Weevils eat pesky buddleia weeds

Introduced Chinese weevils, Cleopus japonicus, are laying waste to large patches of buddleia in the Bay of Plenty, much to the relief of forest growers and biosecurity managers who have been battling the fast-growing, fast-spreading woody weed for decades.

Buddleia (Buddleja davidii), originally introduced to New Zealand as an ornamental plant for gardens, is one of New Zealand’s worst forestry weeds, affecting both indigenous and exotic forests of the North Island and upper South Island. The woody bushes are a tenacious coloniser often seen growing on the sides of roads, bare soil and dry river beds. In a forest, they are a major competitor for light and can cause growth losses and mortality of seedlings in plantation forests. This costs the forest industry an estimated $3 million annually in lost production and control costs.

Weevils used as biological control

In 2006, Crown research institute Scion released 1,000 weevils at each of 3 sites as a sustainable weed control measure. Scientists from Scion identified the cleopus weevil as suitable for biological control because, at least in its China homeland, it only eats buddleia. So far, the weevil is behaving the same way in New Zealand.

The weevil lays its eggs on the leaves of buddleia bushes. The eggs hatch and grow into a yellowish grub up to 5mm in length (like a small maggot), which eats away at the leaves, defoliating the plant, much like monarch caterpillars on swan plants. However, in this case, the grub stunts the buddleia’s growth and can even eventually kill it. The grub pupates in a cocoon on the leaf, eventually emerging as the adult weevil that can fly to a new plant to mate.

The weevils spread slowly in the first few years but scientists studying the populations theorised in a paper published late in 2010 that this relatively slow initial dispersal may help the population to reach high densities. This, in turn, would result in high levels of defoliation on affected trees each autumn from larval feeding (this means more damage to the bushes is noted in autumn). Following this lag phase, the scientists expected that future monitoring would record a rapidly increasing rate of dispersal, and this has proven to be the case.

Defoliation of buddleia plants

Scion scientist Dr Michelle Watson says this is exactly how biological control is supposed to work.

“The weevils are achieving similar damage to herbicides in late summer to autumn, but without the need to spray,” she says.

“We are getting reports from all over the Bay of Plenty that the weevil is spreading naturally and doing a great job of defoliating buddleia.”

Bay of Plenty Regional Council Land Resource Manager (Rotorua) Greg Corbett says weevils are spreading widely throughout the region and causing significant feeding damage to buddleia plants on roadsides and riverbanks.

“This is very promising – if the weevil continues to thrive, buddleia may cease to be such a serious and costly weed in New Zealand.”

People living in areas where the weevil has been introduced and who have various species of buddleia growing in their garden are likely to notice weevil damage.

Scion advises that these plants can be protected using standard pesticides if people want to retain them for ornamental purposes.

Find out more about biocontrol.