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Biological control of St John's wort with the St John's wort mite | LC0151 |
Keith Turnbull Research Institute,
November, 2000 |  |
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This Landcare Note describes the St John’s wort mite, Aculus hyperici, and its use in the biological control of St John’s wort, Hypericum perforatum.
Common names
St John’s wort mite, St John’s wort stunt mite
Scientific names
Aculus hyperici (Liro)
Family Eriophyidae (eriophyid mites, gall mites, bud mites)
Background
St John’s wort, Hypericum perforatum (Clusiaceae), was one of the first weeds targeted for biological control in Australia. The program has resulted in the importation from Europe of eleven natural enemies of St John’s wort since 1930. The St John’s wort mite is the newest of these biocontrol agents.
St John’s wort was introduced to the Ovens Valley as a medicinal plant in the 1860s. It spread rapidly and was well established by the early 1900s. It is now 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 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. St John’s wort is a Regionally Prohibited Weed in the Corangamite and Port Phillip West Regions, and a Regionally Controlled Weed in all other areas of Victoria except the Mallee Region.
Description
The St John’s wort mite is a minute European arachnid that feeds on single plant cells. It is difficult to see with the naked eye, so use a hand lens or low power microscope (20x) when inspecting plant material. Mites are rarely seen unless the leaves at the growing tips are gently peeled back.
Adults – cream in colour, a soft flattened body, without segmentation, with 2 pairs of legs and a feeding unit at the front with 2 pairs of appendages, the chleicerae and pedipalps. Adult females are 0.15 mm long and 0.05 mm wide. Males are very similar to the second stage nymph.
Nymphs – similar in shape to the adults, but smaller. There two nymphal instars (stages), the first 0.075 long and 0.035 mm wide, and the second instar 0.11 x 0.045 mm wide. The thin cast skins of the nymphs are left on the surface of the plant when they moult.
Eggs – approximately spherical, 0.04 mm in diameter.
Figure 1. Adult St John’s wort mite.
Figure 2. Mite (arrowed) on an aphid.
Life cycle
The mite has a generation time of 25 days at 20C and many generations occur per year. Adult mites survive temperatures up to 40C but the optimum temperature for nymphal survival is 15. Males deposit spermatophores (sperm packages) on leaf surfaces. These are picked up by females by means of their genital openings and the eggs are fertilised inside the females. Up to 40 eggs are produced per female. Eggs hatch after 3.6 days. The duration of the first larval stage is 3.6 days and that of the second is 4.2 days. Adult longevity is 14 days. Unfertilised females produce all male progeny.
All life stages are present during the whole year. Nymphs appear in large numbers in autumn and early winter. Highest numbers of mites are usually present in the young leaf buds and developing flowers and fruit. Dispersal mainly occurs in summer, on the wind, but some mites are also carried between plants by bees and other insects. After death of the flowering shoots, mites descend to the basal autumn shoots and during autumn and winter they are concentrated in the terminal and axillary buds of the rosette. Populations decline in winter.
Releases
The mite was first released near Albury, NSW and in the ACT in May 1991. Widespread releases were made from 1995 and over 300 releases have now occurred across south-eastern Australia. The mite established at 73% of sites within three years and is well established throughout most of the weed’s range in New South Wales, Victoria and South Australia, however some sites in NSW have populations of the weed which are resistant to mite attack. Dispersal has occurred at the rate of 1 to 2 km in the first year after release and up to 5 km in the second year.
The mite is easily redistributed by placing infested cuttings of the host plant amongst healthy infestations in spring or autumn. Existing populations should not be harvested unless they have been established for two or more years and there are hundreds of infested plants. Harvesting is best undertaken in autumn. Cuttings from at least 20 rosette plants should be placed in plastic bags with moist tissue paper and sealed to prevent dehydration. Bags should be stored in an insulated container while in transit and the mites should be released within 2 days. A 24 hour rain-free period is required to ensure the mites are able to move from the cuttings to suitable points on the inoculated plants without being washed off. Cuttings can be stored in a refrigerator for up to 4 days if rain threatens. The release should be concentrated in a small area to ensure the new mite population is dense enough to guarantee mating.
