PEST
 
   NAME
     Gall Flies (General)
     Various Gall Flie Species
 
   ORDER
     Diptera
 
   FAMILY
     Cecidomyiidae

Description of the Pest
There is about 100 species of flies that are known as gall gnats or gall midges. They infect particularly the leaves usually forming pimple like galls. The buds and stems are also attacked and the galls can be large.

   Blossom Gall Fly on Acacia species

Blossom Gall Fly (Cecidomyia acaciaelongifoliae) attacks spent flowers heads forming large twisted galls that are made up of tubes containing maggots that feed on the flower heads and causing the seed to become malformed, into galls. Not normally requiring control, but plants appear ugly. 

            Pimple Gall on Eucalyptus species.

Pimple Gall (Harmomyia omalathi) forms small reddish pimple-like galls that may appear along the mid rib.

Callitris Gall (Diplosis frenalae) is a large gall that that splits in to six sections and looks very simular to a Callitris cone. 

Cone Gall (Rhabdophaga strobiliodes) is produced by a small fly that deposits eggs in opening buds. Lave overwinter in the cone shaped galls that inhibit the development of the bud. It is found on Salix species.  

  Fergusonina Galls
Image by B. Sonsie

Fergusonina Galls (Fergusonina species) are the result of a symbiotic relationship between leaf nematodes and gall flies such as (Fergusonina nicholsoni). In all cases the nematodes are deposited along with the fly’s lava and together they feed on the leaves forming galls, which can become large sometimes cover the entire leaf. The nematodes increase in numbers and when the fly emerges it is infested with them, which are then deposited in the new nest completing the cycle. 
  
Appearance and Distribution of the Pest
The adult infect plants from spring to autumn and are distributed by flying with the assistance of wind. The larvae may also be distributed in infected cuttings or leaves that are transported to another site. 

Life Cycle
These insects have a Holometabolous life cycle, ie. When metamorphosis is observed during the pupal stage.
The larvae of these insects are known as maggots. They have chewing mouthparts and the adults have piercing and or sucking mouthparts and one pair of wings.

Period of Activity
Generally the flies are more active during the warmer months of the year and are found in sub tropical to temperate regions particularly on the coast.

Damage Caused
When flowers are attacked the maggot interferes with seed formation and stimulates the growth of galls. Many species of Gall Fly feed off the plant from within the gall and certain species such ass Fergusonina Galls (Fergusonina species) forms a symbiotic relationship with nematodes and both feed from within the gall. The leaves are the normal area attacked, but stems are also infected such as the new stems in Callitris species.

Susceptible Plants
There are many species attacked including Acacia, Eucalyptus, Callitris, and Pittosporum species.
Catalpa species are attacked by the Midge (Cecidomyia catalpae) which lays eggs in the leaves and the emerging lava eat circular sections of the leaf surface causing distortion.

Eucalyptus species are attacked by various Fergusonina Galls which may alter the flowering or seed production of the host.

Viola species are attacked by the Gall Fly (Phytophaga violicola) which attacks the leaves forming galls and distorting its shape. After time the leaves form wet-rot and the plants development is dwarfed.

Cultural Control
Pick off the damaged leaves where possible prior to the emerging adult and destroy. This will reduce build up of the population, but if there is evidence of emergent holes in the gall it is too late. Plants that are repetitively attacked should be removed and the area re-planted with a less susceptible species. Infected trees respond to fertilising and watering to improve vigour against the attack.

Biological Control
Parasitic wasps lay there eggs in the galls and some species of nematode eats the larvae prior to emergence.

Chemical Control
Chemicals may be injected or sprayed but normally has limited effect as the insect is inclosed within the gall and is difficult to monitor. 
Note
Always read the label for registration details and direction of use prior to application of any chemicals.