PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Cabbage aphid
Scientific name
Brevicoryne brassicae (Order Hemiptera).
Common and serious pest of Brassicas
wherever they are grown throughout the world.
Several other aphids will also attack Brassicaceae,
including the green peach aphid (Myzus persicae),
turnip aphid (Lipaphis erysimi).
Host range
Brassicas, including:
Vegetables, eg broccoli, brussell sprouts,
cabbage, cauliflower.
Ornamentals, eg stock, wallflower.
Field crops, eg rape, turnip.
Weeds, eg Indian mustard, shepherd's purse, wild
radish, turnip weed.
Description & damage
Adult aphids are globular and about 2.5 mm
long when mature, slaty grey and covered with a
mealy material. Globules of honeydew may be
seen among aphid colonies. Winged and wingless
forms occur together on the same plant.
Leaves, flower heads. Nymphs and adults
damage plants by piercing plant tissues and
sucking plant juices. Infestation usually starts on
leaf upper surfaces, a single winged female
surrounded by wingless young; leaves curl in and
protect the colony. Aphids prefer to feed on the
tender growing parts of the plant, eg youngest
leaves and flowering parts deep within heads of
cabbage and Brussel sprouts. Large colonies
cause much curling of the leaves and twisting of
tender shoots making the aphids hard to control.
Plants stop growing and leaves become yellowish,
they may suddenly wilt and die.
Transplanted seedlings. Cabbage aphids do
not usually infest seedlings but build up after
thinning or transplanting. Large colonies can
stunt or kill small plants.
White caste skins (left behind after aphids
have moulted), honeydew, ants, sooty mould.
Aphids are most abundant on lower leaves
of established plants.
General. Spread through a crop is rapid.
Transplanted seedlings. Cabbage aphids do
not usually infest seedlings but build up after
thinning or transplanting. Large colonies can
stunt or kill small plants.
Transmission of virus diseases.
Cabbage aphid is a vector of the cauliflower
mosaic virus and possibly other virus diseases as
well in a non-persistent/semi-persistent manner.
The virus is lost during moulting and is not
transmitted directly to progeny.
Diagnostics.
Because of their mealy-gray appearance and habit
of clustering together on leaf undersurfaces to form
colonies, cabbage aphids are easy to recognize.
Pest cycle
There is a gradual metamorphosis (live
nymphs, adult) with many generations each season.
The aphids first appear on the leaves in small
colonies, which may include adults and a variable
number of young. Such colonies reproduce
rapidly, no eggs are laid, active young are born
alive. In warmer areas a generation matures in
2 weeks. However, as the number of aphids in an
area increases, the food decreases and their
individual growth become much slower.
‘Overwintering’
In warmer areas, young are born alive throughout
the year while in cooler areas wingless forms
overwinter. In colder countries, this species has
an egg-laying form and the eggs ‘overwinter’.
Spread
By winged forms flying. Winged forms are
produced when the colony is crowded, spread
through a crop is rapid.
By seedlings.
Conditions favoring
Warm, dry conditions during late summer and
autumn. In coastal areas they may be important
pests in spring. Aphids multiply in colonies at an
incredibly rapid rate depending on temperature, eg
each female can produce 3 young/day at 16 o C and
up to 6/day at 24 o C. Aphids do not thrive in very
hot and dry or very cold conditions.
Fig.104. Cabbage aphid (Brevicoryne brassicae).
Curling and distortion of cabbage leaves caused by
cabbage aphids. PhotoCIT, Canberra (P.W.Unger).
150 Insects and allied pests - Hemiptera (bugs, aphids, etc)