Pachypsylla celtidisgemma [Riemann considers P pallida a variant of this species]
Gall: Formed in the axillary buds of hackberry, usually polythalmic with one to twelve or more cells; sparsely pubescent in the eastern portion of Texas, covered with a dense matt of long white hair in the western part of the state, intermediate forms occurring where the eastern and western forms come in contact.
A few problems relative to this species were discovered during the course of this study.
There are in Texas two forms of bud galls which are distinctly different in appearance (Plate VIII). One of these forms is only sparsely pubescent and most of its pubescence is lost in the course of a season. This form occurs in Central and East Texas, that is, over the part of the state where C. laevigata is the common species of hackberry. The other form is covered with a dense matt of long white hair which remains on the galls at least during the current season. This form occurs In West Texas in areas where C. reticulata is the most common species. Also it apparently occurs along the entire length of the Rio Grande River at least below Del Rio. At Brownsville this was the only pachypsyllld gall which was found in large numbers. In addition bud galls of this form were found on the hackberries in northeastern Oklahoma.
Apparently the two gall forms are not simply modifications due to development on different host trees. At Austin and at other localities in Central Texas the sparsely pubescent form was found on trees having the characteristics of C. reticulata as well as on C. laevigata. Also, near Fredericksburg and in Inks Lake State Park near Bertram a few densely pubescent galls were found on trees with leaves typical of C. laevigata. On the basis of these observations it was concluded that these two types of galls were due to differences in the gallmakers themselves.
If the above conclusion is correct it would appear that the gallmakers having a genotype capable of producing densely pubescent galls are adapted to developing on C. reticulata although not limited to trees having the typical features, at least, of this hackberry. The occurrence of these galls on the population of hackberries along the Rio Grande and on those in northwestern Oklahoma are probably evidence of considerable introgression of C. reticulata characters into the trees of these areas. This is particularly indicated by the fact that other C. reticulata galls are found on the trees of these regions.
In Inks Lake State Park and near Laredo some galls were found which appeared to be clearly intermediate between the two forms, and even at Austin some galls were pubescent enough to approach the western form in appearance. However, the two forms have not been studied in the field in enough detail to determine with any considerable accuracy either the boundaries of the ranges of the two forms or the width of the zone of intergradation.
Collection records do indicate that in Central Texas the areas of changeover from the eastern to the western form correspond at least roughly to those areas where C. laevigata is replaced by C. reticulata as the most common hackberry. Further they indicate that the zone of intergradation between the two forms is relatively narrow.
The above findings and the fact that no differences were found in the genitalia of specimens from the various collection sites strongly indicates that only a single species of bud pachypsyllld occurs in Texas and, in the opinion of the author, this is probably true for the United States as a whole. If this can be shown to be true on the basis of later investigations P. pallida and P. dubia, two allied forms described by Patch in 1912 can likely be considered as synonyms of P. celtidisgemma.
The most plausible Interpretation of the information available in the literature and the observations made in the course of this study would seem to be that there are in the United States two populations of P. celtidisgemma with distinct galls which intergrade in Texas. There is an eastern "typical" P. celtidisgemma gall and gallmaker which occurs on C. laevigata and the other eastern hackberries, and a western P. pallida gall and gallmaker which occur principally on C. reticulata. Between the two groups of gallmakers there is a very broad zone of intergradation, while between the galls there is a much shorter zone. This Intergradation is correlated with inter-gradation in Celtis and could presumably be used to some extent as a measure of this intergradation. Since the form of the gall is a reflection of the physiology of the gallmakers the relatively narrow zone of Intergradation between the gall forms demonstrates that there is a higher degree of host adaptation in the bud gallmakers than would be indicated just on the basis of their coloration.
Possibly P. dubia, on which there is no locality data, represents Intermediates between the P. celtidisgemma and P. pallida forms.
