By PATRICIA SIMPSON
A gall is an abnormal growth caused by certain insects or mites on the leaf, bud, or stem of a plant. A gall may be shaped like a smooth or spiky fruit, a rosette, a cone, a ball of fuzz, a splatter, or a wart, among other things. It may be a bright color (usually a warm color ranging from yellow to red or pink), green, or brown.
While many insects can cause galls (aphids, midges, flies, to name a few), gall wasps (Family Cynipidae) are fairly common perpetrators. At Mission Trails, the most famous is the Live Oak Apple Gall Wasp (Amphibolips quercuspomiformis).
If you ever find yourself wondering why you are seeing an apple growing on the stem of a Coast Live Oak (Quercus agrifolia), you’ll know you just found a gall! If you check the leaves of the same tree, you may find a tiny pumpkin looking growth caused by another cynipid wasp, the Pumpkin Gall Wasp (Dryocosmus minusculus) as seen in this observation: inaturalist.org/observations/62172417.
In this case, the small insect lays an egg on the surface of a leaf. The larva’s secretions will change the genetic growth of the leaf to create a small gall, where the grub will feed and grow, sheltered from most predators. After a while, it will be time for the larva to pupate. An adult will emerge from the gall the following spring and the cycle will go on.
Live oaks are not the only plants sporting galls. At Mission Trails, you may find rosettes created by a midge on Arroyo Willow (Salix lasiolepis), blisters on leaves of Mulefat (Baccharis salicifolia) engineered by a mite, or a bright red disc on a leaf of a Scrub Oak (Quercus berberidifolia) manufactured by another cynipid wasp.
So, while on the trails in MTRP, make sure to look closely at the plants to find these little engineering treasures we call galls.
To find many examples of galls, visit the Galls of California project on iNaturalist (inaturalist.org/observations?project_id=1335) and explore the species list. You can also learn more by listening to the soothing voice of the great David Attenborough and watch the amazing BBC footage on YouTube (youtube.com/watch?v=CzXccvoJThI). For help identifying galls, visit the Gallformers website (gallformers.org/).
—Patricia Simpson is a trail guide at Mission Trails Park.