Insect of the Month: Gall-Making Insects

Yaupon psyllid gall, Gyropsylla ilicis Ashmead Homoptera Psyllidae Photo by Drees. - Closeup

Yaupon psyllid gall, Gyropsylla ilicis Ashmead (Homoptera Psyllidae) Photo by Drees. – Closeup

Editorial note: This is a series highlighting the most interesting facts about a featured insect each month.

Have you ever wondered what makes those small balls that you see underneath leaves and stems of plants, such as trees?

According to Professor and Extension Urban Entomologist Dr. Mike Merchant, the growths, called galls, are often the result of insects laying eggs inside or feeding on the branches or leaves of trees and other plants.

A wasp-caused gall on live oak leaves. Photo by Mike Merchant

A wasp-caused gall on live oak leaves. Photo by Mike Merchant

The galls, Merchant said, were tumor-like growths that are produced by the plant in response to chemicals injected into the plant by adult or larval gall-making insect. The shape and size of the gall is determined by the precise chemicals that are used by each species of gall-maker. The mechanisms of gall formation and how these chemicals are used to make the galls are still poorly understood.

Newly emerging leaves, twigs and flower parts are the most common sites for gall-maker egg laying and gall formation

Newly emerging leaves, twigs and flower parts are the most common sites for gall-maker egg laying and gall formation

Most gall-making insects are tiny wasps that are in the plant-gall-making family called Cynipidae. Other common galls are also called gall midges. Some, like the hackberry nipple gallmaker, are relatives of leafhoppers called psyllids. Merchant said the most interesting fact is that each insect makes a distinctive and unique gall. It is unique enough that it is possible to identify the gall-maker by the type of gall it makes.

vein pocket galls on red oak leaves. Photo by Mike Merchant

Vein pocket galls on the underside of southern red oak leaves, Quercus schumardii, are caused by a tiny midge fly. Numbers of these midges in some years can be severe enough to cause noticeable aesthetic damage to trees–unusual for most galls. Photo by Mike Merchant

Gall formation usually takes place in the spring, when leaves and flowers and stems are rapidly growing.  Only during this time of rapid cell division and growth can these insects bend plant cell division to do their bidding.  Once a leaf or stem has stopped growing, these hormone-like chemicals can no longer affect the plant.

Merchant said the purpose for insect-induced galls seems to be to provide a sheltered feeding site for the gall-maker.  Because galls provide benefit for the insect at little expense to the plant (only a very few galls seem to affect plant growth or overall appearance significantly), this is sometimes referred to as a form of commensal relationship.  The good news for the gardener or tree owner is that galls rarely cause much harm to plants.

Once a gall has formed on a plant, there is no need to kill the insect inside, as whatever energy loss will be suffered by the plant has already occurred.  In addition, short of ripping the galls off of the plant, there is no way to kill gall making insects inside their protective homes.

If there was a need to control galls on a tree, now would be the only time to do it. Sprays applied early in the spring could theoretically kill adult gall-making wasps or midges before they can inject their disfiguring drugs.  But little research has gone into this practice it is not advised, he said.

Instead, as you gaze on the swelling buds and rapidly greening trees in your backyard, just take a minute to consider the gall-making insect. In addition to all the other rituals of spring, these tiny creatures are working like crazy out there to provide little bug caves, or retreats, for their offspring.

For more information and additional photos of gall-making insects, see the Extension publication at: or

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