Jumping Oak Galls by Julie Craves
A move to a new house has been responsible for my lack of productivity here the last few months. Now that we are settled in, we are anxious to get to know our large property. We have eight acres of wet woods, and while my bark identification skills are serviceable, I’ve been spending more time looking down at the ground to unlock some of the secrets of the forest we will now stewards.
Many of the mature trees are Black Cherry, Bur Oak, and Swamp White Oak. I noticed that many of the fallen oak leaves were pocked with dozens, if not hundreds, of round lesions, each the size of a pinhead. On the uppersides of the leaves, they looked just like pimples. On the underside, each lesion was a shallow pit that was either empty or had a tiny nodule nestled in it.
Those nodules are galls, created by very minute wasps in the genus Neurotarus. They’re known as jumping oak galls, because when the galls detach from the leaf and fall to the ground, the wasp larva inside will sometimes wiggle around and cause the gall to move. The fully developed wasp larvae will overwinter in their galls on the ground. Galls that remain on the leaves are probably doomed – they’ll either dry up and die, will expire because they have been parasitized or their galls have been invaded by insect inquilines, interlopers that feed on the gall tissue.
If all goes well, these galls will produce the all-female generation. Neurotarus wasps, like many of the others in their family, have elaborate life cycles that include alternating asexual and sexual generations.
Members of this family of wasps are quite host-specific, usually attacking only one or a few species of oaks; none use hosts in both the red and white oak groups. All the gall-bearing leaves I found were white oaks, with none on the plentiful red oaks present on the upland parts of the property.