|Chu, Chang Chi|
Submitted to: Bemisia International Workshop Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 7/31/2006
Publication Date: 12/3/2006
Citation: Freeman, T.P., Buckner, J.S., Chu, C., Payne, S.A., Moore, J.A. 2006. Ultrastructural characteristics of Bemisia adult and nymph feeding [abstract]. Bemisia International Workshop Proceedings. p. 38-39.
Technical Abstract: Included among the more than 1200 known species of whiteflies are important agricultural pests that can cause significant economic loss for a wide variety of field and horticultural crops. Damage to host plants can be a direct result of obligate phloem feeding, whitefly transmission of virus or non-viral plant disorders, or fungal growth associated with the presence of honeydew. Both adults and nymphs feed on the abaxial surfaces of leaves, and host-plant selection affects egg-hatch success and nymph survival. Important factors associated with successful feeding include stylet length and salivary sheath development. The mouthparts of adult whiteflies are typical of the Homoptera. Whiteflies have a triangular labrum covered with hairs, and a four-segmented labium. The first segment is interpreted as an extension of the thorax. Segments two, three, and four are characterized by a deep labial groove. The stylet bundle enters the labial groove between the first and second labial segments. When the insect is not feeding, the stylet is completely contained within the labium. The stylet bundle consists of two external mandibular stylets and two interlocking maxillary stylets forming separate food and salivary canals. Mandibular and maxillary protractor muscles probably play a role in directional movements of the stylets and perhaps have a limited role in stylet advancement. Adult feeding sites occur both along the margins and through the central portions of leaf abaxial epidermal cells. Both initial probe sites and successful feeding sites can be identified easily by the presence of a glue-like deposit where the tip of the labium was anchored to the host plant epidermis. The mechanical force required for both stylet insertion and withdrawal results from changes in the position of the whitefly head in relation to the anchored labium during feeding. The total length of the stylets and the portion of the stylets inserted into the host plant leaf can be measured easily by examining the position where the stylet bundle is inserted into the labial sheath. During penetration of the host leaf, the whitefly forms a salivary sheath. This sheath is formed by the production of a small bead of material that emanates from the stylet tip, hardens, and is then penetrated by the stylet bundle. Additional beads are formed, giving the sheath a segmented or beaded appearance. The salivary sheath leaves a permanent record of stylet travel from the epidermis to the phloem tissue. Stylets usually do not penetrate directly from the epidermis to the phloem tissue, and salivary sheaths are frequently highly branched as they wind their way through the intercellular spaces of the leaf mesophyll. Branching characteristics of both adult and nymph salivary sheaths provide clear evidence that the stylets can be partially withdrawn and reinserted. Nymphs feed during all developmental instars. The stylets of early nymphal stages are generally shorter that those of adults but they are still long enough to reach the phloem tissue of the host plant. Although the stylets are longer with each new instar, they do not lengthen in direct proportion to the increase in body length. The rostrum on the nymph is short and has a groove similar to the groove on the adult labium through which the stylet bundle slides. In the nymph, unlike the adult whitefly, it is difficult to determine the portion of the stylet used in feeding, as the rostrum and stylet insertion point are below the nymph. Stylets found in all exuviae confirm that the stylets are withdrawn prior to molting and that new stylets must be formed and inserted following the molt. Photographs obtained using light microscopy, confocal microscopy, scanning electron microscopy and transmission electron microscopy will be presented to document whitefly adult and nymph feeding characteristics including