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Beetle control

November 15, 2016

dscn2512Despite its magical properties in folk lore and its current popularity as a medicinal plant, St. John’s Wort (Hypericum perforatum) can be an invasive environmental weed and a serious noxious weed in grazing paddocks. The reason for the latter is that it contains a substance called hypericin which can induce a strong photosensitive reaction in livestock, sometimes leading to a severe loss in condition and even death. It is a declared Regionally Controlled Weed in most of Victoria which means landholders must take all reasonable steps to control it and prevent its spread.

The common means of control is by chemical spraying, but there are some little beetles that can provide an alternative biological control. These are active in small numbers on our roadside at present – see photos. They are Chrysolina beetles which in sufficient numbers can cause heavy defoliation of St. John’s Wort plants, suppressing flowering and seed set. Individual beetles have a metallic sheen varying in colour from green to bronze or blue.
These beetles have an interesting story to tell. Two species, Lesser St. John’s Wort Beetle (Chrysolina hyperici) and Greater St. John’s Wort Beetle (Chrysolina quadrigemina) were introduced from Europe into Australia in the 1930s as biological agents for the control of this weed. (Australian-bred beetles were subsequently introduced to New Zealand and North America.)

Both the larvae and the adults feed on St. John’s Wort leaves, and consequently ingest significant amounts of hypericin, enough to make them photosensitive. But they have adapted their feeding habits to counter this accumulation of toxin. Young larvae feed at night and hide in leaf-buds in the daytime, while older larvae spend the day burrowed in the soil. Adults avoid flying from plant to plant during the day, as their elytra (wing covers) when closed protect them from light, so the toxic hypericin cannot become activated. They will move about in daylight, exposing the cuticle, only when threatened with starvation, although they sometimes drop to the ground and play dead when disturbed.
So, if you find clusters of shiny beetles on your St. John’s Wort, hold off on the spraying and let these little fellows do the control work for you!

[Incidentally, one sure way of identifying young non-flowering St. John’s Wort from superficially similar native Pimelea species which also grow on our roadsides is the presence of numerous translucent hypericin-containing oil glands on the leaves of St. John’s Wort – these can be seen in the photo at left – click for a closer look.]

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