PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Pest cycle
There is a gradual metamorphosis (egg,
nymphs (5 stages) and adult). There may be
3 overlapping generations during spring, summer
and autumn. Females deposit pale yellow bananashaped
eggs in the soil in batches (egg pods) of
30-50. Up to 4 egg pods may be laid by a single
female but rare for females to survive that long. A
concentration of egg pods is termed an egg-bed.
During outbreaks, eggs may be laid in a variety of
situations, including fallow and stubble paddocks
as well as among grass roots in sandy soil. Egg
beds may vary from a few to several hundred
square meters scattered irregularly throughout an
area. Eggs need warmth and moisture to hatch.
Nymphs move away from egg beds as solitary
insects, as numbers increase they form dense
bands of hoppers which vary in size from a few
square meters to several hectares. Hopper bands
may merge and increase to several kilometers
with a distinct front.
After the final moult, adults concentrate in
swarms which make low drifting flights up to
50 meters high and can cover 10-20 km per day,
but can migrate over much larger distances
during the night.
Research into how to prevent locusts from
forming bands and swarming is ongoing.
‘Overwintering’
Usually eggs laid in soil in autumn overdiapause
(in a state of arrested development) and
hatch in spring as soil temperatures increase.
During dry weather eggs become quiescent and
may survive for 8-10 months.
‘Overwintering’ nymphs over-diapause in the
3 rd stage but few survive until spring.
In regions with a more uniform and higher
rainfall (300-500 mm) frequent outbreaks result
from local buildup or breeding by immigrants.
These outbreaks may persist and develop into
plagues.
– Locust numbers multiply rapidly in seasons of
above-average rainfall, particularly when the
previous season had below-average rainfall or
drought conditions. At such times, locust numbers
may increase simultaneously over a vast area.
– Egg laying by 1st generation females may be
concentrated resulting in localized high density
hatchings which then form hopper bands.
– This 2nd generation which is crucial in the
development of an outbreak, produces locusts in
numbers sufficient to form dense swarms.
– The swarms may move long distances, expanding
the infested area and damaging pastures and crops.
Distance traveled by swarms varies greatly and
depends on factors such as age of adults, swarm
size, density of locusts in the swarm, stage of the
outbreak process and prevailing weather
conditions, eg wind speed, temperature.
Long distance migration will occur at night
in suitable weather and if green feed has been
available to enable fat accumulation.
– They may simply disperse within a local area or the
majority may take off at dusk reach considerable
altitudes and travel downwind until conditions for
flight become unsuitable.
– Locusts can travel up to 500 km in a single night,
leading to a sudden ‘overnight’ relocation of an
outbreak.
Swarms flying during the day tend to follow
well-defined pathways determined to some
extent by barriers such as timberlines along
rivers or chains of hills.
The introduction of farming, land clearing and
grazing has provided them with the ideal
environment to build up numbers and swarm.
Spread
Late stage hoppers can travel up to
500 meters/day when in dense bands.
Adults flying in swarms assisted by wind can
extend an outbreak up to 200 km from the
breeding area in a single generation during night
migration.
Outbreaks in areas of intensive agricultural
production are typically the result of an influx
from breeding in inland areas.
Conditions favouring
Locusts have occurred in plagues since the earliest
of times, destroying crops even then. Droppings
were considered lethal. Moderate outbreaks of the
APL occur in most years in the interior of
Australia. Major plagues occur less frequently.
Locust outbreaks occur:
In seasonally arid areas of NSW, Qld and SA
(annual rainfall less than 300 mm) outbreaks are
infrequent and can be the result of successful
breeding in response to widespread rainfall.
Such outbreaks tend to collapse suddenly with
the return to unfavorable breeding conditions or
with emigration. Migrants from these short-lived
outbreaks often start secondary outbreaks in
other areas to the south and east.
Fig. 122. Australian plague locust. Hot northerly
winds ahead of a depression (often associated with a cold
front) may induce mass take-off at dusk and long distance
migration downwind during the evening. Cold conditions
in the wake of the depression stop further migration and
prevent any return movement. Night flights can result in
sudden relocation of plagues. Photo NSW Dept of Industry and
Investment.
Insects and allied pests - Orthoptera (locusts) 183