The Greenbug, Schizaphis graminum.1 1. Mouth Parts and Feeding Habits2

The mouth parts of Schizaphis graminum (Rondani) (Homoptera: Aphididae) consist of 2 mandibular and 2 maxillary stylets. Electron microscope studies of the stylets showed that each mandible is provided with a closed duct which contains a nerve. The maxillary stylets are provided with longitudinal grooves and interlocking ridges on their inner surfaces. The thickness of the maxillae and the arrangement of the inner ridges differ. The musculature of the 4 stylets consists of 4 protractors, 4 retractors, and there are 4 rotator muscles of the mandibles. All the stylets remain encased in the anterior part of the labium in a labial groove. The stylets are held firmly by the sphincter muscle on the distal conical tip of the labium. Two greenbug biotypes have been designated, A and B, based on differential reaction of small grain plants to their feeding. Biotype A refers to greenbugs that infested and damaged small grains for many years and to which resistant wheats were developed, but several years ago biotype A was supplanted by a new strain, biotype B, which destroys all previously resistant wheats. When the feeding habits of these 2 biotypes were studied, it was found that biotype A makes intercellular penetration of stylets in the plant tissues, and invariably feeds in the phloem tissues of the vascular bundles, whereas, biotype B stylets penetrate both intra- and intercellularly and preferentially feeds in the mesophyll parenchyma of the leaf. The nature and extent of injury to plant tissues also differs in the 2 cases. The damage caused by biotype B is much greater than that caused by biotype A.