Insects - Pests of Ornamental Trees, Shrubs and Flowers - page 181

417

418

417 Galls of hornbeam leaf gall midge ( Zygobia carpini ). 418 Drone fly ( Eristalis tenax ).

Family SYRPHIDAE (drone flies or
hover flies)

cluster of terminal leaves. The galled buds, within which
larval development takes place, remain relatively small in
the first year. However, in the next year they become
larger and less compact, and may then exceed 15-20 mm
in diameter; these are commonly described as 'artichoke'
galls. Fully fed larvae eventually pupate within their
galls, and adults emerge in the following spring. Vacated
galls eventually turn brown, and often remain in situ long
after the midges have flown. Although the vast majority
of larvae develop slowly over two years, some
individuals complete their development within just one
year. These, however, do not initiate artichoke galls.

Small to large, often brightly patterned flies, some of
which hover in the air and emit a bee-like hum; body
sometimes very hairy. Larvae of many species are
important predators of aphids, whereas others feed on
living plant tissue, including bulbs and corms, or are
associated with rotting wood or boggy (aquatic or semi-
aquatic) habitats.

Eristalis tenax (Linnaeus) ( 418 )
Drone fly
Adults of this widespread and generally common
syrphid are often attracted in large numbers to flowers,
especially Asteraceae (e.g. Aster and Chrysanthemum )
and Apiaceae, where they feed avidly on nectar. They
also bask in sunshine, darting into the air and hovering
nearby if disturbed. The flies are normally harmless but
sometimes enter greenhouses, particularly in the
autumn, and might then contaminate the petals of
chrysanthemums and other flowers with droplets of
excrement. The insects breed in wet, decaying organic
matter. Drone fly larvae (commonly known as
'rat-tailed maggots') possess a very long, extensible
tube which allows them to breathe whilst submerged
well below the surface of mud or stagnant water. The
bee-like adults (12-15 mm long) are mainly brownish
black, with the thorax clothed in yellowish to brownish-
yellow hairs and the abdomen variably marked with
yellowish or yellowish brown. Species of Eristalis ,
including E. tenax , are distinguished from other closely
related syrphids by the dark bands of hairs across the
eyes and by the simple, unbranched arista.

DAMAGE
The galls abort shoot growth and are disfiguring, host
plants being stunted by persistently heavy infestations.
Attacks are most damaging in nurseries and on plants
already infested by yew gall mite ( Cecidophyopsis
psilapsis ) (p. 420).

Zygobia carpini (Löw, F.) ( 417 )
Hornbeam leaf gall midge
Galls of this widely distributed midge develop on the
leaves of hornbeam ( Carpinus betulus ), either occurring
singly or as a series of swellings along the length of the
midrib. Infestations are sometimes noticed on cultivated
plants but, although disfiguring, they have no apparent
effect on plant growth. The whitish to yellowish-white
larvae (up to 3 mm long), one per gall, develop
throughout the summer; they vacate the galls in
September and eventually pupate in the soil.
Infestations also occur on European hop-hornbeam
( Ostrya carpinifolia ).