Plagiotrichus chrysolepidicola

GALL. — Similar to those of most species of the genus. Swelling up to 20. mm. in diameter by 70. mm. long, usually much smaller; the larval cells distinct, more or less imbedded in the wood, usually not completely surrounded by wood, never separable. On white oaks.

RANGE. — California and Oregon. Apparently confined to the Pacific Coast states.

I have grouped the following varieties into one species because they are all so closely related that further subdivision is not logical (as discussed under diminuens) ; because they all occur on white oaks in the Pacific Coast region, and are abundantly different from the black oak varieties of the same region. The most closely related varieties I have yet seen are the white oak varieties from the Rocky Mountain region, here treated as species frequens. In no case are these varieties as generally dark in color, or the abdomen as naked as in frequens. But inasmuch as in most other respects varieties of both groups are so similar, it must be borne in mind that frequens and chrysolepidicola are closely related, if they should not be considered one species.

The insects emerge very early in the spring, from February 7 in southern California to April 7 in Oregon. New galls were found in some instances at about the same dates, too soon to have been produced by the insects then emerging, suggesting the possibility of the species having an alternate generation which takes a full year for its growth. The galls are abundant wherever white oaks occur, each variety being confined as far as known to a single species of oak, and where that oak ranges over more than one faunal area there will be as many varieties of the insect.

It is unfortunate that the first variety described was named chrysolepidicola, which wrongly defines the habits of most of the varieties. If P. congregatus (Ashmead) should prove to be the bisexual generation of any of these varieties, that name will take precedence for this species.

[Kinsey describes nine varieties that differ in host and gall morphology; see paper for details]

- Alfred Charles Kinsey: (1922) Studies of some new and described Cynipidae (Hymenoptera)^©

Reference: https://www.biodiversitylibrary.org/page/45387508#page/172/mode/1up