Aphids, phloem feeding

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]. 

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]. 

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.). 

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 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. 

---