Agriculture Reference
In-Depth Information
florets, and badly affected leaves are reduced to
shreds of ragged tissue. Some larvae destroy the
ovules (such larvae are termed 'ovuliferous'),
whereas others attack the seeds, and heavy infes-
tations will result in considerable yield loss. In
the British Isles, this species is most abundant in
southeastern England.
BIOLOGY
Adults emerge in the spring, from late April on-
wards. In common with other species of
Apion,
they may feed on the foliage of various non-
fabaceous plants before migrating to clover
where, later in the season, breeding takes place.
The oviposition period is protracted, commenc-
ing in mid-May and lasting for about a month.
Eggs are placed in the green florets, usually sev-
eral per flower head. They hatch in about a week.
The larvae feed within the developing florets and
complete their development within 3 weeks,
passing through three instars. They then pupate
within the flower head. Adults emerge about a
week later but their appearance may be delayed
by very dry conditions. These adults usually
initiate a second generation. However, on late-
flowering crops there is just one generation.
Young adults eventually hibernate under leaves
and other debris in ditches, hedges and similar
situations.
Fig. 225
Clover seed weevil,
Apion trifolii
(x25).
holes (c. 1mm in diameter) or elongated
punctures (c. 3 mm long) in the leaves but are of
significance only as vectors of viruses such as
broad bean stain virus (BBSV) and broad bean
true mosaic virus (BBTMV); these viruses
are also transmitted, but to a lesser extent, by
pea & bean weevil,
Sitona lineatus
(p. 161).
Damage to bean flowers by larvae is of no
significance.
DESCRIPTION
Adult
1.8-2.2 mm long, distinctly pear-shaped
and mainly black, with a long, narrow rostrum
(Fig. 225). Egg
0.5 mm long, oval, pale
greenish-yellow and shiny.
Larva
up to 2 mm
long;
body
dirty white, plump and distinctly C-
shaped;
head
reddish-brown.
NOTE Several other species of
Apion
are associated
with clover crops. These include:
A. assimile
Kirby and
A. apricans
Herbst (both mainly on red clover) and
A.
dichroum
Bedel (mainly on white clover). Larvae of
these species are all ovuliferous.
BIOLOGY
Adults overwinter in woodlands and become
active in the early spring. They occur and often
feed on plants such as
Galeobdolon luteum,
Mercurialis perennis, Rubus fruticosus
and
Urtica,
before migrating to field bean (and other
members of the family Fabaceae) upon which
they are able to breed. Adults may be found
in bean fields from late April or early May on-
wards, where they feed on the leaves and where
eggs are eventually laid in the flowers and flower
buds. The eggs hatch a few days later. Larvae
feed on the pistils and styles for about 2 weeks
before eventually pupating, usually amongst
the shrivelled remains of the keel petal. Adults
emerge shortly afterwards. Development from
egg to adult takes about a month and there are
up to two generations annually.
Apion vorax
Herbst
Bean flower weevil
This weevil is a minor and usually sporadic pest
of field bean. The adults form small, rounded
