Stylet penetration activities by Aphis craccivora (Homoptera : Aphididae) on plants and excised plant parts of resistant and susceptible cultivars ofcowpea (Leguminosae)

Direct current electrical penetration graphs (DC-EPGs) were used to analyze the styler penetration activities of cowpea aphid, Aphis craccivora Koch, on plants of aphid-resistant (ICV-12) and aphid-susceptible (ICV-1) cultiva rs of cowpea, Vigna unguiculata (L.) Walpers. Aphid stylet penetration on w hole plants at seedling, flowering, and podding stages were studied in one experiment, and in another experiment excised leaves from seedling plants, excised flowers, and. excised polls were tested. Electrical signals depicti ng the aphid stylet penetration activities on their host plants were amplif ied, recorded onto a paper chart recorded, and scored for specific waveform patterns. Compared with similar tissues of ICV-1, intact leaves and excise d seedling foliage of ICV-12 plants caused severe disruption of aphid style r penetration activities. This was manifested in frequent penetration attem pts that were abruptly terminated or unsustained, and in shorter' penetrati on times, signifying antixenosis resistance in ICV-12. There was reduced oc currence of E waveforms, which represent stylet activity in plant vascular tissues. Also, prior. exposure of lest aphids to plants of one cultivar did not significantly influence the expected stylet penetration activities on plants of the other cultivar. Overall, ICV-12 exhibited high levels of resi stance against A. craccivora.