Observations on Leafhopper Ingestion-Egestion Behavior: Its Likely Role in the Transmission of Noncirculative Viruses and Other Plant Pathogens12

Abstract

Descriptions of two membrane-feeding systems for direct observation of leafhopper probing and feeding behavior are presented. One system is for use with modified compound microscopes, the other with standard stereomicroscopes. Basically, leaf-hoppers within an insect chamber are allowed to probe through a parafilm membrane into a feeding chamber containing diet comprised of distilled water with 5% sucrose to stimulate ingestion, methylene blue to stain the salivary sheath, and India ink particles to indicate flow of solution into or out of the maxillary food canal of the stylet bundle. Preprobing activities, flange formation, stylet penetration, sheath formation, ingestion, and egestion are reported as they were observed at high magnification from the diet side of the membrane. Detailed observations of the membrane feeding behavior of leafhoppers, especially the aster leafhopper, Macrosteles fascifrons (Stal), revealed that these insects usually egest material from the foregut (maxillary food canal, cibarial sucking pump, pharynx, and esophagus) one or more times during feeding. Initial periods of egestion are nearly always preceded by periods of prolonged ingestion. Periods of intermittent egestion sometimes lasted for as long as 10 min, and insects often egested shortly before terminating probes. When egestion occurs, materials flow out of the stylet food canal in the same steady manner in which they enter it during ingestion, indicating that the sucking pump is able to function normally in either direction. At times, egestion obviously served to free the sheath and food canal openings of blocking particles. Evidence of leafhopper egestion, along with very recent electron microscopical observations of virus retention sites in the foreguts of leafhopper vectors, is interpreted as confirming the previously proposed hypothesis that leafhoppers transmit semipersistent viruses, such as maize chlorotic dwarf virus and rice tungro virus(es), and other noncirculative plant pathogenic pathogens via an ingestion-egestion mechanism.