Agriculture Reference
In-Depth Information
about 6 weeks later, mainly on the upper leaves
of host plants. They usually hatch within 10 days.
The sluggish larvae feed openly on the leaves for
up to 2 weeks and pass through four instars; most
individuals occur on the flag leaf. When fully
grown, each spins a white, cottonwool-like co-
coon (c. 6mm across), usually on the upper sur-
face towards the base of a leaf. Adults emerge in
about 2 weeks, usually from late July onwards.
The total life-cycle from egg to adult occupies
about 5 weeks (cf.
Oulema melanopa,
below)
but, especially at the pupal stage, may be ex-
tended considerably by lower than normal
temperatures. The egg-laying period tends to be
protracted so that, during the summer, all stages
of the pest often occur together. Development of
the pest is favoured by high spring and summer
temperatures; cool and wet conditions are detri-
mental to larval and pupal survival.
in excrement throughout their development. In
common with the previous species, development
is favoured by hot, dry conditions.
DESCRIPTION
Adult
4.0-4.8mm long;
head and antennae
black;
thorax
light reddish-brown;
legs
reddish-yellow
with blackish tarsi;
elytra
bluish-green, elongate
and noticeably broader than thorax.
Egg
1 mm
long, elongate-oval, shiny yellow.
Larva
up to
6 mm long;
head
shiny black;
body
dirty yellow-
ish, coated with brownish or blackish excrement
(Plate 4c).
NOTE
Adults of the polygonum leaf beetle,
Gastrophysa polygoni
(p. 143), are often present in
cereal crops and are frequently mistaken for those
of
Oulema melanopa.
Polygonum leaf beetles are
broader and more rounded in outline than adults of
Oulema.
They feed on
Polygonum aviculare
but do
not attack cereals or grasses.
DESCRIPTION
Adult
3-4 mm long, mainly black to bluish-black;
elytra
distinctly metallic;
antennae
prominent,
11-segmented;
thorax
relatively broad
(Plate 4a)
(cf.
Oulema melanopa,
below).
Egg
1 mm long,
shiny ochreous-yellow.
Larva
up to 5 mm long;
head
black;
body
dirty yellow and shiny, coated
with slimy brownish to blackish excrement
(except in the final stages of development)
(Plate
4b)
(cf.
O. melanopa,
below).
Phaedon cochleariae
(F.)
Watercress beetle
This beetle was formerly a common, widely
distributed pest of brassica crops (Brassicaceae),
including cabbage, cauliflower, mustard and
watercress. Attacks on vegetable crops are gen-
erally of no importance, although the adults
and larvae may be a problem in watercress beds,
particularly during the late spring and summer.
Most significant damage occurs on brassica seed
crops, especially white mustard. Adults and lar-
vae feed on the leaves of host plants and some-
times cause extensive defoliation; they also strip
away the outer tissue of the stems and this has a
deleterious effect on flowerhead development.
Feeding also occurs directly on the buds and
young pods, the pest being capable of causing
considerable loss of yield. Nowadays, however,
attacks tend to be sporadic and this insect is of
only minor importance.
Oulema melanopa
(L.)
Cereal leaf beetle
This beetle is an often common but usually
minor pest of cereals, including sweet corn, and
grasses; minor infestations also occur on orna-
mental grasses. Damage is identical to that
caused by
Oulema lichenis
(above); the beetles
are also virus vectors.
BIOLOGY
The life-history of this species is essentially simi-
lar to that of
Oulema lichenis
(above), except
that pupation occurs in the soil and development
from egg to adult tends to be less rapid, averag-
ing about 6 weeks. Further, larvae remain coated
BIOLOGY
Adults emerge from hibernation in April and
May. They then attack the foliage of host plants.
Eggs are eventually deposited in small cavities
bitten into the leaf surface
(Plate 4d).
The eggs
