The contents of long-chain fatty acids in non-polar root extracts from 1.6 year-old red clover plants of two cultivars (Superqueli INIA and Quiñequeli INIA) and one experimental line (Syn II Pre III), their relationships with H. obscurus populations, and the stimulant or deterrent effects of the root extracts and of individual fatty acids on the weight gain of H. obscurus were investigated. There was an inverse relationship between the insect population size and the palmitic acid content. The Superqueli INIA cultivar contained the highest palmitic acid content and the lowest insect load. When solutions of either palmitic or oleic acids were individually incorporated into an artificial diet, palmitic acid caused a decrease in weight gain, whereas the effect of oleic acid was similar to that of the phagostimulant saccharose. Hence, palmitic acid might be applied directly in H. obscurus control programs to prevent or reduce feeding and oviposition behaviors or might be used as a tool for selecting resistant cultivars.