Callirhytis pomiformis (Ashm)

During the months of February through early April of the years 1955-1958 large numbers of the parthenogenetic female Callirhytis pomiformis (Ashm.) were reared from galls on the native trees and from galls brought into the laboratory. These females were placed in cloth-mesh rearing bags which were tied on the new growth of Quercus agrifolia Nee, both native trees and young trees growing in five-gallon containers. Bags were placed on young leaf buds, new stem growth and on the young staminate flowers. The female wasps showed a decided preference for the young leaf buds and quickly oviposited into this tissue often to such an extent as to kill the developing bud. Wasps were also seen ovipositing into other tissues, however there was no subsequent gall growth from these. Shortly after the new leaves unrolled, a toadstool-shaped gall, Plate 12, fig. a & b, began to develop on the undersurfaces of the leaves instead of the large, apple-shaped structure, Plate 12, fig. c, that is typically associated with Callirhytis pomiformis ( Ashm.) This toadstool gall was known to other workers and had been listed as an undescribed species under Lewis Weld's field No. 1742. A photograph of it appears in "Cynipid Galls of the Pacific Slope," 1957, fig. 121. Adult specimens proved to be exceedingly difficult to rear because approximately 75% were parasitized by chalcids and another 24% were inhabited by the inquiline Synergus agrifolia (Ashm.) which is probably a parasite of the gall-maker. This may be the reason why the toadstool gall-maker had never been described although the gall itself is not uncommon. At the present time, twelve specimens, seven females and five males have been reared. One female was located by Mr. Lewis Weld in the collection of the U. S. National Museum. It had been reared many years ago in Los Angeles and bore Koebele No. 319. As a single specimen it had never been described nor associated with the species Callirhytis pomiformis (Ashm.). Detailed examination showed the sexual females to be remarkably similar to the parthenogenetic females. Such similarity is not necessarily found in alternating generations of the Cynipidae.

It does not seem to be necessary to use a variety or form name to differentiate the sexual generation from the parthenogenetic generation as advocated by Kinsey in 1920. Specimens in the series are here designated as Callirhytis pomiformis (Ashm.), sexual generation and each insect bears Weld No. 1742. Parthenogenetic females bear Weld No. 1621. A male and a female each have been designated as types of the sexual generation and these together with an additional male and female have been placed in the collection of the U. S. National Museum. One female and a male are in the collection at U.C.L.A. and a female and a male are in the collection of the California Academy of Sciences.

Gall: A toadstool-shaped gall arising singly or occasionally in groups of two or three from the lamina of the undersurfaces of the leaves of Quercus agrifolia Nee and Quercus wislizenii A.DC. The galls appear as tiny, bright-green buttons as early as February or March, however most of the young galls make their appearance during the latter part of March and early April. The galls appear ten days to two weeks after the eggs are laid into the growing buds. Many of the galls are tinged with rose or pink when young although most are light-green in color. Growth is quite rapid and unparasitized galls reach a length of 0.8 to 0.9 cm. in May and yield adult insects of both sexes from May 15 through the first week of June. Mature galls are monothalmous with the single cell occupying the space below the flared top of the gall. The elongated stem of the gall is solid.

Temperature experiments in the laboratory showed that the parthenogenetic females were very sensitive to elevated temperatures and emerged from the galls when the temperature was elevated to the high 70's F. and low 80's F. Galls that were collected too far in advance of the early months of the year, yielded few insects and sometimes none at all. This temperature sensitivity would seem to indicate that temperature plays a critical role in correlating the emergence of the insects with the early growth of the host oaks. These oaks must reach a stage that is susceptible to attack otherwise the young galls will not develop. During January and February in Southern California, temperatures in the above ranges are not uncommon. As indicated above, gall growth has been noted on the new leaves in late February and early March.

Preliminary work indicates that some of the emerging parthenogenetic females lay eggs which produce only sexual females while others lay eggs that give rise to the males. Since only twelve specimens have been obtained at this time it is not possible now, to present information to confirm this. In other species of the Cynipidae that produce a sexual generation from a polythalamous gall, galls produced by some females yield only males whereas other galls, produced by other females, yield only female insects.

The biological relationships involving gall-makers, inquilines and parasites in Callirhytis pomiformis (Ashm.) are most interesting and complex. Studies of these relationships are still in progress and the results will be published at a later date.

Summary: An alternating, sexual generation exists in the gall wasp Callirhytis pomiformis previously known from parthenogenetic females only. The gall produced by the sexual generation is a monothalamous, toadstool-shaped, leaf gall that develops when the parthenogenetic females oviposit into the young leaf buds of Quercus agrifolia Nee. Temperature apparently plays a vital role in coordinating the emergence of the parthenogenetic females with the early growth of the host oaks.

- RJ Lyon: (1959) An alternating, sexual generation in the gall wasp Callirhytis pomiformis (Ashm.) (Hymenoptera, Cynipidae)^©

Reference: https://www.biodiversitylibrary.org/item/106892#page/41/mode/1up