Phloem aphids

Dreyer, D. L., Jones, . C. and Molyneux, R. L. (1985). Feeding deterrency of some pyrrolizidine, indolizidine, and quinolizidine alkaloids towards pea aphid (Acryrthosiphon pisum) and evidence of phloem transport of indolizidine alkaloid swainsonine. Journal of Chemical Ecology 11 1045-1051. 

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]. 

Aphids extract sap from plant stems, specifically the phloem tissue. They excrete honeydew which still contains plant sugars. Ants collect this honeydew, often by milking the aphids, and use it as food. In return, they protect the aphids against predators. Ants prey on predators of aphids such as ladybird beetles (coccinellids), thus defending the aphids. Ants also shelter aphids by taking them or their eggs into their nests during inclement seasons. In a sense, ants herd aphids like cows. Furthermore, without removal of honeydew, aphid colonies become fouled. 

Adult grass grubs (Costelytra zealandica)[209] and phloem feeding green peach aphids (Myzus persicae)[210] were observed to die after feeding on Ricinus communis 76 was identified as the responsible toxin. The presence of 76 in the phloem of R. communis indicated a possible role for the toxin in defense of the plant against aphids [210]. 

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. 

Phloem sap may be obtained by a variety of methods ranging from the use of aphid stylets to the cutting of vascular tissue. Recent techniques have involved the use of EDTA to prevent the resealing of phloem elements (King and Zeevaart, 1974) and successive cutting of the pod tip of Lupinus albus (Pate et al., 1974) and Glycine max (Fellows et al., 1978). A summary of some... 

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-... 

Insects such as lygaeid bugs fed on Nerium oleander also accumulate cardiac glycosides, as does a bright yellow aphid, Aphis nerii (Rothschild, 1973). The sugar-rich secretion of these aphids, called honeydew, contains cardenolides found in the host plant (Molyneux et al., 1990). The ability of this aphid to acquire these compounds suggests that they are found in the phloem of Nerium oleander plants (Molyneux et al., 1990). 

Wink, M., T. Hartmann, L. Witte, and J. Rheinheimer, Interrelationship between quinolizidine alkaloid producing legumes and infesting insects Exploitation of the alkaloid-containing phloem sap of Cytissus scoparius by the broom aphid Aphis cytisorum, Z. Naturforsch., 37c, 1081-1086 (1982). 

Basically two lines have been followed in the search for the pathways of hormone translocation (1) surgical methods and (2) autoradiographic methods which have come with the advent of radioactive hormones. Phloem-feeding aphids have been used as a speciality to detect translocation in sieve elements (for references see p lOOf.). 

Table 3.1. Evidence for genuine phytohormones in root exudates (Aylem sap) and phloem sap (partly aphid honey dew) in herbaceous and woody plants... 

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