Donald L. McLean

Pierce's Disease Bacterium: Mechanism of Transmission by Leafhopper Vectors.

The bacterium that causes Pierces disease of grapevines is isolated most consistently from the foregut of its leafhopper vector Graphocephala atropunctata. As seen in light and scanning electron microscopy of infective leafhoppers, the bacteria are attached to the cibarial pump and the lining of the esophagus in the foregut where they appear to multiply. These findings suggest that the bacterium is transmitted from the foregut by egestion during feeding by infective leafloppers.

PROBING BEHAVIOR OF THE GREENBUG (SCHIZAPHIS GRAMINUM, BIOTYPE C) ON RESISTANT AND SUSCEPTIBLE VARIETIES OF SORGHUM

The probing behavior of biotype C of the greenbug, Schizaphis graminum (Rondani) on susceptible and resistant lines of sorghum, Sorghum bicolor (L.) Moench., was electronically monitored. Waveforms corresponding to salivation, phloem ingestion and non‐phloem ingestion are described. The results of a parallel study revealed that the rate of population growth of S. graminum was significantly greater on susceptible lines of sorghum [i.e., NC + 70X. SC423 (Purple). SC423 (Tan)] than on resistant lines (i.e., TAM 2567, IS 809). Aphids probing the resistant lines of sorghum showed a significantly reduced imbibition of phloem sap compared with those aphids which fed on susceptible varieties. Also, increased numbers of separate probes and increased duration of non‐probing were associated with greenbugs feeding on resistant lines. Greenbugs monitored on the nonhost plant, rice, exhibited non‐phloem ingestion, but not phloem ingestion.

The sensory systems and feeding behavior of leafhoppers. I. The aster leafhopper, Macrosteles fascifrons stål (homoptera, cicadellidae)

The ultrastructure of the sensilla, and other structures, within the stylets and precibarium of Macrosteles fascifrons were examined by transmission and scanning electron microscopy. Precibarium is a new term, defined here, for the canal that precedes the cibarium inside the leafhopper head. Within the precibarium are found 20 chemosensilla and a previously undescribed structure, the precibarial valve. Twelve mechanosensilla, three in each stylet, are found within the maxillary and mandibular stylets. The relationship between all of these structures and feeding by the insect is detailed in a feeding mechanism hypothesis. It is concluded that leafhoppers (and probably all homopterans) utilize the precibarial chemosensilla alone for gustatory discrimination, the stylet sensilla for proprioception, and the precibarial valve for regulation of fluid uptake and compartmentalization of the sensilla.

The sensory systems and feeding behavior of leafhoppers. II. A comparison of the sensillar morphologies of several species (Homoptera: Cicadellidae)

The ultrastructure of the sensilla, and other structures, within the precibaria of eight species from three subfamilies of leafhoppers (Homoptera: Cicadellidae) were examined with scanning electron microscopy. The types and grouping of the 20 precibarial sensilla in seven of these species were similar to those observed previously in Macrosteles fascifrons Stål. Oncometopia nigricans (Walker) also displayed similar sensilla groups; however, it had 30 sensilla. The species examined differed chiefly in the exact location and arrangement of the sensilla. The possible significance of the differences relative to leafhopper feeding is discussed. The precibarial chemosensilla may provide chemosensory evaluation of fluid in the food canal and precibarium prior to ingestion or egestion.

Behavioral evidence that the precibarial sensilla of leafhoppers are chemosensory and function in host discrimination

Leafhoppers (Homoptera: Cicadellidae) possess epi‐ and hypopharyngeal chemosensilla within the head. In this paper, we describe the behaviors of leafhoppers (Graphocephala atropunctata Signoret) which have had two of the four main nerves to these sensilla cut. Insects with severed nerves (treated) and control insects (both sham‐operated and normal) were offered a choice of two small mustard leaves infused with either distilled water or a 5% sucrose solution. The control insects showed a preference for the 5% sucrose leaf, whereas the treated leafhoppers did not; they distributed themselves on both leaves with similar frequency. Thus, severing the nerves of only half of the precibarial chemosensilla (epipharyngeal organ) resulted in major changes in leafhopper feeding behavior. This evidence supports the hypothesis that the precibarial chemosensilla mediate gustatory discrimination of chemical compounds within the plant.

Anterior alimentary canal of the pear psylla, Psylla pyricola Foerster (Homoptera, Psyllidae)

Scanning and light microscopy investigations of the anterior alimentary canal of the pear psylla, Psylla pyricola Foerster (Homoptera: Psyllidae), revealed the morphology of the labium and stylets, as well as the presence of sensory structures and a valve in the precibarium. The labium consists of three telescoping segments with an internal labial groove, which surrounds and supports the stylet bundle. Also a part of the labial groove is the internal labial clamp. The stylet bundle is comprised of paired styliform mandibles and maxillae, which interlock to form the food and salivary canals. The stylet bundle proximal to the labium forms a large loop within a membranous crumena. When fully retracted the coiled stylets are under tension. Stylet extension generates increasing tension so that when retracted the stylets readily recoil within the crumena. Penetration of leaf tissues by the stylet bundle is dependent on the interaction between stylet muscles, opening and closing of the labial clamp, the barbed stylet tips, and the ventral position of the labium.

Electronic measurement of probing activities of the green leafhopper of rice.

The electronic measurement system developed for aphids has been modified to measure leafhopper salivation and ingestion activities. Several distinct waveforms were recorded when the rice green leafhopper, Nephotettix cincticeps (Uhler) probed rice plants and into sucrose solutions. These waveforms were correlated with ingestion and salivation activities based on chemical analysis of the excreta.

Isolation of the primary intracellular symbiote of the pea aphid, Acyrthosiphon pisum

Abstract A method for isolating the primary intracellular symbiote of the pea aphid, Acyrthosiphon pisum, is presented. The purity of the preparation, based on an area symbiote to area contaminant and a number to number basis, averages 93%.

CHAPTER 11 – AN ELECTRICAL MEASUREMENT SYSTEM FOR STUDYING APHID PROBING BEHAVIOR

Publisher Summary This chapter describes the electrical measurement system for studying aphid probing behavior. Electrical measurement systems of one type or another are used in virtually every biological research laboratory in the world today. Electronics has enabled biologists to gather more accurate research data and to analyze these data with speed and confidence. Most of the electrical measurement systems come in prepackaged containers, complete with instructions for operation. Two general types of electrical measurement systems are used by biologists today: (1) direct-coupled system and (2) biotelemetric system. In the direct-coupled system, biological signals are generally transduced or converted into electrical signals that are then transmitted through the system via wires to some type of display device. There are a number of different kinds of transducers, including pressure, inductor, capacitor, and resistor types. In the biotelemetric measurement system, the desired signals are transmitted through the air from a transmitter to a receiver. The electrical measurement system used to collect data on the probing activities of aphids is an example of a direct-coupled measurement system. The aphid-activity measurement system must have a solid reference point or ground. All grounded portions of the system must terminate at a conductive metal rod driven into the earth.