Phloem feeding 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). 

The objectives of this paper are broad. Our first objective is to describe the primary mechanism of action of usnic acid on plants as ascertained by our laboratory.33 A second objective is to describe the phytotoxic activity of selected lichen anthraquinone analogues. In addition to the phytotoxic activity, we describe the effects of these secondary metabolites on phloem-feeding insects. Finally, we provide a hypothesis to explain the functional roles of these metabolites in the ecosystem. 

The transport of toxic alkaloids in the phloem can be an advantage for plants against phloem-feeding insects, such as aphids [8], For example alkaloid-rich lupins are avoided by aphids, whereas sweet lupins with very low alkaloid contents are preferred by polyphagous aphids [8,22]. 

Thompson GA, Goggin FL (2006) Transcriptomics and functional genomics of plant defence induction by phloem feeding insects. J Exp Bot 57 755-766... 

Although elicitors from insect oral secretions have received special attention, the quality and quantity of HIPVs may also be affected by insect feeding behavior such as continuous or interrupted leaf chewing, phloem sucking, and even egg deposition.193,194 Recently, it was shown that continuous mechanical wounding was sufficient to induce local as well as systemic emission of volatiles that are emitted as HIPVs.195 Thus, the role of insect elicitors in HIPV production still remains largely an open question. 

The results of these studies examine the difference in the uptake dynamics of bioavaUable inorganic elements from soils into the nectar of flowers versus into the phloem of coniferous trees and passage through the hindgut of aphids into honeydew. The honey from meadow plants would have been obtained by the bees primarily from blossoms. Another insect feeding on coniferous trees produced the honeydew that was then stolen by the bees. 

Penetration of the leaf surface is often not necessarily important to control insects that chew, such as Lepidoptera, but penetration is needed for insects that feed inside the leave such as a leafininer (Liriomyzra trifolii) and the tomato pinworm Keiferia lycopersicelld). Penetration is also needed for compounds that have limited photo-stability such as imidacloprid. Once compounds have penetrated the tissue of the leaf, if they have the appropriate physical properties, can move into xylem and/or phloem systems, otherwise they will remain located at their application site. 

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