PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Pest cycle
There is a gradual metamorphosis (egg,
nymphs and adults) with 2 overlapping generations
per year in southern Australia and 3-4 in northern
Australia (page 55). In southern areas the main
hatchings are usually in spring and autumn. The life
cycle takes 4-8 months in southern districts but less
in warmer areas, eg Qld. Each female may lay up to
2,000 eggs which appear like little heaps of sand
beneath the parent scale. The young nymphs, after
hatching remain beneath the parent scale for 1-2
days, and then crawl actively (crawlers) about
before settling usually along the veins on leaves or
on young shoots. After eggs have hatched, the
body of the female scale seems to shrink and
eventually falls off. After 4-6 weeks the young scale
moults and migrates to the stem of the plant, where
it remains for the rest of its life. After another 4-6
weeks moulting again occurs and the insect reaches
the ‘rubber’ or early adult stage, when the ‘H’
formation on its back becomes obvious. The period
from settling to the start of the next hatch of eggs is
3-4 months. The autumn hatched eggs may mature
on the leaves. Other species of soft scales have only
one generation of crawlers each year, while others
have several.
‘Overwintering’
On the host plant in cooler areas as adults.
Spread
Because of their small size and habit of feeding
in concealed areas, scales are commonly spread
on infested plants (cuttings, nursery stock, etc).
By nymphs crawling from plant to plant if plants
touch. Crawlers also move from plant to plant by
wind dispersal and on clothing and equipment.
By ants and other insects, by birds.
Conditions favoring
Temperate climates with moderate temperatures
and high humidities.
High temperatures (44 o C and above) and dry
conditions kill nearly all eggs and crawlers
beneath parent scales. Settled crawlers are not
so susceptible.
Dense unpruned portions of trees.
Vigorous citrus trees are more likely to suffer
infestation. Not usually a problem of olive trees
in good health but in some regions it seems to
attack trees of all health levels.
Management (IPM)
Are you a commercial grower or home gardener?
1.Prepare a plan that fits your situation.
2.Crop, region. Recognize variations, eg is black
scale really a problem. Host?
3.Identification must be confirmed. If in doubt
consult a diagnostic service (page xiv) as it is essential
to be able to distinguish:
The crawler and adult stage.
When they are likely to occur, are there 2, 4 or
6 generations each year?
Where each stage will occur during the
year, spring crawlers will settle on stems and twigs
while autumn crawlers may settle on leaves.
4.Monitor scales, crawlers and their predators and
parasites, honeydew, sooty mould and ants on fruit,
trunks, branches or leaves using a x 10 hand lens at
regular intervals. Ant presence increases when crawlers
are around. Record your findings:
Stock plants can be a source of scales. Check
mature scales for eggs underneath and check if any
adult scales are still alive.
Check and monitor for signs of beneficials,
eg predators and parasites, holes in scale covering.
Ants, sooty mould and honeydew on leaves,
stems and fruit especially if conditions are humid.
On citrus and other evergreen trees,
depending on your situation, check for crawlers
and natural enemies once in Nov-Dec and again
in Feb-March (Brough et al 1994). Check for
presence or absence of adult female scales on
5 randomly selected green twigs (with 5-10 leaves)
per tree. If trees are tall, take 10% of samples from
the tops of trees.
5.Thresholds vary according to the crop, eg
Citrus. The threshold may be 10% or more of
green twigs infested with one or more scales, while
for mandarins the threshold may be 5%.
Otherwise seek advice or determine your own
threshold depending on how much damage you can
tolerate economically or aesthetically.
6.Action. Take appropriate action when any threshold
is reached, eg sanitation, parasites and predators,
insecticides and controlling ants, if active, at tree base.
7.Evaluation. Review your program to see how
well it worked, recommend improvements if required.
Monitor trees for scale after treatment.
Control methods
Difficult to control because mature scales are
resistant to pesticides and correct timing is
necessary to target crawlers.
Cultural methods.
Maintain trees in good health but do not
encourage excessive vigour.
Provide wind shelter to limit spread of crawlers.
Pruning to provide open airy trees discourages
black scale infestation. Heavily infested plants
should be fertilized to restore vigour.
Harvest fruit at the correct time.
Sanitation.
Discard heavily infested house or stock plants.
Prune out or hand pick isolated infestations on a
few plants or wash with soap using a soft brush
to remove scales and sooty mould. Some scales
are easier to remove than others.
Biological control.
Many natural enemies when not undermined by the
indiscriminate use of sprays. Most common
biocontrols are parasitic wasps and ladybeetles.
Natural controls. Weather, parasites,
predators and diseases in some plantings can
exert some control. However, ants in large
numbers can deter parasitic wasps. Very hot
weather can kill many crawlers.
– Parasites. Several wasps parasitize adult black
scales. Some species of wasp will target a specific
species of scale but some attack a range of scales.
Introduced wasps (Aphytis spp., (common)
Aspidiotiphagus sp., Comperiella bifasciata,
Encarsia perniciosi, Metaphycus sp., Scutellista sp.
Native wasps (Rhopalencyrtoidea dubia,
Aenasoidea varia).
Wasps deposit eggs on or under scales, and
larvae feed on the scale. Parasitized scales are
dark and there is an obvious exit hole.
Wasp parasites together with proper pruning
may provide sufficient control in some areas. In
other regions biological control may be ineffective.
Baker, G and Hardy, J. 2005 Survey Black Scale
Parasitoids in South Austraian Olives. Sardi, SA.
– Predators
Ladybirds and larvae scatter scale eggs and kill
adults, eg mealybug ladybird (Cryptolaemus
montrouzieri), ladybirds (Orcus australasiae, O.
chalybeus), Diomus spp. and steelblue ladybird
(Halmus chalybeus). Also gumtree scale ladybird
(Rhyzobius ventralis), black ladybird (R. forestieri),
and scale-eating ladybird (R. lophanthae).
Insects and allied pests - Hemiptera (bugs, aphids, etc) 165