: Pymetrozine, a pyridine-azomethine, is a selective biocide against aphids, whiteÑies and plant hoppers with a high plant systemic activity. By means of bioassays and autoradiographic techniques it has been shown that this systemic behaviour originates not only from xylem but also from phloem mobility. After foliar application the growing points of plants are protected by pymetrozine imports mainly from leaves. This indicates a high importance of phloem mobility for the systemic activity of pymetrozine against plant-sucking insects.

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:The inÑuence of plant factors on the efficacy of pymetrozine (CGA 215 944), an insecticide against homopteran pests, was investigated. Among all parameters tested, plant species was found to inÑuence pymetrozine activity most. Di †erences in the compartmentation of pymetrozine within tissues of different plant species were found : 33% of extractable compound was measured in the intercellular space of tomato leaves, but only 5% in the same compartment of sugar beet leaves. Foliar uptake after spray application was around 5% in both plant species under a variety of conditions. The persistence of foliar-applied pymetrozine is comparable on tomato and on sugar beet plants. Following drench application, efficacy correlated with plant age and leaf development stage, but not with soil type. In conclusion, the variations in pymetrozine efficacy observed on tomato, sugar beet and pea against Myzus persicae (Sulzer) and Aphis craccivora (Koch) may be attributed to di †erences of pymetrozine distribution within leaf tissues and/or host plant speciÐc feeding behaviour of aphids.

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