Leaf Bumps

Did you know that the bumps under a leaf especially red ones are wasp eggs? Other insects too. So, for every single one you see, KILL IT, unless you like wasps and their stings.

They’re not eggs. Well, they could be eggs, since I don’t know exactly what you’re describing. But insect eggs are tiny. What you’re probably talking about are galls.

Galls aren’t eggs. They’re swellings that a plant creates in response to a gall-inducing organism (often a wasp, though other types of insects and some non-insect species cause galls as well). The gall inducer’s larvae then live inside the gall, feeding on the plant tissue.

Galls are fascinating, and sometimes beautiful. Gall-inducing wasps tend to be tiny; a quarter-inch to a half-inch long. They can’t sting you. By destroying a gall you aren’t protecting yourself. You’re just killing another living thing for no reason.

Here are some galls that look like what you might be describing. These were on the underside of the leaves of an arroyo willow (Salix lasiolepis) at the salt marsh near where I live. I believe these were probably caused by a sawfly from the genus Pontania (not actually a wasp, but the adults look sort of wasplike):

My favorite gall inducers are a tiny midge called Rhopalomyia californica. They live almost their entire lives inside a gall they create in coyote brush (Baccharis pilularis), a common shrub that grows along the coast near here.

Adult R. californica midges leave their galls in coordinated emergence events. We don’t know how they time it so they all emerge at once, but it seems to be related to rainfall. What we do know is that about 24 to 48 hours after a rain, adult midges emerge from their galls around sunrise. The adult midges look kind of like small mosquitoes, though they can’t bite. They don’t have mouths. The females are already full of eggs; you can recognize them by their swollen orange abdomens.

Within an hour of emerging, their wings have hardened and they begin looking for a mate. After mating, the males die. The lifespan of an adult male is about two hours.

Females begin laying their eggs on the growing tips of nearby coyote brush stems. They lay as long as they can. The lifespan of an adult female is about eight hours.

It’s easy to find R. californica galls once you know what to look for. Here’s a picture I took of one. You can see the leftover exoskeletons (called exuviae) of the emerged adults:

Finding an adult midge is harder. I’ve never seen an emergence event in-progress, though I keep looking. The closest I’ve come is finding the body of a dead female a day or two after emergence:

She was still laying eggs when she died. The tip of her abdomen was stuck to the plant, an orange line of eggs trailing from it.

Bugs can be scary. A few of them can bite or sting, especially if you bother them. But most of them don’t want to hurt you. They’re just living their lives.