Probing behavior of Diuraphis noxia (Mordvilko), Russian wheat aphid, and Rhopalosiphum maidis (Fitch), corn leaf aphid, was measured on barley lines resistant and susceptible to D. noxia grown under low and high soil moisture. R. maidis reproduces similarly on both barley lines. Probing behavior was interpreted from waveforms of an alternating current electrical penetration graph (AC EPG) system of the Oklahoma design during 6-h monitoring periods. Significant effects were observed, particularly the aphid species by barley line interaction. Averaging across moisture levels, D. noxia took longer to first enter sieve element phase when probing D. noxia-resistant ‘STARS-9301B’ (306 ± 19.9 min [mean ± SEM]) than when probing susceptible ‘Morex’ (180 ± 21.6 min). In contrast, R. maidis relatively quickly entered sieve element phase on the two barley lines (average of 132 ± 13.7 min), with no detectable difference between lines. When measuring the total duration of sieve element phases, the stylets of D. noxia were in contact with phloem sieve elements of STARS-9301B for a shorter period (27 ± 10 min) than with sieve elements of Morex (111 ± 21 min). In contrast, stylets of R. maidis were in contact with sieve elements of the two barley lines for similar time periods (average of 176 ± 15.8 min). Any mediating effect of soil moisture was slight, if at all measurable, using the AC EPG system, making any interpretation of probing behavior relative to previous observations of aphid population growth affected by plant water stress untenable. In contrast, monitoring probing behavior was beneficial in assessing how plant resistance may affect aphid species differently. The differences in probing behaviors between the two aphids fed barley resistant and susceptible to D. noxia corresponded well with previous observations that D. noxia population growth was lower on D. noxia-resistant barley than on susceptible barley, while R. maidis population growth was similar on the two barley lines.
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