Why are there popping, jumping 'seeds' around my valley oaks?

Leimone Waite
Master gardeners
Oak galls contain insect larvae, usually wasps.

Q:  We have oaks all around our property and it sounds like someone is popping corn in the woods when we walk out among the trees. When we investigated further we found these little brown “seeds” falling from the oak tree and jump all around on the ground.  The noise is coming from these “seeds” moving the dry leaves. They do not look like acorns but they also do not look like an insect. Can you tell me what this is?

A: It sounds like you have jumping oak galls. These are tiny, seed-like galls that form on the undersides of valley oak leaves. They are caused by the tiny gall wasp, Neuropterus saltatorius. They are not considered harmful to the tree but with large infestations of these galls the oaks will have some leaf drop. Oak leaves infested with these galls have a lighter color green on the upper surface of the leaf and will often have a blister that is ringed with a yellow halo. If a tree has a large population of these wasps forming galls it may cause the entire crown of the tree to defoliate. However, this usually happen late in the season, so it does not cause too much harm to the tree.

An oak gall found at the Ridgefield National Wildlife Refuge

There are over 200 insects that cause oak galls including midges, mites, aphids, flies, and wasps. The undisputed champs of oak galls are a big family of little wasps called Cynipids. These wasps are very small, rarely exceeding the size of a mosquito. There are 90 different species of native California Cynipid wasps that cause galls on oaks. “These tiny wasps cannot sting,” says Dr. Kathy Schick, assistant specialist/curatorial assistant at the Essig Museum of Entomology at University of California, Berkeley. “Gall-inducers are fascinating in that they are very specialized to their organ of the host plant.” Oak galls form when an insect, or its larvae, introduce chemicals into a leaf or stem that cause the plant’s growth hormones to produce in overabundance. These chemicals induce a profusion of plant cells to form on the leaf, or they increase the size of existing cells, causing the formation of the galls.  In some cases, this profusion of cells growth gets so out of control that they form large, alien looking shapes. 

In the case of the jumping oak gall, each gall contains a single wasp larva that feeds on the inner lining of the gall. The galls fall to the ground once the wasp larvae are mature. The movement that you describe is the larva inside the gall moving and making the gall jump around on the ground. This movement helps the larvae burrow into the ground or under leaf litter. It then overwinters inside the gall burrowed into the ground.

Oaks prefer to grow in natural soil without any mulch or cover in the wild and in gardens.

In the spring, the females emerge and lay their eggs in newly opened leaf buds. The leaf or twig galls form in response to chemicals in the larva’s saliva. The adult female wasp, in order to be able to create a gall that protects her egg, must sting the leaf at precisely the right time.  If the leaves are too fully expanded and hardened off, the galls will not form.  That is why you might see one tree with millions of these galls and yet another tree nearby might not have any.  It was because the unaffected trees leafed out at a slightly different time thwarting the wasp’s attempt to lay eggs on the leaves.

The Shasta Master Gardeners Program can be reached by phone at 530-242-2219 or email mastergardener@shastacollege.edu. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.