PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Pest cycle
There is a complete metamorphosis (egg,
larva or ‘spitfire’, pupa and adult) with several
generations each year. Adult females emerge from
pupal chambers in the soil in late summer. They
lay eggs in slits on leaves, larvae feed on foliage
during autumn, winter and spring, when they
descend from the tree in a slow moving mass (as
many as 250) and may wander about on the ground
for several days before burrowing into soft ground
to a depth of 5-10 cm, usually about the base of the
tree. They spin large cocoons in rows against each
other, usually with their heads all facing one way.
Cocoons are dark brown, thin-walled, cylindrical
about 25 mm long and 12 mm across. Timing of
the cycle varies according to species, subspecies
and geographic location.
‘Overwintering’
As larvae in cocoons in the soil. Sometimes odd
small colonies are observed even in winter.
Spread
As adults flying and as larvae crawling.
Conditions favouring
Weather has most impact on sawfly numbers.
Long term weather cycles determine numbers.
Warm weather. In mild winters the pest cycle
continues though at a slower rate. In some
winters colonies appear particularly damaging.
Hot and dry weather in early spring kills many
mature larvae when they are about to enter soil
which is too hard for them to dig into, to pupate.
Larvae can survive heavy frost in winter.
Attack declines once trees achieve canopy closure.
Management (IDM)
Are you a commercial grower or home gardener?
1.Obtain/prepare a plan that fits your situation.
2.Crop, region. Recognize variations.
3.Identification of pest is easy, though the exact
species can be more difficult to determine. Consult a
diagnostic service if necessary (page xiv). Damage is
often not noticed until it is severe, late in the season and
larvae are preparing to enter the ground to ‘overwinter’.
4.Monitor pest and/or damage to trees and record
results as recommended (page 39). If sawfly damage
is anticipated, young trees can be inspected for
clusters of young larvae in autumn before any major
feeding has occurred. Techniques for assessing
impacts in forest areas are available.
5.Threshold. How much damage can you accept to
young trees? Have any thresholds been established? If
so, what are they, eg economic, aesthetic?
6.Action. Take appropriate action when any threshold
is reached. Larvae can be eliminated either by physical
removal or by applying a chemical insecticide (see
Table 20 below).
7.Evaluation. Review IPM program to see how well
it worked. Recommend improvements if required, eg
replacing susceptible species/provenances.
Control methods
Biological control. Exudate produced by the
‘spitfires’ might deter predators and parasites.
Natural controls.
– Parasitic flies and wasps parasitize larvae on
leaves and pupae in the soil and seem to stabilize
sawfly populations from year to year.
– Vertebrate predators have only a limited impact
on sawfly abundance as numbers of larvae tend to
remain surprisingly constant throughout winter.
Currawongs, cockatoos, gang-gangs and other
birds feed on larvae but most find them distasteful.
Gang-gangs pull off and discard the head and oil
sac before eating the rest.
Resistant varieties.
Susceptible species in some areas include
Blakely’s red gum (E. blakelyi), river red gum
(E. camaldulensis), yellow box (E. melliodora),
snow gum (E. pauciflora), manna gum
(E. viminalis), swamp gum (E. ovata), wandoo
(E. wandoo), others.
Non-hosts include ironbark (E. sideroxylon),
scribbly gum (E. rossi), grey box (E. microcarpa).
Variation. Within a susceptible eucalypt host
species there is little evidence that individual
trees vary in their susceptibility.
Terpenoid oils. Larvae feed on a wide variety of
eucalypts with different amounts of terpenoid
oils in the leaves. The ability of the larvae to
utilize and store leaf oils for their own defense
suggests they may be relatively immune to the
effects of terpenoid oil defences in host plants.
Physical & mechanical control.
If only a few trees are affected and clumps of
‘spitfires’ are within reach, they may be knocked
from the tree with a long stick or hosed off with a
strong jet of water and destroyed. Branches
bearing clumps may be cut off.
Insecticides. If it is not practical to remove
and destroy clumps by hands, clumps on small
trees less than 3 m high can be spot sprayed. Apply
directly to the cluster of larvae using a good
wetting agent.
Table 20. Steelblue sawfly – Some insecticides.
What to use?
SMALL TREES, LESS THAN 3 METRES
Group 1B, eg Malathion (maldison); Rogor (dimethoate)
Group 3A, eg Tempo Residual Insecticide (beta-cyfluthrin)
Group 4A, eg Initiator Systemic Plant Insecticide and
Fertiliser (imidacloprid/fertilizer) - steelblue
sawfly larvae are not specifically listed on
the label
VERY LARGE VALUABLE TREES
When and how to apply?
Spray when first noticed, the use of a wetting agent is
considered essential when spraying eucalypts.
Initiator improves the establishment of young
eucalypts trees, enhancing growth and protection against
damage caused by various insect pests, including some
defoliating insects.
Seek specialist advice. If tree injection is being
considered, larvae must be feeding and the tree actively
growing (sap moving) when tree injection is carried out.
126 Insects and allied pests - Hymenoptera (ants, bees, sawflies, wasps)