PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Pest cycle
There is a gradual metamorphosis (egg,
4 nymph stages and adult) with many generations
each year. One generation from egg to adult takes
3-8 weeks, depending on the temperature and the
host plant. Each female lays several hundred eggs
in circles or arcs on smooth-leafed plants, or
scattered about if leaves are hairy. Each egg has a
short stalk which is embedded in leaf tissue,
supporting the egg in an upright position; a cluster
of eggs looks rather like a group of minute pegs.
After hatching, the 1 st stage nymph crawls about on
the underside of the leaf for up to 3 days before
settling down to feed. Later nymphal stages
complete their development at this site chosen by
the 1 st stage nymph. These oval shaped immature
forms look like scale insects. The adult emerges
after the 4 th nymphal stage.
‘Overwintering’
Outdoors in cooler climates as unhatched eggs on
leaf undersides and as adults in sheltered places.
In warm climates and in greenhouses the cycle is
continuous.
Spread
As adults flying assisted by wind.
Movement of infested plants carrying eggs,
nymphs and/or adults.
Conditions favoring
Mild moist conditions. Optimum temperature is
about 30 o C when life cycle takes about 18 days.
Outdoors, sporadic pest in protected humid sites
in late spring, summer and autumn.
High nitrogen levels.
Management (IPM)
Are you a commercial grower or home gardener?
1.Plan well in advance, consulting professionals if
necessary, eg. National Spiralling Whitefly
Consultative Committee.
2.Crop, region. Recognize that variations in climate
and species will affect your plan. The Cotton CRC
website has a Cotton Pest Management Guide.
3.Identification. Consult a diagnostic service (page
xiv) if necessary, to ensure correct identification of the
whitefly in your crop.
4.Monitor to detect early infestation of whiteflies and
their parasites before applying an insecticide (page 39).
Group susceptible plants to maintain regular monitoring
and treatments. GHWF nymphs become black and SLWF
nymphs become brown about 2 weeks after successful
parasitism. Monitor weekly mainly during autumn and
spring as GHWF is most active during that time (this
may vary depending on the crop).
Plant inspections for nymphs. Crops very
susceptible to whiteflies are useful for pest scouting,
eg poinsettia, gerbera, rose, hibiscus. (Bodman and
Hargreaves 2000). Examine 5 compound leaves
(5 simple leaves = 1 compound leaf) on 20 plants
widely spaced through the crop.
Yellow sticky traps trap adults (1 per 100 m 2 ).
Monitor GHWF weekly. Desire sticky and InSense
Lure traps are used to monitor adult whitefly:
http://insense.com.au/products.htm
Sooty mould and honeydew also be monitored.
Whitefly on leaf undersurface can be hard to see but
look for honeydew dripping down on leaves below.
5. Thresholds vary with area, host, etc, and have
been determined for some commercial crops, but seek
up-to-date advice about the need to monitor, eg
Specialist propagators may have a nil tolerance
for whitefly. Treatment is applied as soon as whiteflies
(particularly SLWF) are detected.
Roses. Treatment may be required if more than
10% plants are infested.
6. Action/control. Small populations in a home
garden can often be ignored. Commercially suppression
using Encarsia sp. is preferred. When any threshold is
reached commercial growers should apply control in the
early stages of infestation. If Encarsia is used, an
insecticide program prior to shipment may be required.
Insecticides can be useful to:
Reduce whitefly numbers to a level at which Encarsia
can be successful. Control whiteflies in hot spots.
If temperature is not suitable for Encarsia.
Continue to monitor plants after treatment.
Destroying nests of ants feeding on aphid honeydew
will allow parasites and predators do their job.
Avoid broad spectrum insecticides.
8.Evaluation. Review IPM program to see how well
it worked and implement improvements if required.
Control methods
GHWF and other whiteflies may be common in some
areas but are easy to control compared with SLWF
which has a wider host range, higher reproductive
rate, develops resistance rapidly to insecticides and
is adapted to high temperatures. Where populations
are a mix of SLWF and GHWF, consider treating as if
all are SLWF.
Cultural methods.
Reducing humidity can help to control whitefly
in glasshouse and outdoor situations.
Some plants, when used as companion plants are
reputed to repel whiteflies, eg nasturtium.
Fig. 116. Greenhouse whitefly (Trialeurodes vaporariorum). Left: Adults on undersurface of leaf.
Centre: Adult about 1 mm long. Right clockwise: Egg on stalk (about 0.24 mm long); Top and side views of
1 st stage nymph (when fully fed about 0.3 mm long); 4 th stage nymph (about 0.75 mm long) with wax rods on the
upper surface which are not always obvious, may be mistaken for scale. Photo NSW Dept of Industry and Investment.
172 Insects and allied pests - Hemiptera (bugs, aphids, etc)