By Kelcie Brown
Mar 9 2022
If you’ve ever marveled at an odd growth on a leaf or bulge in a stem, chances are, you have seen a plant gall. Galls can come in many forms including witches brooms, blisters, leaf edge rolls, cones, balls, pouches, and patches or tufts of hairs called Filz galls (Redfern, 2011). On first glance, galls can sometimes be hard to distinguish from a plant’s own physiological or developmental abnormalities, but the key to understanding galls is that they are tissue thickenings induced by another organism. What further separates galls from leaf mines or normal herbivory is that any tissue growth by the plant is due to the gall inducer’s direct influence, rather than natural responses to damage, and the gall inducer feeds off of or in another way benefits from this modified tissue. These relationships are almost always parasitic, except in rare cases, such as the relationship between figs and fig wasps.
Certain species of bacteria, fungi, nematodes, mites, insects, and even other plants are known to induce plant galls (Redfern, 2011). The majority of gall inducers are insects, although the exact number of species is unknown. There are anywhere from 21,000 to 211,000 estimated species of insects worldwide that rely on galls to complete their life cycle (Espírito-Santo & Fernandes, 2007). For insect and mite gall inducers, galls provide shelter for developing larvae and provide ample food source without having to scavenge (Redfern, 2011). Gall-inducing plants are much less common, but you are likely already familiar with them–they include mistletoe, dwarf mistletoe, and dodder. At first glance, these plants may seem like your typical parasites, since they do not create galls as obvious as insects do, but they do induce tissue proliferation from which they draw nutrients, which classifies them as gall inducers.
Evolutionarily, galls have arisen independently many times (Redfern, 2011). The relationship a gall inducer must have with its host is highly specialized, and so it makes sense that galls can often be found on the dominant plants of a region, increasing their chances of finding a suitable host. Plants in the family Fagaceae (namely Quercus and Betula) in much of Europe and North America, as well as parts of Asia, are especially good hosts to gall inducers (Redfern, 2011; Insect and Mite Galls, n.d.). Likewise, gall inducers often find their homes in plants of the family Fabaceae in South America, Africa, and India, and Myrtaceae in Australia (Redfern, 2011).
Take a look at some of the weird and wacky galls that have hitchhiked into the herbarium!
If you would like to try your hand at identifying galls, visit https://gallformers.org/, a website made for gall enthusiasts by gall enthusiasts!