St. John’s wort (Hypericum perforatum) is a perennial plant that produces a chemical called hypericin. It is responsible for the red liquid that exudes when flowers are crushed between the fingers. When livestock ingest the plant it causes photosensitisation which combined with sun exposure can result in painful lesions developing on the animal causing weight loss and in severe cases death. In sheep producing areas the seeds add further issues causing wool fault. The plants is widespread in southern Australia (see map). Orange shows the distribution of St. John’s wort
St Johns’s wort is easily recognised by the small yellow flowers (about 20 mm in diameter) that appear from October – February. The leaves when held up to the light appear perforated, hence the name Hypericum peforatum. Each seed capsule is packed with seeds around 1 mm long and a single plant can produce around 30,000 seeds in a year. There are two strains, the narrow leaf and broad leaf. While the narrow leaf produces more hypericin both strains are problematic and need to be controlled so we do not differentiate between them.
The Greater St. John’s wort beetle (Chrysolina quadrigemina) and the Lesser St. John’s wort beetle (Chrysolina hyperici) both feed selectively on the plant and are currently our frontline biocontrol insects. The beetles are not very mobile so unless it has been released in your region it may not have established. Like all animals it is impacted by drought, flood and fire so monitoring and reestablishment is frequently required.
New growth may be observed in summer after beetle activity stops, however the production of flowers, where the majority of the problematic hypericin is localised, and where the seeds develop is significantly supressed, or stopped entirely. A healthy plant may contain hundreds of seed pods (≈5-8 mm long), with each pod containing around 60 seeds (≈1 mm long), resulting in an annual production of around 25,000-35,000 seeds. A plant impacted by St. John’s wort beetle may be devoid of seeds.
Beetles will usually emerge from aestivation in March and enter a breeding cycle where females will lay eggs on the fresh leaves of St. John’s wort. Each female can lay hundreds of eggs. Those of the greater St. John’s wort beetle hatch in autumn and larvae feed through winter, while those of the lesser St. John’s wort beetle
Control of St. John’s wort across infested landscapes presents a challenge and we advocate an integrated pest management (IPM) approach. This includes release of St. John’s wort beetles as soon as available in spring, targeted grazing in late autumn throughout winter, spot spraying in spring/summer and mechanical removal at any stage of the year. Slashing will not stop St. John’s wort and may encourage spread. Cultivation where all roots are brought to the surface or hand removal can assist. Grazing, spraying and mechanical removal, including cultivation, needs to be done with the understanding of where biocontrol beetles have been released to not adversely impact their development and beneficial activity.
