When you were a child, did you ever see what were called Mexican jumping beans? They used to be sold at the ends of the checkout counters of variety stores, before there were dollar stores. These were places that sold school supplies, drooping vases of colored glass in various colors, odd kitchen gadgets, hankies, and other small household sundries. The “beans” were sold in small clear plastic boxes, inside of which they bounced and jumped, thrilling children who often persuaded their parents to purchase a box (at least once). The “beans” came from Mexico, and were actually the seeds of a large shrubby plant native to that country. The seeds had been parasitized by a small wasp, the larvae of which caused the beans to move when warm.
We have our own version of jumping beans here in Oregon, although they occur on Oregon white oaks and are called jumping oak galls. The galls are also caused by a wasp, Neuroterus saltatorius. These tiny insects have an outsized life cycle, with alternating generations that produce different types of galls, depending on whether it is mom or the daughter who lays the eggs.
The wasps overwinter in the small galls (more on these later) and emerge in spring, where the females lay eggs in the expanding buds of the oaks, causing blister-like swellings on the leaves. Both males and females develop from the eggs, eventually fly, mate, and the females lay eggs in expanded oak leaves, one to a small gall. These galls look like mustard seeds, and are not much larger. When the egg hatches and eventually matures into an advanced larva, the gall, which is formed from the plant tissue, detaches from the leaf, and can bounce around. The jumping action may help the gall fall into a small crevice or other protected place where it can safely overwinter.
The galls of the first generation of wasps do not resemble those of the second generation, and are so different that the wasps forming them had been given a different name. They are now known to be different generations of the same insect.
The wasp itself does not sting people, and is likely to be too small to even notice: it would take around 35 of them laid out in a line to measure up to one inch.
Trees can have hundreds of galls on an individual leaf, and in hot areas badly affected leaves may look scorched, but the damage these little insects cause is only periodically noticeable and doesn’t require any management.
I say: keep an eye out for affected trees, and see if you can spot affected leaves before the galls fall off – perhaps you’ll have your own jumping galls.
- California jumping gall wasp – Univ. of Florida – IFAS
- New Scientist: The secret hop of the Californian flea seed