Agriculture Reference
In-Depth Information
developing inflorescences within the shelter of
the leaf sheaths. Symptoms of attack show
clearly after the ear emerges, as sections of
spikelets are either partly or totally grazed away;
these damaged areas are often arranged spirally.
Severe seed loss is reported mainly from the
northern parts of continental Europe, especially
France, Germany and Scandinavia (where
the range of these pests extends into the Arctic
Circle).
be followed by the complete collapse of plants.
On older plants, the larvae (maggots) may bur-
row within and destroy the inner tissue of the
actual bulbs; there may be as many as 30 larvae
inside a heavily infested bulb. The oldest leaves
protruding from infested bulbs may wilt, turn
yellow and then whitish; such leaves are readily
detached. In the British Isles, this pest is most
important in central and eastern England.
BIOLOGY
Adult flies appear in May. Eggs are deposited on
the neck or young leaves of onion plants or
placed in the soil immediately adjacent to host
plants, each female fly depositing up to 200 eggs
in her life-time. The eggs hatch within a few days,
and the larvae then attack their hosts. Larvae
may move from one bulb or plant to another in
order to complete their development, passing
through three instars and becoming fully grown
in approximately 3 weeks. Pupation occurs in the
soil, a few centimetres from the surface, and a
second generation of adults emerges 2-3 weeks
later. In favourable situations, there may be
three generations annually. The pest overwinters
in the pupal stage.
BIOLOGY
Adults are active in late April or early May.
They occur in association with timothy grass,
upon which pairs of mating adults may be found;
they also rest on the leaf tips, characteristically
head downwards. Eggs are deposited close to the
stem on the upper surface of the uppermost
leaves, parallel to the veins. They hatch 5-7 days
later. The larvae then bore into the grass stem to
feed on the developing, but still enclosed, inflo-
rescence. After 2-3 weeks, when fully grown,
they reappear and usually drop to the ground
before pupating in the soil. There is just one
generation annually.
DESCRIPTION
Adult
5.0-5.5mm long, mainly dark grey;
legs
partly pale and reddish-brown.
Egg
1 mm long,
pale yellow, elongate, seed-like with four longi-
tudinal ridges.
Larva
up to 8 mm long, lemon-
yellow, maggot-like;
anterior spiracles
prominent
and bifid, with several lobes;
posterior spiracles
slightly sclerotized and 3-pored.
Puparium
5 mm long, dark brown.
DESCRIPTION
Adult
5-7 mm long; mainly grey, with numerous
long black hairs;
thorax
yellowish-grey with four
brownish, longitudinal stripes;
abdomen
grey
with a dark stripe down the mid-line;
wings
yellowish.
Egg
1.0-1.3 mm long, white,
elongate-oval and ridged longitudinally.
Larva
up to 10 mm long, dirty white, tapered anteriorly
and blunt posteriorly
(Plate 9d);
posterior papil-
lae
relatively prominent, the small hind-most
(median) pair distinctly separated
(Fig. 266a)
(cf.
bean seed fly,
Delia platura,
p. 199).
Puparium
6-7 mm long, oval, dark brown.
Family ANTHOMYIIDAE
Delia antiqua
(Meigen)
Onion fly
The onion fly is a widely distributed and often
serious pest of onion; leek and shallot are also
attacked. Most important damage occurs in June
and July, with seedlings and salad crops most at
risk. Infested seedlings soon wilt, and the leaves
appear flaccid and may turn brown; such symp-
toms often occur in patches within crops and may
Delia coarctata
(Fallen)
Wheat bulb fly
Wheat bulb fly is a local and sporadically impor-
tant pest of winter wheat; infestations also occur
on winter barley and on early-drilled crops of
