Citrus trees in Perth backyards are under threat from a small invasive wasp, which experts warn will destroy trees if left untreated.
Citrus gall wasp has been found in more than 50 suburbs north of the river since it first detected in Western Australia five years ago.
As it spreads north on winds it is encroaching on the state’s $23 million commercial citrus industry orchards.
The wasp attacks all citrus types including mandarins, oranges, grapefruit, cumquats, lemons and limes, including the popular native limes.
It forms distinctive gall nests on stems which look like woody bulges up to 250mm long and 25mm thick.
Responsible tree ownership a must
Perth-based citrus tree consultant Scott Moore said many people were unaware their backyard trees were infested with gall wasp.
“Almost nobody knows about this problem,” Mr Moore said.
“It’s most important that people know what it is, if they don’t know what it is they can’t address it.
“Responsible tree ownership is a must; it is fundamental.”
$23m commercial industry at risk
WA Citrus biosecurity representative Helen Newman said while wasps were unable to fly well, they were prolific breeders and spread easily on winds, through relocation of potted plants, or even through gifted bags of fruit.
“It’s really important that if you have gall wasp in your tree that firstly control it in your tree, but you go around, meet the neighbours and just have a chat to them about their infestations and help them control theirs as well,” she said.
“It is spreading quite rapidly and our concern is that it could get into [commercial] citrus orchards.”
Ms Newman said the wasp was initially difficult to detect.
“It’s quite possible that the orchardists aren’t going to pick up the trees in their early infestations,” she said.
Regular inspection and pruning
Mr Moore said, once infested, a tree would survive for approximately four to five years before being unable to bear fruit.
“One wasp will lay about 100 eggs, so if you do the maths after three or four years you have complete infestation, the tree is full,” he said.
“This will reduce your crop yields, your yields will go down, and the biggest problem with this is you are infesting other people’s trees.”
He said regular inspection and pruning nests out of trees was the best method of control.
“Home gardeners can take steps to prune all visible galls off the tree as soon as possible and solarise the pruned stems by placing them in a black plastic bag (double bagging is best) and baking in the sun for four weeks,” Ms Newman said.
Mr Moore said home gardeners should keep their trees small and manageable.
“They’re much easier to look after, you can net them, you can care for them, you’ll get good food from them and you can easily find gall wasp, scale, leaf minor or any other problems you may have with your tree,” he said.
The Department of Primary Industries and Regional Development is trialling release of beneficial wasps to combat gall wasp.