Agriculture Reference
In-Depth Information
damage is often followed by bacterial rotting of
tissue and tends to be most severe in June, par-
ticularly under dry conditions. Fully grown lar-
vae 'jump' to the ground and eventually pupate
in cocoons in the soil, the next generation of
adults appearing shortly afterwards; some larvae,
however, do not pupate but remain in a state of
diapause for up to 1 or more years. There are
usually from three to five generations each year,
depending on temperature.
30 per female) are laid in batches in the pods
of brassica plants in late May or early June.
The adult females are unable to penetrate an
unblemished pod wall and usually deposit their
eggs through feeding or egg-laying punctures
previously made by adults of the cabbage seed
weevil,
Ceutorhynchus assimilis
(p. 154). Sites of
other mechanical injury are also utilized; later
in the season, for example, second-generation
midges may deposit eggs through holes (exit
holes) formed in pod walls by fully fed seed wee-
vil larvae. Midge eggs hatch within a few days.
The larvae then feed gregariously on the pod
wall
(Plate 6c),
without damaging the central
septum, causing premature ripening and split-
ting. (In prematurely split, bird-damaged pods,
the septum is usually broken.) When fully grown,
the larvae escape and fall to the ground. They
then enter the soil and pupate, each in a small
silken cocoon. A second generation of adults
appears about 2 weeks later. Some larvae, how-
ever, do not pupate but remain in a state of
diapause until the following or subsequent sea-
sons. There are usually up to three or more gen-
erations per year, depending upon temperature,
although any one crop is likely to sustain only
two generations in a season.
Contarinia pyrivora
(Riley)
Pear midge
This univoltine midge is a damaging pest of pear
but tends to occur mainly on garden trees rather
than in commercial orchards. Adult midges are
active in the spring, when they deposit eggs in the
open blossoms. The relatively large, up to 5 mm
long, yellowish-white larvae feed gregariously
within the developing fruitlets and become fully
grown in about 6 weeks. They cause severe dis-
tortion; affected fruitlets also turn black and, on
reaching approximately 15-20 mm in diameter,
usually crack open and decay. Fully fed larvae
overwinter in the soil, and adults emerge in the
spring.
DESCRIPTION
Adult
2 mm long, greyish-brown, with a pinkish
(male) or red (female) abdomen.
Egg
minute,
sausage-shaped and translucent.
Larva
up to
2 mm long, whitish; the younger stages are
translucent.
Dasineura brassicae
(Winnertz)
Brassica pod midge
Infestations of this potentially major pest occur
on oilseed rape and other brassica seed crops but
not on white mustard. The larvae feed within the
developing pods and cause premature ripening
and splitting, which may result in significant
seed loss. Infested pods often swell (the so-called
'bladder pod' symptom) but this symptom is
not expressed on all hosts. Midge damage is of-
ten concentrated on headlands and decreases
markedly further into the crop.
Dasineura alopecuri
(Reuter)
A foxtail midge
This widely distributed pest occurs on meadow
foxtail throughout northern Europe and can
have a very significant effect on seed crops. The
larvae feed within the florets, affecting germina-
tion; they also cause direct damage to the seeds,
which often develop a distinct lateral depression.
Adult midges are red and appear in May. The
females deposit reddish eggs singly in the florets,
the long ovipositor readily penetrating beneath
BIOLOGY
Adult midges emerge in May. They are weak
fliers, and the egg-laying females migrate down-
wind to seek host plants, usually within a few
hundred metres of emergence sites. Eggs (up to
