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Aphelinus
mali was successfully introduced to New Zealand from
U.S.A., beginning in 1921, in a classical
biological control programme. It overwinters in the
larval or pupal stage within the black dead body
('mummy') of the aphid. The tiny adult wasps (0.7-1.0 mm
long) emerge through September/October, tending later in
cooler southern areas. The emergence leaves
characteristic circular holes in
the mummies. This coincides with the spring
production of nymphs by the aphid. Females lay their eggs individually
inside the lower part of the abdomen in young nymphs. The
female wasps also sometimes feed on the aphid juices
which exude from the punctures. Development to adult
inside the aphid takes from 45-50 days in spring down to
about 20 days in mid-summer. This allows about 7-8
generations per year in the main fruitgrowing regions
compared to 8-10 generations of the aphid. Adults are
rare in the field after mid April, although woolly apple
aphids may continue to reproduce. The adults may be seen
on the fruit, leaves and bark, particularly amongst
woolly apple aphid colonies. In warm weather, they have
characteristic rapid short flights which appear like
jumping movements. |
