COMMITTED PHLOEM INGESTION OF APHIDS (HOMOPTERA, APHIDIDAE) - DIFFICULTIES IN ITS APPLICATION TO HOST-PLANT RESISTANCE STUDIES

The application of the concept of committed phloem ingestion (CPI) of aphids was studied for different aphid species feeding on resistant an d susceptible plants. Greenbugs, Schizaphis graminum (Rondani) (biotyp e E), were monitored electronically on wheat, Triticum aestivum L., an d Sorghum bicolor (L.) Moench, cultivars, and Russian wheat aphids, Di uraphis noxia Mordvilko, were monitored on wheat cultivars. The durati on of phloem ingestion (PI) events of these aphids was not randomly di stributed and almost-equal-to 53% of them were <60 min in duration. Th e minimum PI duration to be considered as committed feeding (threshold of CPI) was determined for each aphid-host combination. Thresholds va ried depending on the aphid species and host plants, indicating the di fficulty of assigning a single threshold value that could be useful as a measure of host acceptability. Also, if a given threshold duration is chosen as a criterion for having achieved CPI (e.g., 15 min), the p roportion of PI events that continues for a long time period varies fr om host to host. Therefore, we recommend that use of this parameter be discontinued, and that measurements of the time from the beginning of the experiment to PI and the time from the initiation of a probe to P I be substituted for time to first CPI.