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Over the Garden Fence

Get to know plant galls

It is that time of year for plant galls (or things that look kind of like a gall anyway) to be more obvious in the home landscape. They got a start long ago when bud swell was going on earlier this spring, but they are more noticeable now.

What is a gall?

Plant galls, or abnormal growths, form around insect eggs deposited when the leaf tissue can adapt during spring expansion. The list of plant galls is just about limitless. You name a tree species and chances are there is a matching gall-forming insect. Absolutely all of our more common trees planted in home landscapes have an associated gall and that includes the needled evergreens. In fact, oaks support something in the order of 500 different insects. Other species fall in line with that number, but white oak is the winner.

A gall typically starts out thick-walled so there is something for the hatching larvae to feed upon. Some galls can form on plant stems, causing a thickening as well. Galls can be formed by very, very tiny wasps (no threat to us at all), aphids, pod gall midges, and gall forming psyllids.

If you study galls, it is interesting to see that the galls are unique to the insect or the host plant. Some galls will transform the entire leaf like the oak apple gall, or the gall will follow the veining of the leaf. Do some citizen science research by carefully cutting open some galls to reveal the larvae. They will be quite small to start, but easily visible as they grow. Since the larvae are consuming green leaf tissue, you can guess what color they will be. As a side note, as long as the galls remain green, that gall tissue is producing energy for the tree. Only when it turns brown, as the adult leaves the gall, does it stop.

What else could it be?

Things that may look like a gall could be caused by a fungus, as is the case for peach leaf curl, or an insect that causes whole leaves to cup or curl like boxwood leaf psyllid.

What to do about galls?

Unless you have had a young tree that is totally covered with heavily distorted foliage from gall forming insects in the past, there is really no need to treat. If you feel the need is there for a preventative treatment, it will all be about timing. In general, these gall forming insects are out there as bud swell begins each spring and any treatment has to be in place before they arrive. There is no wait and see scenario that works with gall forming insects.

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.