It has been suggested that salicylic acid (SA) is a signal in acquired resistance to pathogens in several plants. Also, it has been suggested that infestation of plants causes an increase in the activity of phenylalanine ammonia-lyase (PAL), a key phenolic biosynthesis enzyme. The purpose of this work was to investigate whether the induction of SA and PAL activity is related to the susceptibility of barley to aphid infestation. The induction of free and conjugated SA in two barley cultivars that differ in susceptibility to aphids was analyzed. Analyses of several physiological parameters showed that cv. UNA-80 was more susceptible to the aphid Schizaphis graminum than cv. LM-109. Salicylic acid was not detected in noninfested plants. Levels of free and conjugated SA in cv. LM-109 and of conjugated SA in cv. UNA-80 increased with aphid infestation, whereas the levels of free SA in cv. UNA-80 remained high under all infestation degrees. Maximum values reached in both cultivars were not significantly different. With respect to PAL activity, cv. LM-109 showed a significantly higher specific activity than cv. UNA-80, the more susceptible cultivar. The relationship between the susceptibility of a plant to aphid and SA induction and PAL activity is discussed.

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2,4‐Dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one (DIMBOA), the main hydroxamic acid isolated from maize extracts, increased the mortality of Schizaphis graminum when fed in artificial diets. Electrically‐monitored feeding assays showed that DIMBOA acted as a feeding deterrent at concentrations as low as 1 mM. On 12 mM DIMBOA diets, feeding by aphids was completely inhibited. Additional feeding experiments showed that when DIMBOA was ingested there was an increase in aphid mortality relative to that of aphids which did not ingest the compound. Thus, the deleterious effects of DIMBOA on aphids are due to feeding deterrency and toxicity. The 2‐ß‐0 glucoside of DIMBOA (DIMBOA‐Glc), the form in which DIMBOA naturally occurs in Gramineae, had a slight effect on lowering aphid survival and an appreciable feeding‐deterrent effect on diet‐fed aphids. The relevance of the effects of DIMBOA and DIMBOA‐Glc on aphids to resistance of certain graminaceous crops against aphids is discussed. RÉSUMÉ Toxicité et répulsion provoquées par des acides hydroxamiques de Gramineae dans des régimes synthétiques sur le puceron Schizaphis graminum La 2,4‐dihydroxy‐6‐metoxy‐1,4‐benzoxazin‐3‐one (DIMBOA), acide hydroxamique le plus abondant dans des extraits de maïs, accroǐt la mortalité du puceron Schizaphis graminum nourri artificiellement. Des essais d'alimentation contrǒlés par des moyens électriques démontrent que DIMBOA agit comme un répulsif, měme à la concentration de 1 mM; dans des régimes à 12 mM, l'alimentation des pucerons est supprimée. Des expériences complémentaires indiquent une augmentation de la mortalité des individus nourris avec DIMBOA par rapport aux témoins. Par conséquent, les effets nuisibles de DIMBOA sur les pucerons sont dus à sa répulsion. Le 2‐ß‐0‐glucoside de DIMBOA (DIMBOA‐Glc) a un effet important de répulsion sur les pucerons nourris artificiellement, mais affecte peu leur survie. L'importance des effets des DIMBOA et DIMBOA‐Glc sur les pucerons par rapport à la resistance des céréales contre ces insectes est discutée.