Elm Leaf Beetle
By James Martens-Mullaly | Director | Tree Logic
Elm Leaf Beetle (Pyrrhalta luteola) is an introduced species of insect native to Europe, North Africa and
Eurasia, which as the name suggest is a beetle that feeds almost exclusively on elm trees. Untreated, it can
defoliate a mature tree in a matter of weeks, and presents a serious threat to the elm tree population of
Australia.
The beetle was first detected in Australia in 1989, about 40 km south of Melbourne on the Mornington
Peninsula. However, based on the numbers of beetles initially observed, it is believed the actual arrival of the
beetle may have been up to 10 years earlier. In Tasmania the presence of Elm Leaf Beetle was confirmed in
Launceston in 2002 and in Hobart in 2008, in southern New South Wales in 2008, and most recently several
cases have been reported in the Australian Capital Territory.
Originally from Europe, North America and Asia, elm trees were once a popular choice of ornamental tree in
south-eastern Australia. Planted extensively in public and private gardens, elms in particular were used to
create park and roadside avenues in honour of fallen soldiers after the First World War 1. It has been
estimated that there are approximately 70,000 mature elms in Victoria alone and over 5,600 of these trees
are registered as significant by the National Trust of Australia (Victoria). The majority of these significant elm
trees comprise avenues including Victoria and Royal Parades, Melbourne, Finlay Avenue, Camperdown,
Western Highway, Ballarat, and Bacchus Marsh Road, Bacchus Marsh.
In the wake of the high mortality inflicted upon elm populations in the Northern Hemisphere by Dutch Elm
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Disease , Melbourne now has one of the largest and most significant populations of mature elms in the
world. This heritage and botanical significance of the Australian Elm population has largely driven efforts by
land managers to control the Elm Leaf Beetle population.
As the name suggests Elm Leaf Beetle feeding is predominantly restricted to Elm trees. The European
species’ of elm trees, the most widely planted species in Australia, are most susceptible to infestation;
however Asian and American species and closely related genera occurring in Australia are also at risk as
illustrated in Table 1.
Table 1. Susceptibility of Selected Elms and Elm Substitutes to Elm Leaf Beetle (ELB)
TREE
SUSCEPTIBILITY
Common name
Scientific name
ELB
English elm
Ulmus procera
HS
Dutch Elm
Ulmus x hollandica
HS
Scotch elm
U. glabra
HS
Golden Elm
Ulmus glabra ‘Lutescens’
HS
Camperdown Elm
Horizontal Elm
Ulmus glabra ‘Camperdownii’
Ulmus glabra ‘Pendula’
HS
HS
Smooth-leaved Elm
Ulmus minor
HS
Silver Elm
Ulmus minor ‘Variegata’
HS
American elm
U. americana
S
Siberian elm
U. pumila
S
Chinese elm
U. parvifolia
*R
Zelkova
Zelkova serrata
**R
Hackberry
Celtis spp.
NS
Hornbeam
Carpinus spp.
NS
HS = highly susceptible
MR = moderately resistant
NS = not susceptible
S = susceptible
R = resistant
*Can be susceptible if ELB numbers high or ELB emerge prior to Elm leaf burst and ELB numbers
high
** Can be susceptible if beetle number high.
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Dutch Elm Disease, responsible for the decimation of elms populations in the northern hemisphere is not present in
Australia, and is spread by the Elm Bark Beetle and NOT the Elm Leaf Beetle.
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Importance as a pest
Unsightly damage to elm trees results from the feeding of adult beetles and larvae on leaves. In severe
cases, infestation causes the leaves to dry up and die. This foliage feeding by itself generally will not kill the
tree; however repeated seasonal defoliation can weaken the tree resulting in reduced growth, a decline in
tree health, and an increased susceptibility to other biotic and abiotic stresses.
The Elm Leaf Beetle can become a nuisance to householders living nearby elm trees. In the autumn,
beetles migrate into nearby homes searching for over-wintering sites protected from the elements. The
beetles may also be a problem in the following spring when they emerge from hibernation and become
conspicuous inside houses as temperatures increase and they attempt to reach nearby elm trees.
Identification
Adults are approximately 6mm long and bronze, yellowish to olive green with a black stripe on each wing
margin. The stripes are sometimes indistinct in the dark green forms. There are 3-4 black spots on the region
behind the head. These stripes are sometimes indistinct in the dark olive forms. The eyes are black.
Eggs have the appearance of miniature lemons, are pointed, orange-yellow and about 1.5mm long, and are
laid on the underside of elm leaves in a double row.
Larvae, when full grown, are approximately 12mm long and have a dull yellow colour with a black head and
two black stripes on the back.
Pupae are about 6mm long and bright orange-yellow.
