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Biological control of St John's wort with the chrysolina leaf beetles | LC0152 |
Keith Turnbull Research Institute, Frankston
June, 1999 |  |
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Common and scientific names
St John’s wort leaf beetles
Chrysolina hyperici (Förster)
Chrysolina quadrigemina (Suffrian)
Background
St John’s wort, Hypericum perforatum, was introduced in the Ovens Valley of Victoria as a medicinal plant in the 1860s. It spread rapidly and was well established by the early 1900s. It is a serious weed of improved pastures, roadsides and neglected areas in north east Victoria and is an increasing problem in dry forests and woodlands. In natural areas it is a serious environmental weed which can out-compete other ground storey plants. St John’s wort is a Regionally Prohibited Weed in the Corangamite and Port Phillip West Catchment and Land Protection Regions, and a Regionally Controlled Weed in all other areas of Victoria except Mallee CaLP Region.
St John’s wort contains the chemical hypericin which causes weight loss, reduced milk and wool production, reduced reproductive performance and photosensitisation when ingested in large quantitities by stock. Horses and cattle are more susceptible to hypericin poisoning than sheep, with goats being the least susceptible.
St John’s wort was one of the first weeds targeted for biological control. Five insect species were introduced to Australia from Britain in the early 1930s but only C. hyperici became established. Later four species from France were released including C. quadrigemina.
Description
Adults - Adults of the two species are very similar in appearance and behaviour and are usually found in mixed species populations. They are black with bronze, dark-blue or purple reflections, 5 to 7 mm long and oval in shape. C. quadrigemina is slightly larger (6.0 to 7.1 mm) than C. hyperici (5.3 to 6.1 mm). Some individuals of C. quadrigemina are distinctly bluish.
Eggs - singly or in small clusters on the undersides of leaves or leaf buds.
Larvae – Pinkish-grey to orange in colour, with dark heads; found in litter beneath the plant during the day and feeding on the plant mainly at night.
Pupae - in globular cells in the soil at up to 5 cm depth.
Life cycle
Females lay eggs on the undersides of leaves or leaf buds in autumn. C. quadrigemina larvae emerge after about 3 weeks and overwinter as larvae. C. hyperici overwinters in the egg stage. Larvae consume the young leaves and buds of procumbent autumn and winter growth. Larger larvae leave the plant during the day and return to feed at night. When mature, they pupate in the soil at a depth of a few centimetres. The pupal stage lasts 2 to 3 weeks and adults emerge in the spring. Adult beetles defoliate the erect spring plants and enter a resting stage (aestivation or diapause) under the bark of trees during summer.
Figure 1. St John’s wort leaf beetles.
Figure 2. Larvae of St John’s wort leaf beetles.
Impact
Beetles eat the leaves and defoliate plants along a front which expands each year. Larvae attack the winter growth and adults attack the spring growth. After a few years at favourable sites the beetles reach densities which are high enough to cause complete defoliation. Heavy damage suppresses flowering and seed production.
Figure 3. Leaf beetles defoliating St Johns wort.
The damage produced by Chrysolina beetles can appear spectacular but the impact tends to be sporadic and inconsistent. They can provide effective control in open, unshaded situations, but without follow-up pasture improvement, the weed frequently re-establishes. The beetles are not effective in timbered country as they mate only in sunlight.
Releases
Chrysolina hyperici was released in Victoria from 1930 to 1934 and had become established by 1939. C. quadrigemina was released in Victoria from 1937 to 1939 and was recognised as established in 1942. Over 1 million beetles were released in 1946. New strains of C. quadrigemina were released in the 1980s.
Because the two species have low mobility it may be worthwhile is to assist their movement by collecting batches and transferring them to infestations from which they are absent. Larvae or adults can be shaken into a bucket or beating tray and transported in large bags. Harvested insects should be kept cool and out of direct sunlight and released as soon as possible.
Integrated control
Clean areas should be kept free of St John’s wort and managed to prevent reinfestation. Lightly infested areas should be cleaned up as soon as possible to prevent spread. Extensive areas are best quarantined and tackled as finances permit. Pasture improvement programs using clovers and superphosphate have been used to control St John’s wort since the 1950s. Rotational grazing of sheep with the stock removed after one week exposure and returned after five weeks can allow infested pastures to be grazed with little effect on the stock. Goats are affected less by St John’s wort toxicity than sheep. Establishment of tree plantations will eliminate the weed which does not survive in dense shade. Herbicides and biological control are the main control options in natural areas.
A number of other biocontrol agents are having an impact on St John’s wort. The gall midge, Zeuxidiplosis giardi, was released in 1953. Larval midges burrow into leaf buds which develop into galls. The Hypericum stem aphis, Aphis chloris, was released in 1986-87 and is now well established. Unlike the leaf beetles, it has no preference for sunny areas, however it only stresses the plant during summer. The St John’s wort stunt mite, Aculus hyperici, was first released in 1991 and has high potential for controlling the weed. It has established well and spread widely on the wind. The mite stunts the growth of rosettes and flowering stems, gradually weakens plants and reduces vigour and seed production.
Biological control cannot eradicate a weed, it can only reduce the spread and density of infestations. In some cases control is achieved to the level where the weed is no longer of concern and no other control is necessary. More commonly, other methods are still required to achieve the desired level of control, however these need not be used so frequently or intensively. Use of herbicides is not advisable when high numbers of St John’s wort beetles are present because partially defoliated plants are unlikely to absorb enough herbicide to kill them. Biological control should not be considered the complete answer to a St John’s wort problem, but can be used in conjunction with other control measures in an integrated weed management program.
Further information
Please refer to the Landcare Note St John’s Wort.
For further information on biological control contact:
Keith Turnbull Research Institute,
PO Box 48, Frankston, Victoria, 3199.
Tel (03) 9785 0111
Fax (03) 9785 2007
Acknowledgments
Prepared by Ian Faithfull. Nigel Ainsworth, Raelene Kwong and Franz Mahr provided comments. Figure 2 courtesy of CSIRO Entomology. Current biological control programs for St John’s wort are supported by the Cooperative Research Centre for Weed Management Systems.
This note replaces note number BC0018.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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