Buddleia Takes a Beating
Buddleia weevil adults, larvae and feeding damage. Image - SCION
The impact of the buddleia leaf weevil (Cleopus japonicus) continues to impress with some buddleia (Buddleja davidii) bushes being completely defoliated.
Many people are beginning to notice and comment on the dramatic and widespread damage, especially in the Central North Island. Toni Withers and Michelle Watson from Scion in Rotorua have found that most damage occurs in the autumn following the main egg-laying period in late summer. They have also looked at the rate of weevil dispersal from the original release sites. Toni says, “It is very important that the weevils invade newly planted pine forests quickly and give the pines a chance to gain some height.” Buddleia can withstand repeated attacks from defoliators and it is important that enough weevils hang around until the next growing season to keep up the pressure on the plants. “We are starting to see evidence of sustained control, with some large woody plants having died as a result of 2−3 seasons of repeated defoliation,” said Toni. Forestry companies are reporting a lot of damage to plants, equivalent to what you would expect from herbicide application. Toni adds, “If this level of impact continues, this biocontrol agent could have positive economic implications for the long-term management of buddleia in forestry and natural areas in New Zealand.”
Scion is developing a case study to quantify the economic and environmental benefits versus the historical costs of releasing this agent. The case study will make predictions based on whether the weevil is achieving 25%, 50%, 75% or 100% control of existing buddleia populations. Simon Fowler from Landcare Research supports this approach and agrees that more analysis needs to be done to demonstrate how cost-effective successful weed biocontrol really is (see Issue 61).
Regional councils and forestry companies are now successfully introducing the weevil to other sites throughout the Central North Island. So far, the weevils have established at every site that they have been released at. Scion is keen for people to report back on the presence and performance of the weevil, especially any new outbreaks. So keep an eye out for adults in old seed heads and new flower buds next spring or look for damage on the leaves from the slug-like larvae.
Contact: Toni Withers
toni.withers@scionresearch.com