Agriculture Reference
In-Depth Information
sprout, producing brown markings on the outer
leaves, and on developing cauliflower curds,
causing the tissue to turn pinkish; the weevils
may also feed on the young leaves of other veg-
etable brassicas, and check the growth of recent
seedlings or transplants.
BIOLOGY
Overwintered adults emerge in May. They are
attracted to flowering brassicaceous weeds, such
as
Sinapis arvensis,
and also to brassica seed
crops, especially oilseed rape and white mustard.
Here, they feed on the buds, flowers and devel-
oping pods. After mating, the females deposit
eggs singly in the young pods, usually from late
May onwards. The weevils make distinct egg-
laying punctures in the pod walls and then mark
the pod with a pheromone to deter other
ovipositing females. The eggs hatch in 1-2 weeks
and the larvae then attack the developing seeds.
Larvae feed for 3-6 weeks, depending upon tem-
perature. When fully fed, usually in late June
or July, each larva bites its way out of the pod,
leaving a pinhead-sized exit hole in the wall
(Plate 5a),
and drops to the ground. Individuals
then pupate in earthen cells a few centimetres
below the surface. Young adult weevils appear
about 2 weeks later, usually from mid-July to
August. These weevils feed on brassicaceous
weeds, and may also attack vegetable brassicas,
before eventually taking up their winter quarters
amongst debris in nearby woodlands or
hedgerows.
Fig. 229
Cabbage seed weevil,
Ceutorhynchus
assimilis
(x20).
by their small size (1.5-1.8 mm long) and somewhat
Apion-like
appearance; the antennae of this species
have just six funicular segments (there are seven in
C. assimdis).
Ceutorhynchus napi
Gyllenhal
Rape stem weevil
This weevil is a major pest of winter rape in
France, Germany and Switzerland, and in other
parts of continental Europe, but does not occur
in the British Isles; infestations also occur on
other crops, including cabbage and turnip. Most
significant damage is caused by the process of
egg laying within the elongating stems, rather
than by larval feeding. Main stems typically twist
and split open, especially following heavy rain
or frosty conditions, and considerable distortion
and disruption of growth results. Infested stems
are often killed and plants then survive only
by producing lateral shoots. Damaged plants
are particularly liable to invasion by secondary
organisms such as canker.
DESCRIPTION
Adult
2.2-3.0 mm long, lead-grey, unicolo-
urous, with a long, slender rostrum
(Fig.
229). Egg
0.6 x 0.4 mm, oval, creamish-white
and translucent.
Larva
up to 5 mm long;
body
creamish-white and sac-like;
head
light brown.
NOTE Adults of
Ceutorhynchus floralis
(Paykull). a
greyish-black weevil, appear to be increasing in num-
bers on oilseed rape crops. Although of no pest status
(it breeds on weeds such as
Capsella bursa-pastoris
and
Erysimum cheirantholdes),
the adults are some-
times mistaken for
Ceutorhynchus assimdis
and can,
therefore, contribute during pest monitoring to over-
estimates of the number of seed weevils present in
a crop. The weevils are most readily distinguished
BIOLOGY
Adults appear in the early spring and then
invade rape crops, eventually depositing eggs
