Agriculture Reference
In-Depth Information
petioles and mid-ribs split open, and shoots be-
come blackened. Larvae may also occur as con-
taminants in crops sent for processing. In the
British Isles, attacks occur most frequently in
southern and southeastern England.
BIOLOGY
Adults are active from May onwards. Eggs are
deposited in the soil, usually in the presence of
decaying organic matter. The eggs hatch within a
few days, and the larvae then tunnel inside ger-
minating bean seeds, young bean stems and
other suitable hosts. They feed for approxi-
mately 1-3 weeks and then pupate in the sur-
rounding soil. New adults appear 2-3 weeks later
and, after mating, females initiate a further gen-
eration; each fly deposits about 50 eggs. There
are usually three to five generations each year.
BIOLOGY
Adults first appear in the spring, and eggs are
then deposited singly along the veins on the
upper surface of young leaves. The eggs hatch
within a few days. Larvae immediately burrow
into the leaf tissue to feed within expansive
blotch mines; fully fed individuals eventually pu-
pate in the soil. Occupied mines occur from May
to late September or early October, and there
are three or four generations annually.
DESCRIPTION
Adult
6 mm long, greyish-brown;
wings
5mm
long.
Egg
1.0 x 0.3 mm, elongate-oval, white.
Larva
up to 8mm long, white, relatively robust;
head
only partly retracted into the prothorax;
posterior papillae
relatively prominent, the very
small hind-most (median) pair contiguous and
inconspicuous
(Fig. 266c)
(cf. onion fly,
Delia
antiqua,
p. 197).
Puparium
4-5 mm long, oval,
reddish-brown.
DESCRIPTION
Adult
4-5 mm long, yellowish-grey with a
slightly darker median stripe on the thorax and
abdomen;
legs
black;
wings
hyaline, iridescent,
with yellow veins.
Larva
up to 7 mm long,
white, with plant sap often congealed on the
body to form distinctive black rings;
head
deeply
retracted into prothorax;
anterior spiracles
8-
lobed and fan-like;
posterior spiracles
relatively
large and prominent (cf. mangold fly,
Pegomya
hyoscyami,
p. 200).
Delia radicum
(L.)
Cabbage root fly
The cabbage root fly is a major pest of brassica
crops (Brassicaceae) and causes considerable
damage to summer cabbage and autumn cauli-
flower; infestations are also of significance on
Brussels sprout, calabrese, Chinese leaf, radish,
swede, turnip and ornamentals such as wall-
flower. Most damage results from eggs laid by
first-generation flies in late April and May. Seed-
lings or recent transplants wilt and die, as the
fibrous roots are eaten away and much of the tap
root destroyed; damaged root systems are also
liable to subsequent attack by fungal pathogens.
Older or less heavily infested plants survive, but
are usually stunted, and the outer leaves fre-
quently become discoloured (often blue-green or
reddish) and may wilt.
Brussels sprout:
larvae
often feed within the buttons to cause extensive
damage, and much of the inner tissue turns
brown. Such larvae, and the damage they have
caused, often remain undetected until after
Delia platura
(Meigen)
A bean seed fly
This fly is an increasingly important pest of
French bean and runner bean, and various other
vegetable crops, including Brussels sprout, cab-
bage, cauliflower, lettuce, onion, pea, potato and
radish; infestations also occur on sunflower and
certain ornamentals. The larvae often hollow out
the seeds, which then fail to germinate; they also
cause death or severe stunting of seedlings.
Damage is particularly severe when germination
is slow. In trashy conditions (especially following
crops such as mustard, oilseed rape, parsnip
and sugar beet), larvae may attack germinating
cereal plants. On potato, the larvae will also act
as vectors of the bacterial disease known as
'black leg'.
