PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Green peach aphid
Scientific name
Myzus persicae (Order Hemiptera) is a common
and major pest of many plants.
Host range
Primary. food plants. Peaches and nectarines,
also apricot and plums, rarely almonds.
Secondary. food plants. Wide range of plants
including ornamentals, eg Iceland poppy, rose;
fruit, eg plum, strawberry; vegetables, eg
cabbages, peas, potatoes; weeds, eg capeweed,
dock, sowthistle. Many ornamentals, vegetables,
fruit and weeds may host several aphid species.
Description & damage
Aphids have sucking beaks and feed by puncturing
plant tissues and extracting sap.
Adults are globular, mostly green with dark
green markings but may be pale yellow or pale
pink and about 2.5 mm long. On the abdomen
there is 1 pair of cornicles with distinct dark tips.
Aphids live in colonies formed from the young of a
single female, single or small numbers of aphids
are sometimes found on shoots. Nymphs are like
adults except smaller and wingless, colour varies
from green to pale yellow and pale pink. White
papery nymph skins are shed as they moult and
grow and are found on leaves and buds.
Leaves. The green peach aphid sucks sap from
young leaves causing them to wrinkle. It produces
copious amounts of honeydew, which attracts
ants and on which sooty mould grows reducing
photosynthesis, making plants shiny, unsightly and
unsaleable. Honeydew is sticky and may drip on
onto underlying leaves and plants, floors, seats,
etc. Ants protect and care for the aphids, move
them around and keep away predators and
parasites. In return ants feed off the honeydew.
Transmission of virus diseases.
Over 100 virus diseases of secondary hosts
may be transmitted from plant to plant by the
green peach aphid during feeding, eg cucumber
mosaic virus (the most common plant virus in
the world), turnip mosaic virus, potato leaf roll
virus, pea mosaic virus, bean yellow mosaic.
Green peach aphid does not transmit virus
diseases of stone fruits.
Ornamental plants affected by virus diseases
transmitted by green peach aphid include
carnation, chrysanthemum, gladiolus, tulip,
lilium, hyacinth, iris, narcissus, daphne, lilac.
DAMAGE TO STONE FRUITS.
Flowers/Fruit. Aphids feeding on swelling
buds often cause premature opening of flowers.
A single petal emerging from a bud indicates its
presence. Later generations feed on flower parts
before the petals unfold fully. Opening buds and
flowers are distorted and fall readily, reducing fruit
setting. Young fruit may be attacked and fall.
Leaves/shoots. Aphids feed on spring growth
and inject toxic saliva into the developing plant
tissue producing shoot, leaf and flower distortion.
Aphids infest young leaves and laterals of fruit
trees, causing the leaves to curl, shrivel and fall
(page 153, Fig. 106; page 31, Fig. 16). Trees may
become unproductive and may take several years
to recover from repeated severe attacks.
DAMAGE TO ORNAMENTALS.
Green peach aphid attacks new growth in spring
causing shoot, leaf and flower distortion, wilting
and retarded growth in a range of ornamentals.
Small numbers affect plant appearance and
distorted leaves cannot be fixed. Large numbers of
live young may occur in hot spots in a crop.
Diagnostics.
On stone fruits green peach aphids are easy to
recognize from other aphids. Cherry aphids and
black peach aphids are black.
If aphids have moved to secondary hosts,
look for white nymph skins on leaves and buds of
these plants, drops of honeydew, ants, sooty
mould, shriveled leaves. Check leaf undersurfaces.
On stone fruits do not confuse green peach aphid
damage to leaves with the fungal disease peach
leaf curl ( pages 152, 358). It is possible to have
both problems together in spring.
Comparison of symptoms:
Leaves
Other
features
Parasitized aphid
Green peach
aphid injury
Leaves wrinkled (no
blisters or thickening),
aphids, nymph skins,
honeydew, infested
leaves die and fall
Infested leaves die
and fall trees become
unproductive
Peach leaf curl
symptoms
Blistered, may be
thickened, affected
areas blacken and
die, affected leaves
fall
Fruit may be
infected, reduced
fruit crop, reduced
tree vigour.
Unparasitized aphid
Fig. 105. Green peach aphid (Myzus persicae).
Note distinctive cornicles at rear of aphids.
Left: Parasitised aphid with exit hole through which the
adult wasp has emerged. Right: Unparasitised aphid.
152 Insects and allied pests - Hemiptera (bugs, aphids, etc)