Resistance of Alfalfa to the Spotted Alfalfa Aphid12

Abstract

Single sterns with1 trifoliolate leaf from susceptible (C-Al) and resistant (C-84) alfalfa clones were placed in aqueous solutions of amino acids, sugars, or juices expressed from the 2 plants. After a day the trifoliolates were charged with adult Therioaphis maculata (Buckton) and insect behavior was noted for the succeeding 3 days. Most of the sugar and amino acid solutions were no different than water in their effects on the plants or insects. Hydroxyproline, leucine, and methionine were toxic. 4-Aminobutyric acid, cysteine, homoserine, proline, tyrosine, and melezitose solutions enabled the aphids to stay longer on the stems from the resistant clone than when these stems were placed in water. Clarified and autoclaved The resistance of certain alfalfa clones to infestation Juices from the 2 clones were infused into the excised stems of each clone. No juices from the resistant plants rendered the susceptible stems more resistant, thereby ruling out a soluble toxin or feeding deterrent as a mechanism for resistance. Juices from both the susceptible and resistant plants rendered the resistant cuttings more susceptible, thereby ruling out the presence of a selective nutrient in the susceptible plants. The results indicate that the alfalfa clone resistant to the spotted alfalfa aphid probably contains a level of some nutrients that is too low to support the aphid, and that is why the insect cannot live and reproduce on this plant.