Stylets, insects

The apparent orientation of insect stylets to the vascular bundles caused by hydrogen ion concentrations was first reported by Fife and Frampton ( ). The leafhoppers fed mainly in the phloem tissue, which had a substantially higher pH than the surrounding plant cells. Other discriminating criteria for phloem-feeding insects have been indicated, including carbohydrate concentration (10) and positive hydrostatic pressure (11). 

Hyperdermium bertonii is believed first to infect and necrophyhze a scale insect that has parasitized the plant host. Afterward, the fungus acquires nutrients that leak from the plant s phloem tissues through the stylet wound (Sullivan et al., 2000). 

Aphids have flexible, stylet-like mouthparts adapted for probing of plant tissues. By this means, the aphid must use chemical cues from the plant to determine if the plant is a suitable host (host plant quality), where the aphid is on the plant, the location of the stylet within the plant tissues, and the direction to probe to locate the plant phloem (Campbell and Dreyer, 1990 Dreyer and Campbell, 1987). Aphids avoid many of the toxic compounds stored in plant cells by probing between the cell walls. These insects are able to penetrate the intercellular spaces by producing a variety of digestive enzymes which depolymerize the pectins and hemicelluloses forming the intercellular-cell wall matrix. Aphid-host plant compatibility is associated to the extent that these enzymes depolymerize their respective substrates, and a reduced rate of depolymerization is often associated with host plant resistance. Differences in methoxylation, acetylation, or neutral sugar composition in the matrix polysaccharides are often involved in this reduced depolymerization. Further, specific breakdown products from depolymeri-... 

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