This time of year we get to see a lot of interesting leaf galls. A general rule of thumb is that if they are on leaf or petiole tissue, they are likely not significantly damaging to tree health. Above is an oak (Quercus macrocarpa) leaf gall caused by a tiny wasp in the Cynipidae family. The juvenile stage of the wasp is spent inside the galls, feeding on plant tissue. After 4-5 weeks, the wasp maggots pupate and tiny adult wasps will fly out from a hole they make in the gall. At this point, the galls will turn brown but the leaves will usually stay at least partially functional. Leaves are distorted and undersized, so there could be a reduction in growth due to reduced photosynthetic capacity if a significant percentage of leaves are colonized by these galls.
However during years of high populations, the galls can displace functional leaves and lead to a reduction in growth. There are so many different kinds of oak leaf galls that early botanical illustrations of Quercus sp. often included a leaf gall.
Maple trees (Acer saccharhinum and A. saccharhinumx rubrum hybrids) are host to several species of leaf galls, including this “bladder gall” (caused by an Eriophyid mite). The galls rarely interfere with the photosynthetic capability of the leaves and do not negatively affect tree health. Though they do freak homeowners out sometimes.
Like other insects, leaf gall makers go through population cycles, usually because of corresponding population changes with their predators and parasites. Some years, the level of infestation can be quite significant. But for the most part, the tree health is not adversely affected as leaf area for photosynthesis is still significant.
Hackberry (Celtis occidentalis) are also host to species of galls. Here is the hackberry nipple gall caused by a psyllid which feeds from inside the leaf galls in the juvenile stage (nymph). It’s quite common in southern Ontario, as is the tree in our urban landscape. The trees do not seem to suffer seriously from the galls unless a high proportion of leaves are infested over several years. The nipple gall may cause some premature leaf drop in summer.
You can see why they call it the hackberry nipple gall, although personally I think the scientist had men in mind when they thought of this common name. If you are laughing right now, it means that you are reading my blog (and not just looking at the pretty pictures). 🙂
When galls are found on twigs or branches, they can be more serious since they often result in twig mortality, which is more significant to tree health. Again, most established trees can withstand light infestations. Juvenile trees and nursery stock are much more susceptible to twig galls.
This twig gall on oak is also caused by a Cynipidae wasp, but the damage is more serious to the trees. Juvenile nursery stock is more vulnerable than large, established landscape specimens. Again, pruning out twig galls and destroying them can help.
Putting out yellow sticky cards in mid-summer can help trap flying adults and interrupting their life cycle. Ontario nursery growers use sticky traps as an important part of their Integrated Pest Management program.