PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Pest cycle
There is a complete metamorphosis (egg,
larva, pupa and adult) with several overlapping
generations each growing season. In warmer areas
there are 10 or more generations/year. Each female
moth lays about 1000 eggs which are white, domeshaped,
finely striped about 0.5 mm across, singly
on upper parts of plants, eg growing tips, sepals,
petals, young fruits and flower buds. Eggs change
to yellow then brown prior to hatching. When fully
grown larvae leave the host and burrow 8-10 cm
below the soil surface to pupate. Pupal stage may
be as long as 2-3 weeks in warm weather and up to
6 weeks in cooler conditions. Life cycle from egg
to adult can be about 4-6 weeks in summer or up to
12 weeks in cool weather.
‘Overwintering’
As pupa in soil. Moths do not emerge from
pupae formed in mid to late autumn until the
following spring and early summer. In cooler areas
they have fewer generations. The pupae enter a
diapause (resting state) in autumn and adult
moths emerge in spring.
Spread
Moths can fly only for short distances up to
50 meters but can be carried up to 100 km, by wind
to new hosts in bloom. They are attracted to lights.
Movement of infested produce.
Conditions favoring
Warm, moist, weather. Damage may be severe
and widespread during periods of good summer
rainfall when moisture stimulates emergence of
moths and food plants are plentiful. Long dry
cool spells delay emergence of moths.
Amount of damage varies from year to year.
Corn earworm (H. armigera) is more common in
coastal, sub-tropical and northern areas. Native
earworm (H. punctigera) is widely distributed
throughout the inland and southern states.
Usually there are 2 main periods of infestation,
spring-early summer and autumn. Most common
in late summer to autumn.
Plentiful hosts starting to flower and fruit.
Management (IPM)
Are you a commercial grower or home gardener?
1.Prepare a plan that fits your situation. Growers
should obtain local information on scouting and
recommendations on control.
2.Crop, region. Recognize variations and regional
susceptibility. Resistance and Best Management
Strategies for Helicoverpa have been developed for
some crops, eg cotton, sweetcorn.
3.Identify the exact Helicoverpa species causing the
damage. Consult a diagnostic service if necessary
(page xiv). Fact sheets for your crop.
4.Monitor pest and/or damage and record results as
recommended (page 39), before deciding to use a
biological or chemical insecticide or release beneficials.
Monitor adults using pheromone traps to detect
presence of moths and indicate population size.
Monitor for very small larvae and eggs
regularly at the appropriate times depending on
weather, especially after heavy rainfall.
Trap crops associated with cotton crops can be used
for predicting Helicoverpa populations.
5.Thresholds which differ, depending on the crop
and/or crop value, precise species of caterpillar,
region, season, climate, planting date, have been
developed for some crops. How much damage can
you accept? Remember that the threshold is the break
even point where the cost of control equals the cost of
likely damage, so you are no worse of if you spray
and no worse of if you don’t.
6.Action. Spraying thresholds are unlikely to be more
than guidelines for timing sprays. Examine crops at
least twice per week during danger periods. Before
deciding to spray consider:
Likely extent and severity of infestation.
Ability of crop to either tolerate caterpillar damage
without any significant loss or to replace leaves or
fruiting parts lost to caterpillars.
Estimated value of likely loss if crop is left
untreated against anticipated cost of treatment.
Only spray eggs and very small caterpillars (up to
5 mm long). Larger caterpillars are unlikely to be
controlled.
7.Evaluation. Review IPM program to see how well
it worked. Recommend improvements if required
based on records of infestation in the current and
previous seasons. Seek advice if necessary.
Control methods
Cultural methods. Cultivation will damage
pupae, survivors may be eaten by birds, mice or
earwigs. Hot wet conditions favour disease in
larvae and may sharply reduce populations. Heavy
rainfall may wash eggs off leaves; heat may kill up
to 50% of the eggs and larvae.
Sanitation. Attack in corn cobs can be
prevented by cutting the tips off cobs and the silks
after the latter are brown and beginning to dry out.
For small infestations caterpillars can be handpicked
off the plant. Remove alternative weed
hosts. Destroy infested plant material and debris to
prevent development of the pest.
Biological control.
Natural controls are of limited effect:
– Predators feed on eggs and larvae. Most abundant
predators are birds, ladybird beetles, pirate bugs,
black mired bugs and spiders which eat about 60%
of eggs on unsprayed plants. Night stalker
spiders are season-long predators of Helicoverpa
eggs on cotton. Ants are early season predators of
Helicoverpa eggs at the edges of cotton fields in
Australia and in the USA are being considered for
the biological control of insect pests of cotton.
– Parasitic wasps and flies parasitize eggs, larvae
and ‘overwintering’ pupae.
– Diseases (viral, bacterial, fungal) infect caterpillars
and are favoured by hot wet conditions. Some mycoinsecticides
(based on fungi) are being researched for
commercial use against Helicoverpa spp.
Commercially available agents include:
– Parasitic wasps (Trichogramma pretiosum).
Parasitized eggs may be purchased and released.
Eggs attacked by Trichogramma turn black
3-4 days of attack. Trichogramma can be:
Encouraged in crops by avoiding broad
spectrum insecticides or using pesticides not
toxic to Trichogramma www.goodbugs.org.au/
Purchased. Microplitis wasps also parasitize
larvae. List of suppliers www.goodbugs.org.au/
Released after Helicoverpa eggs have been
collected from sorghum and maize crops and
assessed for levels of parasitism.
– Food attractants.
Natural enemies in bush around crops can be
attracted to the crops by Envirofeast (yeastbased)
which attracts > 20 species of beneficial
insects into cotton crops to feed on Helicoverpa
spp. For Envirofeast to work effectively a source of
beneficial insects or a ‘refuge’ to draw them from
is essential.
Insects and allied pests - Lepidoptera (butterflies, moths) 87