Contributions to the Description of the Instars of the Planthopper <i>Hyalesthes obsoletus</i> (Fulgoromorpha, Hemiptera)

Hyalesthes obsoletus, a planthopper of the family Cixiidae, is a very efficient vector of the phytopathogenic bacterium ‘Candidatus Phytoplasma solani’, which causes diseases in a variety of crops, including grapevines. The larvae are oligophagous and infect their host plants, in Central Europe mainly Convolvulus arvensis and Urtica dioica, the polyphagous adults transmit the pathogen both to the hosts of the larvae and to the crop plants, which are dead-end hosts in the infection process. In this study, characteristics of the larval stages are analysed comparatively. Body length, length of the antenna, the labium and the metathoracic limb were determined for the five instars and conclusions drawn about changes in proportions. Furthermore, the morphology of the antennae was described, the development of the eye primordium, the distribution of the sensory pits on the pro-, meso- and metanotum, the alteration of the tarsi and other features of the hind legs (‘gear-teeth’ as in Issus are present) and the structure of the wax plates of the abdomen during individual development. The characteristics of these features were summarised in a table and allow a reliable instar classification. The easiest way to do this is with the aid of the wax plate, which is divided mediolaterally into several sections by sensory pits. This results in a simple relation: Instar = number of wax plate sections – 1. The results are compared in particular with the description of the larval stages by Sforza et al. 1999. Larval development is relevant for the beginning of the flight period and thus also for the start of pathogen spread in the course of the year.