PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Pest cycle
There is a gradual metamorphosis (egg,
nymphs, adult) with many generations each year.
In spring aphids multiply rapidly on peach and
nectarine trees, producing wingless young-bearing
females and cause serious damage. In early
summer, as the peach foliage hardens off, the
winged forms migrate to their secondary food
plants (ornamentals, vegetables and weeds), where
they spend the summer. In late autumn, some
females and males migrate back to the peaches and
nectarines. Eggs are laid about the bases of the
buds from May to mid-July. Eggs may also be laid
on cherry trees but fail to survive. Eggs hatch in
late July to mid-August, but the aphids remain in
the buds until bud burst when they multiply rapidly
as the trees come into leaf. In warm districts (eg
coastal areas) the autumn migration to the peach
and nectarine trees does not occur and so eggs are
rarely seen.
‘Overwintering’
In mild climates there is no egg stage. Aphids
breed throughout the year on secondary host plants
occasionally producing winged forms which fly to
other secondary hosts. Where winters are cold,
eggs ‘overwinter’ about the bases of buds on peach
and nectarine trees. Sheltered conditions may
allow aphids to survive throughout the year.
Spread
As winged forms flying assisted by wind.
Movement of infested plants, young nursery
trees may carry over-wintering eggs. Seedlings
and container plants may carry wingless forms.
Conditions favoring
Aphids generally do not like hot dry weather.
Aphids are seasonal pests and may occur in
large numbers for a relatively short time usually
during spring and autumn.
Abundant growth of herbaceous weeds in the
previous summer and autumn,
Late leaf-fall from the peach trees.
Cool wet weather in spring favours severe
damage in spring due to the slow hardening
of early peach growth and delayed
appearance of natural enemies.
Researchers in the UK have found that GPAs
that had become more resistant to pesticides
were less able to withstand the British winter.
They also became less sensitive to their
environment and didn’t notice warnings from
alarm pheromones secreted by other aphids and
became victims to ladybirds and other predators.
Management (IPM)
Are you a commercial grower or home gardener?
1.Access a plan in advance that fits your situation.
2.Crop, region. Recognize variations as weather can
affect severity of damage and appearance of natural
enemies. GPA is a serious pest.
3.Identification must be confirmed. Consult a
diagnostic service if necessary (pages 152, xiv).
4.Monitor at least weekly for aphids and beneficials
when they are expected, seek advice (page 39). Know
when and where to look, what and how to monitor.
Stone fruits. 'Overwintering' eggs on peach and
nectarine trees in cool climates; can be monitored
by inspecting bark of all trees while they are being
pruned. Mark all trees infested with the tiny black
'overwintering' eggs (Brough et al 1994.
Ornamentals and vegetables crops. Monitor
populations on commercial crops. Control measures
can be implemented before damage is obvious.
– Yellow sticky traps can be used to monitor winged
adults; other insects are also attracted so accurate
identification of the trapped insects is important.
– Examine say 10 plants at say 5 locations in the
crop for aphids, caste skins, honeydew and black
sooty mould, predators and parasitized aphids
(mummies). Also look for ants.
5.Thresholds will vary depending on the crop.
Economic damage to ornamentals and vegetables can
be considerable. There may be a nil threshold on
peach trees, ie all infested trees should be treated.
There may be a complaint threshold for honeydew.
6.Action/control. Unless aphids are present in high
numbers on some crops, eg potatoes, they may not
cause major damage. Biological control agents are
commercially available. Spray all marked infested
peach trees with winter oil either while trees are
dormant or at budswell. Brough (1994) suggested
checking results at flowering by inspecting 5 flower
clusters from each infested tree. If aphid colonies are
found spray with a recommended insecticide when all
petals have fallen. Control ants if present.
7.Evaluate the program, are improvements needed?
Fig. 106. Green peach aphid
(Myzus persicae). PhotoNSW Dept of
Industry and Investment (E.H.Zeck).
All enlarged about x14
1. 1 st stage nymph
2. Adult wingless viviparous female
3. Pre-adult nymph of winged
viviparous female
4. Adult winged viviparous female
5. Overwintering egg when first laid
6. Eggs several days old
Actual size
7. Peach shoot showing curling and
distortion of leaves
8. Peach twig; arrows indicate
where eggs are generally laid
Insects and allied pests - Hemiptera (bugs, aphids, etc) 153