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.

Gall wasps nest in new foliage in fruit trees and create hard lumps in trees. ( ABC: Chris Lewis )

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.

"If you have trees in your backyard and they are abandoned or you do not care for them, do everyone a favour please remove them.

"Responsible tree ownership is a must; it is fundamental."

Helen Newman from WA Citrus says gall wasps in commercial orchards would increase costs of production. ( ABC: Chris Lewis )

$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 wasps in your tree that firstly [you] control it in your tree, but [also that] 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.

"Biosecurity is everyone's responsibility.

"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.

"Once that infestation starts to ramp up they'll start managing it, and it's going to add an extra management cost onto production."

Gall wasps create large woody lumps on citrus trees. ( ABC: Chris Lewis )

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.

"Another option is to shred the pruned stems using the fine setting on a mulcher or a pair of secateurs, or by burning or deep-burying to one metre."

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 wasps, 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 the release of beneficial wasps to combat gall wasps.