Identification by injury
Two distinct types of injury occur. Adult beetles chew small round holes through leaves. This type of damage
is referred to as ‘Shot Hole’. This is usually the most noticeable damage soon after leaf burst.
The subsequent generation of larvae live on the underside of leaves, and having different mouthparts use a
rasping action to remove leaf tissue from the underside of the leaf while leaving the venation and upper leaf
surface intact. This damage is referred to as skeletonising. The larval stage is the Elm Leaf Beetle's most
destructive. In severe infestations the upper surface of the leaf is also eaten but the leaf veins are not.
Resulting from this the leaves turn brown and shrivel, often falling prematurely. A large mature tree can be
defoliated in a matter of weeks!
Life cycle
Elm leaf beetles overwinter as adults in tree bark crevices, buildings, wood piles, debris at the tree base and
other protected places. During early spring, beetles leave their overwintering quarters, become active and fly
to elms where they begin feeding. Eggs are laid in a cluster on the undersides of leaves, and after around 710 days small black larvae emerge to feed on the leaves. Young larvae feed in groups and older larvae
sometimes singly. The larvae skeletonize foliage, leaving only the vein network intact. The larvae mature in
two to four weeks and migrate to the base of the tree to pupate in tree bark crevices or on the ground in
sheltered places, with new adults emerging about 10 days later. In south-eastern Australia there can be up
to two generations per year, depending on the season and geographical location.
Control
Biological Control
Natural enemies that parasitize the Elm Leaf Beetle were trialed in Victoria during the late 1990’s and early
2000’s. Trials were conducted using the Parasitic Wasp (Tetrastichus gallerucae) that parasitizes Elm Leaf
Beetle eggs, and the Tachinid Fly (Erynniopsis antennata) that parasitizes the larval stage of the Elm Leaf
Beetle. Released populations of these insects failed to establish as neither species was able to successfully
overwinter in southern Australia, and therefore neither parasite was has been effective in controlling the Elm
Leaf Beetle.
Non-Chemical
Trapping the larvae as they migrate down the trunk to pupate between December and early February using
bands of horticultural glues or adhesive tape (wrapped sticky side out) applied around the trunk(s) is a safe
non-chemical means of control. Banding is aimed at breaking the beetle life cycle. Monitoring of the larvae
activity is important as banding requires application prior to larval migration, the timing of which can vary
depending on the weather. Banding is an annual treatment and can require re-application several times each
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season if trapped beetle numbers are high. Banding does not reduce the initial damage caused by feeding
adults and may not be effective against heavy infestations.
Chemical Control
There are a number of chemicals registered for the control of Elm Leaf Beetle that are applied as a foliar
spray. These are best suited to use only on trees where adequate coverage of the foliage can be achieved.
Soil and trunk injection treatments are the most common method of control. Imidacloprid, a neonicotinoid
based systemic insecticide is the only chemical registered in Australia for use against Elm Leaf Beetle in soil
and trunk injectable formulations. When applied, the chemical is translocated via the xylem into the plant
leaves and pollen and acts as an effective neurotoxin targeting insects that eat the leaves, or plant fluids.
Minor levels of leaf damage are required for the beetle to ingest sufficient quantity of the insecticide. Soil
injection treatment is applied during late winter early spring, requires adequate soil moisture levels and can
be applied to trees of any size. Trunk injection is applied after spring leaf burst and is best suited to trees
having a trunk diameter greater than 150mm. Soil and trunk injection typically provide effective control for 2 –
3 years.
There is ongoing debate over the alleged impact to beneficial insects and soil biota surrounding
neonicotinoid class insecticides. Earlier this year the European Commission adopted a proposal from the
European Union to ban for a period of 2 years certain applications of 3 neonicotinoids class pesticides due to
the alleged impact on honey bees. Among other exclusions, the moratorium will not apply to winter cereals
or crops non-attractive to bees, and therefore should exclude it use on the wind pollinated Elm in affected
countries. Chemical control using Imidacloprid remains the most effective treatment for the control of Elm
Leaf Beetle.
Cultural practices
Maintaining good tree health is important to assist tree resilience to environmental pressures including the
impact in of Elm Leaf Beetle. The key principles include:
•
Maintaining adequate soil moisture.
•
Avoid direct and indirect damage to roots such as compaction of soil or root severance.
•
Avoid wounding the tree or inappropriate pruning / lopping.
•
Encourage the development and maintenance of healthy soil through the use of mulch or leaf litter
beneath the canopy rather than lawn to reduce soil moisture loss, encourage macro invertebrate
activity and enhance nutrient cycling.
When treating trees affected by Elm Leaf Beetle also consider removing unmanaged food sources by
removing thickets of unmanaged and wild elm suckers where present, thereby reducing the beetles’ habitat.
The chemical control of these thickets, particularly where dense and overgrown can be ineffectual with
inadequate coverage achieved as well as being cost prohibitive.
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