Galls on native broom no cause for concern
The emerging success of the broom gall mite (Aceria genistae) has people starting to notice galls on other plants too. The highly conspicuous galls now appearing on Scotch broom (Cytisus scoparius), in many areas, are deformed lumps of plant tissue that form at the base of the stem buds, where the gall mites attack the plant in spring. The galls range in size from 5 to 30 mm and stunt the growth of the plant. “We are seeing, and hearing reports of heavily galled Scotch broom plants dying, which is very encouraging,” said Lynley Hayes. “But we are also getting regular queries from people saying they have seen galls on native broom (Carmichaelia spp.) and are concerned that the broom gall mite could have jumped host. But rest assured,” she said. “Specimens sent in have been passed on to a mite specialist to confirm their identity, and the galls on native broom are the result of a native gall mite (Aceria carmichaeliae), which people just haven’t noticed before.”
Host range testing undertaken before the broom gall mite was introduced into New Zealand in 2006 showed convincingly that this exotic mite does not attack native broom and is unable to complete its life-cycle without access to Scotch broom. “It is extremely unlikely that the highly host-specific broom gall mite will damage any other plants, although spill-over damage can sometimes occur on the close relative, tree lucerne (Cytisus proliferus), if broom gall mites are present in big numbers nearby,” confirmed Lynley. The broom gall mites are not able to persist solely on tree lucerne though.
“A similar thing happened about 20 years ago, after we released gorse spider mite (Tetranychus lintearius), and people then noticed mites on Coprosma a few years later. In this case it turned out to be a very similar-looking mite (Tetranychus sp. near pacificus), which had self-introduced and was attacking Coprosma,” said Lynley. It is great for biosecurity in New Zealand that people always report seeing anything unusual, just in case it is a new unwanted invasive species needing to be dealt with promptly or some issue requiring further investigation . “We are always happy to follow up if people have any concerns about weed biocontrol agents being found on other hosts. So please keeping the queries coming,” said Lynley.
CONTACT
Lynley Hayes – hayesl@landcareresearch.co.nz