Stylet penetration and feeding damage of Eupteryx melissae Curtis (Hemiptera, Cicadellidae) on sage

Abstract

The feeding of Eupteryx melissae Curt. (Cicadellidae) on the leaves of ssge gives rise to the appearance of chlorotic spots, as a result of damaged mesophyll cells becoming filled with air. The length of the mandibular stylets in this species (as averaged from the adult and five nymphal instars) is 67 per cent. of that of the maxillaries, both types of stylet being slightly larger in females. The mandibular stylets when inserted do not penetrate much below the leaf surface where they are held by barbs and a salivary sheath, and the total extent of their insertion is only 29 per cent. of that of the maxillary stylets. The latter are the main penetrating organs; the stylets may move together or independently and are not surrounded by a sheath. Mandibular and maxillary penetration is greatest in fifth-stage nymphs, the mean (and maximum) distances being 97 (205)μ and 177 (340)μ respectively. It increases progressively from the first nymphal stage, and in the adult shows mean values near those of nymphs in the third or fourth stage. Rate and depth of stylet penetration is very variable, due to repeated maxillary protraction and retraction, and there is little correlation between the lengths to which the mandibular and the maxillary stylets are extended. Exuvial stylets, particularly the mandibulars of fifth-stage numphs, consistently show a graeter degree of extruction than is to be found in nymphs when feeding normally: such deeper insertion may help to anchor the insect more firmly during ecdysis.Stylet insertion is by pressure, intracellular, and very rapid (attaining rates up to 115μ per minute). The sex of the individual insect and the leaf-surface attacked have no effect on depth of penetration, at least in certain nymphal stages, and all tissues of the lamina are available for feeding, even to first-stage nymphs, in consequence of the relatively greater development of the stylets in the younger insects. During penetration the mandibular stylets move alternately and initially lead, but quickly come to rest, and further penetration is by the maxillaries which follow a mainly intracellular path. Protrusion of one maxillary stylet beyond the other (up to a maximum distance of 112μ.) was observed in plant tissue and observations of feeding on stripped epidermis show that such a stylet can empty cells at its apex. Protoplasts are removed from a cell within a few seconds, and as this appears to be too short a time to allow digestion, it is assumed that chloroplasts must be mechanically fragmented on passage into the food canal. Removal may be a two-stage process, and if it is assumed that sometimes only the first stage is completed, and is accompanied by saliva injection, it is possible to explain the occurrence, as unimbibed residue, of plasmolysed degenerating chromophilic cells scattered throughout damaged tissue, which occurs mainly in the palisade and spongy parenchyma, and only seldom in vascular tissue. There is no evidence for diffusion of any salivary toxin from damaged cells or from the sheath. Stylet-tracks in damaged areas are unbranched and short and rarely give any indication of the cells fed upon or of the repeated probing by the maxillary stylets that is known to occur.