Date of this Version
This review concerns the relationship between chemical components found in sweetclover [Melilotus officinalis (L.) Pall. and M. alba Desr.] tissues and insects feeding on these tissues. Coumarin and its related compounds did not appear to influence the feeding of sweetclover aphids [Therioaphis riehmi [Börner)], pea aphids [Acyrthosiphum pisum (Harris)] or potato leafhoppers [Empoasca fabae (Harris)] , however, these materials appeared attractive to sweetclover weevils (Sitona cylindricollis Fåhraeus) and to adult sweetclover root borers [Walshia miscecolorella (Chambers)]. Conversely the feeding of four species of blister beetles (Epicauta sp.] was inhibited by cis-o-hydroxycinnamic acid glucoside and coumarin, but was not affected by trans-o-hydroxycinnamic acid glucoside. An artificial method of feeding sweetclover weevils was used to bioassay substances extracted from sweetclover. Three classes of feeding stimulants were found: (i) three sugars (sucrose, glucose, and fructose); (ii) a chloroform-soluble stimulant(s) (possibly a lipid); and (iii) adenosine. Two feeding deterrents were found. Deterrent "A" was partially characterized as a substituted phenolic compound, and deterrent "B" was identified as the nitrate ion. The relative concentrations of the stimulant and deterrent substances in the resistant plants were found to change as the plant matured. Thus, no one substance is responsible for resistance throughout the life of the plant. In seedlings of resistant plants, the nitrate ion plays the principal role in resistance to weevil feeding, but in more mature plants resistance seems to result from the relative amounts of other deterrents and stimulants. It may be possible to use this knowledge to screen for resistance without using insects or to protect the plants artificially by increasing levels of deterring substances, such as nitrate ion, by means of fertilizer applications.