Developmental Anatomy of the Horned Oak Gall Induced by Callirhytis Cornigera on Quercus Palustris (Pin Oak)

Abstract

The horned oak gall forms on twigs of Quercus palustris Muench. and is initiated when the wasp Callirhytis corrigera O.S. oviposits into the periderm or cortex of the twigs. Injury to phellogen as a consequence of oviposition results in dedifferentiation of phelloderm and underlying phloem tissue to form a wound‐response phellogen. The wound‐response phellogen encircles and compartmentalizes the wasp ovum and is continuous with the normal stem phellogen. This tissue arrangement forms the framework for the ensuing stages of gall development. Development of the insect larva coincides with the formation of a gall‐matrix phellogen. This additional meristem is derived from locally hyperplastic stem phellogen where adjacent to the wound‐response phellogen. The essentially parenchymatous, roughly spherical body of the gall, the gall matrix, is formed chiefly from derivatives of the gall‐matrix and wound‐response phellogens. In the immediate area of the wasp larva, the wound‐response phellogen produces cells toward the larva that differentiate to form a vascularized, externally sclerified, horn‐shaped chamber containing in its base the developing insect larva. The involvement of the vascular cambium is apparent during intermediate gall development stages. Vascular strands, produced transverse to the stem axis, penetrate the gall‐matrix tissue and terminate near the base of the horn‐like larval chamber. Vessel elements in the region of the gall that are not included in this tissue alteration often become heavily tylosed. Disruption of the vascular tissue through the galled area appears to be a primary cause of twig death.

Stem galls affect oak foliage with potential consequences for herbivory

“…The gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae), was the model herbivore. Pin oak is a favoured host of the gall wasp, which forms large (∼40 mm), globular woody stem galls and small leaf galls (Taft & Bissing, 1988; Johnson & Lyon, 1989). The gamogenetic, stem‐galling generation requires ∼33 months to develop, and wasp emergence from stem galls coincides with budbreak in early spring.…”

Spatial Distribution and Parasitism of Leaf Galls Induced byCallirhytis cornigera(Hymenoptera: Cynipidae) on Pin Oak

“…Development of the agamic and sexual generations requires about 33 mo and 9 wk, respectively (Eliason and Potter 2000b). Stem galls on tree branches are aesthetically disÞgur-ing, cause branch dieback, and heavy infestations may kill trees either alone or in combination with other environmental stresses (Whyte and Ford 1980, Johnson and Lyon 1988, Taft and Bissing 1988. Upon emergence from stem galls in early spring, females selectively oviposit into juvenile leaves within swelling buds, resulting in leaf gall formation along the larger leaf veins.…”

Biology ofCallirhytis cornigera(Hymenoptera: Cynipidae) and the Arthropod Community Inhabiting Its Galls

“…Galls remained succulent and green at least until adults exited, but soon thereafter became very hard and woody. Taft and Bissing (1988) provided a more detailed description of the morphology of C. cornigera stem galls.…”

“…Several cynipids attack pin oak, but probably the most damaging is Callirhytis cornigera (Osten Sacken), which makes the "horned oak galls." Endemic populations of C. corni-gera can build to damaging levels on isolated shade trees or in forests in eastern and midwestern United States (Whyte and Ford 1980, Taft 1982, Taft and Bissing 1988. Unlike gall systems in which annual plants or plant reproductive structures are infested, stem galls on tree branches remain aesthetically dis-Þguring for years after the wasps have emerged and abandoned them.…”

“…Males and females emerging from leaf galls have not been described, although they have long been recognized as the alternate, sexual generation of C. cornigera (Felt 1940). Taft and Bissing (1988) examined stem gall morphology, and Johnson and Lyon (1988) suggested a tentative life cycle that was not based on empirical study. In 1997, we found a population of C. cornigera and its galls that was large enough to provide samples for rearing adult wasps (Fig.…”