FORUM Committed Phloem Ingestion of Aphids (Homoptera: Aphididae): Difficulties in Its Application to Host-Plant Resistance Studies MELAKU GIRMA,l GERALD E. WILDE,l JOHN C. REESE,l AND ELAINE A. BACKUS2 Environ.Entomol. 23(5): 1061-1065 (1994) ABSTRACT The application of the concept of committed phloem ingestion (CPI) of aphids was studied for different aphid species feeding on resistant and susceptible plants. Greenbugs, Schizaphis graminum (Rondani) (biotype E), were monitored electronically on wheat, Triticum aestivum L., and Sorghum bicolor (L.) Moench, cultivars, and Russian wheat aphids, Diuraphis noxia Mordvilko, were monitored on wheat cultivars. The dura- tion of phloem ingestion (PI) events of these aphids was not randomly distributed and "'53% of them were <60 min in duration. The minimum PI duration to be considered as committed feeding (threshold of CPI) was determined for each aphid-host combination. Thresholds varied depending on the aphid species and host plants, indicating the difficulty ofassigning 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 proportion of PI events that continues for a long time period varies from host to host. Therefore, we recommend that use of this parameter be discontinued, and that measurements ofthe time from the beginning of the experiment to PI and the time from the initiation of a probe to PI be substituted for time to first CPI. KEY WORDS aphids, probing behavior, electronic monitor COMMITTEDPHLOEMINGESTION(CPI) has been a widely used variable in the quantification of feeding behavior of aphids. It was first intro- duced by Montllor et al. (1983). They chose 15 min of PI as a threshold for an aphid commit- ment to long-term feeding because "an aphid which ingested from the phloem continuously for at least 15 min tended to remain feeding in phloem for up to several hours." Subsequently, this threshold has been used to estimate other feeding parameters such as time to the first CPI (Montllor et al. 1983, Ryan et al. 1987) and po- tential phloem ingestion index (PPII) (Montllor et al. 1983, Girma et al. 1992) as measures of host acceptability. Unfortunately, publications have not always made it clear whether CPI was mea- sured from the beginning of the experiment or the beginning of the probe, also known as pene- tration. Time to the first CPI refers to the time taken for an insect to reach a phloem sieve ele- ment and ingest its sap for at least 15 min. Time to the first x-wave, or phloem contact, measures only the relative difficulty in penetrating plant tissue en route to the phloem. However, time to the first CPI includes both plant morphological and physiological factors that affect aphids be- 1 DepartmentofEntomology, KansasStateUniversity,Man- hattan,KS66506-4004. 2 Department of Entomology, University of Missouri- Columbia,Columbia,MO65211. fore they reach the phloem as well as sap quality while ingesting from the phloem. The CPI parameter has been used widely with greenbugs, Schizaphis graminum (Rondani), feeding on sorghum, Sorghum bicolor (L.) Moench (MontHor 1985, MontHor et aI. 1983); on wheat, Triticum aestivum L. (Niassy et al. 1987, Ryan et al. 1987); on barley, Hordeum vulgare L. (Peters et al. 1988); on corn, Zea mays L. (Mc- Cauley et al. 1990) and on oats, Avena sativa L. (MontlIor & Gildow 1986). It also has been used in feeding behavior studies of other aphids, in- cluding the hop aphid, Phorodon humuli (Schrank) (Dorschner & Baird 1989); Russian wheat aphid, Diuraphis noxia Mordvilko (Girma et al. 1992); and bird cherry-oat aphid, Rhopal- osiphum padi (L.) (Montllor & Gildow 1986). Although CPI has been used by many research- ers, its 15-min threshold value has not been quantified using experimental data. MontUor (1991) and Backus (1994) have suggested several interpretational problems with the concept. However, until our work, no one has attempted to test CPI quantitatively. Dorschner & Baird (1989) assigned a posteriori a 60-min CPI thresh- old to the ingestion of the hop aphid. Studies by Girma et al. (1992) and Montllor (1991) showed' that the threshold for CPI can vary among aphid species and even among individuals within spe- cies as they feed on different hosts. The actual threshold value researchers use to determine 0046-225X19411061-1065$02.00/0 © 1994 Entomological SocietyofAmerica Downloaded from https://academic.oup.com/ee/article/23/5/1061/2394657 by guest on 26 February 2021