Submitted to: Zoological Journal of the Linnean Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/29/2000
Publication Date: N/A
Interpretive Summary: Psyllids or jumping plant lice are serious pests of pears, apples, citrus, and ornamentals and cause millions of dollars in losses in the United States each year. Species of gall-making phyllids in the genus Pachypsylla may become very abundant in the fall to the point of becoming a nuisance in houses. A new gall-forming psyllid in this genus was discovered that is unable to form a gall itself, but feeds near other species of gall formers and is incorporated into their galls. Evidence is presented that the new species is unable to survive without the presence of a different species of gall former. The new species is described and its unusual life history is given. Gall cohabitation of multiple species of insects in the same genus is known in other groups, but it is recorded for the first time in psyllids. The descriptions and illustrations of the new species will assist federal and state identifiers to recognize it when it becomes a nuisance in the fall. This research also is important for ecologists interested in studying cohabiting gall-forming organisms.
Technical Abstract: The two largest lineages of holometabolous gall-forming insects, cynipid wasps and cecidomyiid flies, have given rise to numerous obligate inquilines, species which are unable to form galls themselves and survive only by inhabiting galls formed by other species. In contrast, only a single obligate inquiline, an aphid, has heretofore been known in the sternorrhynchous Hemiptera, the memimetabolan lineage in which galling is best developed. We describe the first known inquiline in psyllids (Sternorrhyncha: Psylloidea), Pachypsylla cohabita sp. n. All other members of this genus produce closed galls on hackberries, Celtis spp. (Ulmaceae). Newly-hatched nymphs of P. cohabita feed next to nymphs of several species of leaf gall makers, becoming incorported into the gall as the stationary nymphs are gradually enveloped by leaf tissue. In the mature gall, the inquilines occupy separate, lateral cells surrounding a central cell containing a single gall maker. Pachypsylla cohabita is similar in morphology to leaf gallers, but differs in nymphal and adult color, allozyme frequency, especially in the malic enzyme, and in adult phenology. Laboratory reared progeny of side-cell females, then caged alone, never form galls, while progeny of center-cell individuals alone only form galls comprised of single individuals. Multiple-cell galls are formed only when adults of side-cell and center-cell individuals are caged together. Experimental removal of center-cell nymphs in early stages of gall initiation leads to small galls or death of side-cell individuals. We conclude that the side-cell individual is an obligate inquiline species that is incapable of forming a gall on its own but is derived from a leaf-galling ancestor.