Agriculture Reference
In-Depth Information
(gynoparae) then produce a wingless generation
of oviparae that, after mating with the winged
males, deposit the winter eggs. Under warm
conditions (including heated glasshouses in tem-
perate regions) this species breeds partheno-
genetically throughout the year and lacks a
sexual phase (i.e. is anholocyclic); the clover-
inhabiting form
B.h. warei
is also believed to be
anholocyclic.
damage, however, is largely cosmetic and rarely
of significance.
BIOLOGY
Although commonly overwintering in the egg
stage on old wild and cultivated brassica plants,
wingless aphids may also survive the winter if
conditions are suitably mild. Overwintered eggs
hatch in April, and breeding on the original host
plants is then resumed. Winged forms appear in
colonies from May onwards. These spread infes-
tations to newly planted brassica plants, where
colonies develop on the underside of leaves and
on flower heads. Colony development reaches its
peak in the autumn, when sexual forms are pro-
duced, by which time host plants may be heavily
infested and covered in aphids and secreted wax.
Winter eggs are laid in the autumn, particularly
during October.
DESCRIPTION
Aptera (on primary host)
small (up to 2 mm
long), rounded, shiny (with a slight waxy bloom),
varying in colour from brownish (= the late
winter/early spring form) to yellowish-green;
siphunculi
short, pale;
cauda
short, tongue-like
(Fig. 191b). Aptera (on secondary host)
very
small (often <lmm long), pale greenish or
whitish-yellow to pale pinkish-white.
NOTE Peach trees are colonized by
Brachycaudus
persicae
(Passerini) (black peach aphid) and
B.
schwartzi
(Borner) (peach aphid); these two species
are distinguished from
B. helichrysi
by their dark
siphunculi.
DESCRIPTION
Aptera
1.6-2.6 mm long, greyish-green, with
small paired black patches on the abdomen and
coated with greyish-white, mealy wax;
siphunculi
short, barrel-shaped;
cauda
short, triangular
(Fig. 188d).
Brevicoryne brassicae
(L.)
Cabbage aphid
Cavariella aegopodii
(Scopoli)
Willow/carrot aphid
This notorious aphid is a widespread and gener-
ally abundant pest of brassica crops, including
broccoli, Brussels sprout, cabbage, cauliflower,
oilseed rape and swede. Attacked leaves may
become discoloured (often reddish or purplish)
and distorted, severely weakening plants and
checking growth; heavily infested seedlings or
young plants wilt and may die. The presence of
the aphids, along with quantities of secreted
wax, can also be a problem at harvest, rendering
otherwise edible parts of plants useless for
human consumption. The aphids may also
transmit certain viral diseases, such as cabbage
black ring spot, cabbage ring necrosis and cauli-
flower mosaic. Damaging spring populations
on overwintering brassica seed crops, such as
winter oilseed rape
(Plate 2b),
occur only fol-
lowing mild winters. Heavy infestations cause
distortion and, often, a reddish or purplish
discoloration of both leaves and pods; such
This aphid is an important pest of carrot; infesta-
tions also occur on other umbelliferous crops,
including celery, parsley and parsnip. The aphids
cause significant damage to early and mid-season
crops, infested leaves turning yellow and plants
becoming stunted. The aphids are also important
vectors of persistent viruses (e.g. carrot motley
dwarf, carrot red leaf), semi-persistent viruses
(e.g. parsnip yellow fleck) and non-persistent vi-
ruses (e.g. celery mosaic), the foliage of affected
plants often becoming mottled with yellow.
BIOLOGY
Winter eggs occur on
Salix
(especially
S. alba
and
S. fragilis).
In spring, colonies of aphids de-
velop on the young shoots, culminating in the
production of winged forms that migrate in May
