Jumping Galls: Why and How They Move

Right now in certain parts of California and other parts of the world, thousands of greenish-brown spheres - looking like oversized seeds - are falling from trees, only to seemingly jump on their own to more suitable ground.

Now, in a timely release, a study on these unusual little bits of tree has been published in the Journal of Insect Behavior. The study details how these balls, called galls, are actually the homes of wasp larvae. The little baby wasps thrash around within these small spheres, causing them to erratically jerk and jump.

According to the study, Neuroterus saltatorius "are gall forming wasps endemic to western states in which a variety of oak trees act as hosts." Every summer, unusual formations swell on oak trees after adult female N. saltatorius lay their eggs on the surface of the oaks' traditionally wide and thick leaves.

There are a handful of wasps, mites and other organisms that can reproduce asexually via gall formation, but how this process occurs may differ. In all cases, however, a chemical or mechanical stimulation of a leave's plant cells causes them to grow in size and number until they utterly encapsulate each egg.

What makes N. saltatorius part of a very unique club is that after the galls reach such a size that they begin to fall to the ground, they then start to jump.

Saige Manier and David Deamer, the authors of this latest study, suggest that these larvae jump to move their gall homes to suitable conditions where they can complete their development before hatching after several weeks. In fact, in what the pair believe is the first comprehensive field study of N. saltatorius, they found that the galls only jumped at temperatures between 68 and 104 degrees Fahrenheit (20 and 40 Celsius), and in dry climates.

The hope, they believe, is that the jumping will help increase the likelihood that the gall will roll into shade or a moister region, as to avoid drying up.

Still, it's unclear exactly how the jumping occurs. Manier and Deamer suggest that the larvae contracts its body, literally sloshing the fluids in its body around to create a rocking motion.

However, that is still largely under speculation, and only one of a number of probably theories. You can catch this intriguing phenomenon for yourself, dreaming up you own theories, as a dry and hot summer - apparently prime jumping season for galls - presses in on the west.