Impact
The mites concentrate on the growing tips of plants where they feed and are believed to release a toxin which damages the plant. Over 20 mites per bud can be found when mite populations are dense. Damaged leaves often have yellow streaks or mottling. Mite infestations cause stunting of rosettes, leaves and flowering stems, dwarfing of plants and deformation of flowering parts (Figures 3 and 4) and gradually weaken plants and reduce their vigour and seed production. Over a two to three year period mites can exhaust the root reserves and kill plants. Biomass of individual plants is decreased, along with their stem and root diameters. Reduced vigour makes plants more vulnerable to pressures such as weather extremes and competition from other plants that often result in death.
A reduction in height and density of St John’s wort infestations becomes noticeable within a few years of establishment of the mite. Shading does not effect establishment. Intensive impact studies are being undertaken at five sites across south-eastern Australia using miticides to exclude mites from test plots to enable comparison of unaffected plants.
The mite has the potential to dramatically decrease the density and vigour of St John’s wort populations through much of south-eastern Australia. Growth of populations of the mite is inhibited by factors of 100 to 10,000 on the mite-resistant forms of St John’s wort present in localised areas of New South Wales. These infestations are probably derived from different genotypes that were introduced separately to Australia. The mite is also able to survive on Hypericum gramineum, the small St John’s wort.
Figure 3. More heavily damaged rosettes (right) are more dwarfed and show greater reduction in the root system.
Figure 4. The foliage and flowering stems of St John’s wort are stunted and dwarfed by mite attack and the biomass of the plant is reduced. Most heavily attacked plant at far right.
Integrated control
Use of the St John’s wort mite is best reserved for infestations that are unecomic to control by other means. Clean areas should be kept free of St John’s wort and managed to prevent reinfestation. Lightly infested areas should be cleaned up with herbicides, cultivation or manual methods as soon as possible to prevent spread. Extensive areas are best quarantined and tackled as finances permit. Pasture improvement programs using clovers and superphosphate has been used to control St John’s wort since the 1950s.
Properly timed rotational grazing of sheep can allow infested pastures to be grazed with little effect on the stock and to gradually reduce the extent and density of the weed. 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 most effective agent is the leaf beetle Chrysolina quadrigemina, which causes heavy, but sporadic defoliation. It is not effective in shaded areas, but does control St John’s wort locally in favourable years. This beetle controls St John’s wort in Western Australia where it would be a serious weed without biological control.
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, but only stresses the plant during spring and summer. A number of other agents have been established in Australia but are restricted to small areas or cause little damage to the weed.
Biological control can reduce the spread and density of infestations and 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. Biological control should be used in conjunction with other control measures in an integrated weed management program.
References
Briese, D.T. (1997) Biological control of St John’s wort: past, present and future. Plant Protection Quarterly 12(2), 73-80.
Mahr, F., Kwong, R.M., McLaren, D.A. and Jupp, P.W. (1997) Redistribution and present status of the mite Aculus hyperici for the control of St. John’s wort, Hypericum perforatum, in Australia. Plant Protection Quarterly 12(2), 84-88.
Mahr, F.A., Mayo, G., Ainsworth, N. and Jupp, P. (1999) Monitoring the impact of the biological control agent Aculus hyperici on Hypericum perforatum across south eastern Australia. Pp. 335-338 in A.C.Bishop, M.Boersma and C.D.Barnes (Eds.) 12th Australian Weeds Conference Papers and Proceedings. Devonport, Tasmanian Weed Society.
Further information
Please refer to the Landcare Notes St John’s Wort and Biological control of St John’s wort with the Chrysolina leaf beetles available from NRE.
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 and Raelene Kwong. Figure 1 Helen Geir, SEM Group, CSIRO Entomology. Figure 2 Linda Iaconis. Figures 3 and 4, John Green 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 BC0017.
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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