Citrus Gall Wasp | Mandarins.com.au

Citrus gall wasp (Bruchophagus fellis) is a native Australian wasp that causes woody swellings on citrus stems. It is one of the most common problems Australian backyard citrus growers face, and its range is expanding. It was historically confined to Queensland and New South Wales, but it has now spread to Victoria, South Australia, and the southern parts of Western Australia.

What it is

Citrus gall wasp is a small, shiny black wasp about 3 mm long. It is native to Australia, where its original host plant was the native finger lime. It has adapted to use introduced citrus trees, particularly lemons and grapefruit, though mandarins are also attacked.

The tree’s reaction to the wasp larvae developing inside the stem produces the characteristic woody gall. The gall itself does not kill the tree but weakens affected branches, reduces canopy productivity, and over many seasons can significantly impair a heavily infested tree.

Identifying galls (autumn through winter)

Galls are easiest to spot from December through winter. They appear as swollen, corky lumps on young stems, typically green stems from the previous spring’s flush. They grow gradually through autumn and become increasingly visible as the season progresses.

Old galls from previous years show tiny exit holes where adult wasps have already emerged. These old galls are no longer active and do not need to be removed, though they can be cut out during routine pruning.

Fresh galls with no exit holes still contain developing larvae. These are the ones to remove before adults emerge in spring.

Life cycle in Australia

The citrus gall wasp completes one generation per year. Adults emerge from galls in spring or early summer. The timing varies with temperature. Warmer springs bring earlier emergence. In Melbourne and Sydney, peak emergence is typically October, with warmer years pushing it into late September.

Adult wasps live only five to seven days. In that time, a female lays up to 100 eggs under the soft green bark of new citrus growth. The eggs hatch after two to three weeks. Larvae feed inside the stem for nine to ten months, stimulating the tree to produce the woody gall around them. A short pupation period in spring precedes adult emergence and the start of the next cycle.

Adults generally do not travel far but can be carried on wind or on infested plant material.

Cut and burn before September

The core management strategy for backyard growers is pruning out fresh galls before September. This removes larvae before they complete development and emerge as adults.

Cut the galled stem at least 10 cm below the swelling. Seal the pruned material in a plastic bag and dispose of it in the general rubbish bin. Do not compost it. Do not leave it beside the tree.

The timing matters. If you cut galls after adults have already emerged (indicated by tiny exit holes in the gall), removing the gall achieves nothing for this season’s generation.

ABC Gardening Australia magazine recommends checking for gall wasp during winter pruning sessions: look for unusual swellings on stems, and cut them off and seal them in a bag before they emerge.

Wasp traps and Surround

Yellow sticky traps can capture adult wasps during the emergence period. They are not sufficient on their own to control heavy infestations but can help monitor emergence timing and reduce adult numbers.

Calcined kaolin clay, sold under the brand name Surround, is the main registered product for gall wasp management. NSW DPI has trialled it in commercial orchards with promising results. Applied as a spray during adult emergence in spring, it irritates and repels adults, disrupting egg-laying. The WA Citrus industry recommends two Surround applications during the October to December emergence period.

Surround has some downsides. It can affect beneficial insects and may exacerbate other pests such as red scale and two-spotted mite. Use it within an integrated pest management approach, not as a standalone solution.

The neonicotinoid insecticides Confidor (imidacloprid) and Samurai (clothianidin) have historically been used by commercial growers. As of the 2022 Strategic Agrichemical Review Process, both were under regulatory review. Check current registration status before use.

Where it is worst

Citrus gall wasp is most established and most damaging in the Sydney basin, the coastal strip of NSW, and urban Melbourne. The WA Citrus industry notes it has reached the Perth metropolitan area and asks growers in regional WA to report any finds outside Perth using MyPestGuide.

Lemon and grapefruit trees in densely planted suburban gardens are the most commonly infested. Mandarins are attacked but are somewhat less preferred than lemons.

Heavily fertilised trees with abundant spring flush are more attractive to egg-laying wasps. This is one reason the Leaf Root & Fruit approach in Melbourne suggests being cautious about timing heavy spring nitrogen applications.

Mandarin trees and gall wasp

Mandarins are attacked but tend to be less severely infested than lemons. A mandarin tree with a moderate number of galls can still crop reasonably, but heavy galling on young stems weakens branches and reduces productive wood over time.

New trees should always be inspected before purchase or planting. NSW DPI notes that galls are often introduced to new properties on infested nursery stock. Look for swollen stems before you buy and again when the tree arrives.

NSW DPI guidance

NSW DPI has published detailed guidance on citrus gall wasp as part of the Citrus Plant Protection Guide. The key practical points for home gardeners align with this page: prune galls before adults emerge, bag and bin pruned material, consider Surround applications during emergence, and monitor new planting material carefully.

Biological control using parasitic wasps (Megastigmus brevivalvus and M. trisulcus) is under investigation by NSW DPI. These wasps can parasitise more than 90 per cent of gall wasp larvae where established, but populations sufficient to provide meaningful control do not yet exist in southern Australian urban areas.