence on both the abundance and phenology (life cycle timing) of apple leafcurling midge. Rainfall softens the leafrolls and makes it easier for mature larvae to escape; delays of up to10 days can occur in this process in the absence of rainfall.

The apple leafcurling midge overwinters as a dormant cocooned larva, mainly on or just beneath the soil surface under the apple trees. The larvae pupate in spring, and adult midges emerge from budburst in the north, from just pre-flowering in central New Zealand, or from flowering in the south. This discrete adult emergence and associated egg-laying is completed over the flowering/post flowering period. The eggs are laid in batches mainly on the edges of unfurling leaves in the tips of growing shoots, although this spring generation also uses bracts and flowers for egg laying. The larvae feed primarily on the edges of the upper surfaceof the leaves, where their feeding causes tight inward rolling or galling. Development is completed in these protected sites and at maturity, the larvae leave the rolls and descend to the ground, bark, and occasionally leaves and fruit, to spin a cocoon and pupate. Like the first generation, the second is also discrete in all regions, but overlapping of generations increases subsequently, particularly in the north (see life cycle charts). Depending on temperatures, the eggs and larvae take respectively 3-10 days and 20-30 days to develop.