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The probing behavior of biotype C of the greenbug, Schizaphis graminum (Rondani) on susceptible and resistant lines of sorghum, Sorghum bicolor (L.) Moench., was electronically monitored. Waveforms corresponding to salivation, phloem ingestion and nonphloem ingestion are described. The results of a parallel study revealed that the rate of population growth of S. graminum was significantly greater on susceptible lines of sorghum [i.e.. NC + 7OX. SC423 (Purple). SC423 (Tan) I than on resistant lines (i.e.. TAM 2567, IS 809). Aphids probing the resistant lines of sorghum showed a significantly reduced imbibition of phloem sap compared with those aphids which fed on susceptible varieties. Also. increased numbers of separate probes and increased duration of nonprobing were associated with greenbugs feeding on resistant lines. Greenbugs monitored on the nonhost plant, rice, exhibited non-phloem ingestion, but not phloem ingestion.Resistance in sorghum to greenbugs is associated with the phloem. There appear to be no physiai differences between the sorghum varieties in the location of or extent of lignification around the vascular bundles. Hence, resistance probably involves plant natural products associated with the phloem sap.

The effect of the tomato resistance gene, Meu1, on feeding, longevity, fecundity and developmental rate of the pink biotype of the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera, Aphididae), was determined using nearly isogenic tomato (Lycopersicon esculentum Mill, Solanaceae) lines. Aphid mortality was significantly higher on resistant plants, with 60% of the aphids dying by the 4th day of exposure. By the 10th day, all the aphids on the resistant plants were dead whereas 100% of the aphids on susceptible plants were alive. Meu1‐mediated resistance resulted in significantly decreased fecundity with a ten‐fold decrease in the net fertility rate (4.5 and 45.7 progeny per aphid on resistant and susceptible tomato, respectively). A qualitative analysis showed that honeydew was produced by aphids on resistant and susceptible plants, suggesting that aphids initiate feeding on both plant types. However, significantly lower quantities of honeydew were present when aphids were caged on resistant plants. There were also significant differences in aphid location on resistant and susceptible leaves. Experiments evaluating behavior in less than 24 h showed that aphids left resistant leaves after relatively short exposure (3–6 h). Aphids transferred from resistant to susceptible tomato at intervals between 3 h and 24 h resumed feeding as evidenced by presence of honeydew. Although the mechanism by which Meu1‐mediated resistance operates is not yet known, our data suggest that resistance factors act rapidly after initiation of feeding and that lower fecundity and longevity are related to reduction in aphid feeding.