The probing behaviors of two leafhoppers, Empoasca fabae (Harris) and E. kraemeri (Ross & Moore), feeding on susceptible ('Porrillo Sintetico') and resistant ('EMP-84') common beans, Phaseolus vulgaris L., were studied using an AC electronic monitoring system (EMS) (McLean & Kinsey 1964). The behaviors of the two species were generally similar. For both species, three probing behaviors were dominant in duration and number of probes: (1) multiple-cell laceration probing, represented by the I(a) waveform, (2) single-cell puncture probing, I(b), and (3) still-stylet ingestion, I(c). On the susceptible cultivar, multiple-cell laceration was dominant for both species, in duration and number of probes. Both species made more probes, all of medium to long duration, in interveinal areas. When feeding on veins, they chose larger over smaller veins and made fewer probes (all of short duration). However, E. kraemeri made significantly more, short-duration laceration probes than did E. fabae. When feeding on the resistant and susceptible cultivars was compared, there was no significant difference in the duration of probing by an average insect in either species. Yet, there were significant differences in types of probing behavior exhibited on the two hosts, primarily because of a decrease in the number of short-duration probes containing multiple-cell laceration probing on the resistant cultivar, 'EMP-84'. There was also a proportionate increase in the number of longer probes containing single-cell puncture probing and still-stylet ingestion. Fewer mechanical punctures were made and, possibly, less saliva was injected into 'EMP-84' than 'Porrillo Sintetico' plants. Thus, on 'EMP-84', a switch in behavior occurred from multiple-cell laceration probing, which was more abundant on the susceptible cultivar and may be more damaging, to still-stylet ingestion or single-cell puncture probing, currently thought to be less damaging probing styles. The results show that Empoasca spp. leafhoppers did not reject the resistant plant, nor to our knowledge, was the resistant plant itself tolerant to the effects of feeding in the customary sense. Instead, 'EMP-84' may exhibit-a new mechanism of tolerance, wherein the attacking leafhoppers were stimulated to feed in a less damaging manner.
