Gall-inducing insect

A gall-inducing insect is any insect that can cause the growth of galls within plants.[1] The term gall fly is also used to cover these species, although most are not true flies.

Parthenogenetic generation female gall wasps Neuroterus albipes, from galls on Quercus robur

There are several groups of insects that meet this description. They include the gall wasps, scales, gall midges, aphids, psyllids and certain species of leafminer flies.

Galls are growth deformities induced in certain plants by various insects which are mostly species-specific.

The gall's form or type depends on what organism is attacking the plant and where the plant is being attacked.

Based on the form, there are two classification systems used to identify the cause of galls: causative agents located outside plant tissues, and those agents located inside plant tissues.

Causative outside agents include:

• Krebs gall is caused by surface agents.
• Filz gall is caused by agents among surface hairs.
• Fold/roll gall is caused by agents within turned-over leaf blades.
• Pouch gall is caused by agents within a cup-like structure that occurs when opposite ends of the infected structure arch upward and form a spherical oval.

Causitive inside agents include:

• Covering gall is caused by agents embedded within a gall when plant tissues rise up and surround the parasite.
• Lysenchyme gall is caused by agents that sink into the plant when the plant cells dissolve away and close around the parasite.
• Mark gall is caused by agents burrowing within plant tissue before gall develops.

This symbiotic relationship is rather one-sided, whereas not much research has been done to verify any benefit given to the plant species. The insect that causes the gall formation gets an entire microenvironment or microhabitat provided to it, safe from climate and predation to grow within, and with a rich supply of food formed within the gall for the parasitic insect to feed upon.