Very hungry caterpillars set to chomp hairy horehound

An army of very hungry caterpillars incubating in the Barossa Valley will soon be ready to gorge on a noxious weed across the Northern and Yorke region.

The horehound plume moths (Wheeleria spilodactylus), currently in a nursery in Rosedale, are expected to be ready to attack in September.

Known as host specific, plume moths only have a taste for the invasive horehound weed (Marrubium vulgare), with the larvae feeding on the growing tips, weakening the plant and reducing the flowers and burrs.

They have been used successfully in the region before, first as a trial in Rosedale in October 2018, with a batch eliminating 75 per cent of a five-hectare infestation in less than a year.

Horehound plume moth caterpillar. Photo credit: Dragos Moise

“Not only did our 60 original very hungry caterpillars increase and decimate the horehound on the host property, but we found the caterpillars had spread to three adjoining properties and roadsides where they were busy attacking the weeds, turning them into sticks,” said Northern and Yorke Landscape Board Landscape Officer David Hughes.

“Some farmers have suggested these moths act like a thousand sheep that have come in and eaten the weed back to its roots.”

On Brian Sambell’s sheep property near Burra, a 50 square-metre patch of horehound has completely disappeared as a result of releasing the ravenous plume moths about two years ago.

“It certainly works. I couldn’t show you a horehound plant where I released the caterpillars now. They’re all gone,” said Mr. Sambell. “It took about five months before the horehounds’ colour changed and they started to look like they were dying. All it took was a small jar of caterpillars.”

Horehound has a distinct odour that taints milk and meat when consumed by stock. Seeds are dispersed when its tenacious burrs get caught in wool, fur or clothing and large mature plants can produce in excess of 20,000 seeds per year.

Landscape officers from the Northern and Yorke Landscape Board will be ready to supply free plume moth larvae to landowners with horehound infestations in September. To express interest, please call the Northern and Yorke Landscape Board on 8841 3444.

Horehound plume moth. Photo credit: Dragos Moise