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A program is now under-way to introduce natural enemies of horehound from Europe to Australia. The first of these, the horehound plume moth, Wheeleria spilodactylus (Curtis), was first released in southern mainland states during the summer of 1993/94. The successful establishment and spread of the moth at mainland release sites resulted in the Tasmanian Institute of Agricultural Research initiating a mass rearing/release program for horehound plume moth in Tasmania. Rearing stock were first introduced from Keith Turnbull Research Institute in Victoria in October, 1997. The first field release of horehound plume moth in Tasmania was in November, 1997 at Sorell. The moth has since become permanently established at this and other release sites.
 Description
The moth (adult) is about 10 mm in length with a wingspan of 20 mm. It is cream or greyish with fine dark markings on the wings. The forewing is divided into two lobes and the hindwing into three feather-like plumes, hence the name plume moth.
The eggs are green, flattened and oval in shape and about 0.5 mm long. The larvae or caterpillars are green and hairy with a green head capsule. They mimic the colour and texture of the horehound leaf. The pupae are also well camouflaged and are also hairy and green with darker V-shaped markings visible along the dorsal surface.
Life cycle
The plume moth has about three generations each year, the length of each generation depending on environmental conditions. First generation adults emerge in spring with the second and third generations emerging during summer and autumn.. Each female moth lays up to 120 eggs, usually singly on the undersurface of younger leaves, over about a fortnight. The majority of eggs are laid during the first few days after emergence. The eggs hatch in about a week and the larvae begin feeding on the developing shoot tip. As the larvae feed they gradually increase in size, progressively feeding on the leaves and gradually defoliating the stem.
When fully fed the larvae pass into the pupal stage. The pupae can be seen on the upper leaf surfaces.
The larvae produced by the autumn generation of moths overwinter in the leaf buds and finish their feeding when temperatures start to increase in the spring.
Damage
Larval feeding can severely damage the growing tips and leaves of horehound. This weakens the plant and reduces the number of flowers and seeds produced.
High plume moth populations may restrict the spread and density of horehound infestations enabling the establishment of more desirable plant species.
Release and establishment
The plume moth is currently being mass reared in glasshouse cultures and released at field sites in the larval stage. Once plume moth densities become high enough at the field sites it is intended they be used as nurseries from which larval infested foliage will be collected and transferred to new sites. This will accelerate the spread of the moth.
All release sites are selected in areas heavily infested with horehound that will not be treated with insecticides or herbicides.
Initially all new release sites are being fenced over an area of about 20m x 20m primarily to prevent sheep grazing. This is because sheep may sometimes feed on horehound and, in the process, remove large numbers of plume moth larvae thereby restricting or even preventing the establishment of populations.
Potential for control
Biological control does not involve the eradication of a pest species and is not expected to provide the complete answer to the horehound problem. It is intended that the moth be eventually used in conjunction with traditional control methods as part of an integrated management program. Once the horehound plume moth is well established in Tasmania a study on its effectiveness in reducing the vigour and spread of horehound and the best strategies that can be utilised to maximise the impact of the moth will be undertaken.
Meander Valley Region
Although horehound is not a problem weed in this region, only appearing in isolated small infestations, the objectives of the Meander Valley Weed Strategy are to introduce biological control agents for any weeds posing a risk within the region.
The first release of the Horehound Plume moth was made in the Meander area in early January 1999.
The technical text and image of the horehound plume moth on this page were supplied by Dr. John Ireson, Senior Entomologist, Tasmanian Institute of Agricultural Research.
Funding support for this project is being provided by the Co-operative Research Centre for Weed Management Systems and the Natural Heritage Trust.
 
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