Plagiotrochus suberi, new species

Host.—Quercus suber, the European cork oak introduced into California.

Gall (fig. 10). — Cells imbedded in the wood immediately under the normal bark on small slender twigs, sometimes occurring in such great numbers that the twig is uniformly hypertrophied to twice its normal diameter for several inches. The individual cells are elongated, 2.5 mm. long by 0,7 mm. in diameter, lying parallel to the axis of the twig, the exit hole through the bark 0.5 mm. in diameter.

Habitat. — The type is selected from material from a cork oak tree on the grounds of the Cottage City Nursery Co., north of San Jose, Calif. Paratype locality, Pasadena.

Biology. — When the San Jose tree was visited in company with Dr. H. E. Burke on May 16, 1918, it was almost in full bloom, the young leaves not yet half grown. Some of the cells were in the previous season's growth but most were in the two-year-old wood and a few in that still older. Almost every twig on the tree was more or less infested. When badly infested so as to be hypertrophied the twigs are killed and fully half of the small twigs in the upper part of the tree seemed to be dead. No other kind of gall was seen on the tree. A few adults had already emerged and were seen resting on the foliage but most were still in the pupa state. From twigs collected on that date and sent to the Eastern Field Station for rearing (Hopkins U. S. No. 15G08«) adults emerged May 21 to June 5. Adults emerged at Evanston June 3-11. This tree had a trunk diameter of about 15 inches but nothing was learned as to its history.

On May 7, 1924, A, O. Larson collected more material of this species from a cork oak tree on North Orange Grove Avenue, Pasa- dena. Flies were emerging when this was received at Washington, May IT. When he revisited the tree on May 22 practically all had emerged and many of the twigs had died but still retained their leaves, badly disfiguring the tree. This tree is about 18 inches in diameter and undoubtedly planted about 25 years ago by the architect, not by the gardener who planted most of the ornamentals on the place.

As the gall resembles that of a European Fioriella on cork oak, galls and flies were sent to Prof. J. S. Tavares in Spain, who is familiar with the galls of the Iberian peninsula, thinking possibly this might be that European species which had been brought over years ago in the twigs of a young tree imported from Spain. But he did not know the gall, nor was the insect a Fioriella. No record is at hand of the introduction of the cork oak except by importation of acorns. There are said to be over 1,000 cork oak trees in the San Gabriel Valley descended from two trees on the Richardson farm at Alhambra grown from acorns sent to Don Benito Wilson by the Commissioner of Agriculture in 1860. It seems probable that this is a native Cynipid which has come over from the native oaks on to this introduced exotic ornamental tree, although all the galls of this type known to the writer on the native oaks are produced by entirely different genera.

Another tree infested with the galls of this species was discovered by Louis Kroeck in May, 1924, standing in a lumber yard — once a recreation park — in Santa Clara on land that once belonged to the mission. In April, 1924, Eric Walther found another infested tree in Golden Gate Park in San Francisco. He writes:

This tree is about 35 years old and has evidently been suffering severe injury for years, the 2-year-old twigs being so heavily infested that they die and the first heavy storm causes them to break off, and so the tree looks as if it had been pruned annually.

- LH Weld: (1926) Field notes on gall-inhabiting cynipid wasps with descriptions of new species^©

Reference: https://www.biodiversitylibrary.org/page/7610635#page/315/mode/1up