Oak Gall Wasp

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Oak Gall

Oak Gall, Andricus californicus,
and Ozognathus cornutus.
Images and text by Don McIntire

Order Hymenoptera/ Family Cynipidae – Gall Wasps

Oak Gall, Andricus californicus, and Ozognathus cornutus

Adult: 4-5 mm

This little California Oak Gall Wasp (Andricus californicus) is probably among the most widespread of the micro-hymenoptera, specializing in laying its eggs within the young twigs of various oaks which causes the growth of the familiar galls often referred to as oak apples. These, the largest insect galls in California, are usually noticed only after they turn the bright red color and the wasps have emerged. The cross-section pictured shows the larvae clustered near the center in their individual cells with one having emerged through its boring. The spongy interior of the gall is a result of the deformation of the plant’s nutritious phloem cells after oviposition into the, usually, new-growth twig.

The wasp may lay many or just a few eggs, which might be the reason for the varying sizes of galls. The wasp pictured was reared from such a gall with two others. All three emerging wasps were females. And such has also been the case that other galls may yield only males. Why this is has not been fully explained by science, but probably, among other plausible reasons, helps to insure genetic diversity. The tiny white grubs have no legs and move through contractions and extensions of their segmented bodies within the cells as they feed until they finally pupate to become adults.

Other kinds of micro-wasps specialize in laying their eggs in these galls so their young may feed on the defenseless young of the Cynipid wasp. And still others, like the tiny beetle Ozognathus cornutus in the family Anobiidae, live as a commensal and inquiline, or animal which lives in the nest of another, feeding on the tissues of the gall. The beetle is pictured beside the Cynipid female at about a comparable ratio with the host, ranging in size from 1.5 to 2.5mm. The little males are adorned with a delicate pair or horns which arise from the base of the mandibles. These are seen in the lateral view projecting up from the anterior part of the head. Sometimes these little beetles may be attracted to the catkin flowers of willows or oaks in great numbers.

Next example: Spittle Bug