Feeding behavior analysis of the soybean aphid (Hemiptera: Aphididae) on resistant soybean 'Dowling'

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an exotic pest of soybean, Glycine max (L.) Merr., in North America. Plant resistance to the soybean aphid was recently discovered in 'Dowling' soybean in 2005. Dowling exhibits strong antibiotic effects on the soybean aphid by limiting colonization and effectively controlling aphid population development during all soybean growth stages. In addition, aphids feeding on Dowling experienced significantly reduced survival, longevity, and fecundity and increased developmental times. An assessment of soybean aphid feeding behavior was compared on Dowling and the susceptible check 'Glenwood' by using the electrical penetration graph technique. The effects of plant resistance on aphid feeding behavior were analyzed by comparing the amount of time soybean aphid spent feeding on various plant tissues on resistant and susceptible plants. There was no significant difference in the initial time for the aphid to begin probing on resistant and susceptible plants indicating no adverse effect of plant exterior on the soybean aphid attempting to feed. However the time from the beginning of plant penetration by the stylets to the first phloem sieve element phase was significantly different. In aphids that successfully reached phloem on Dowling, the time taken to reach this phase was triple the time of aphids reaching the phloem sieve element on the susceptible line. In addition, the total number of probing events and the duration of nonprobing were not significantly different between Dowling and Glenwood. When comparing feeding between lines, a greater proportion of aphids showed ingestion of xylem when feeding on Dowling, whereas successful feeding on phloem on Dowling was significantly shorter than when feeding on the susceptible line. Therefore, the antibiotic effect of Dowling resistance seems to reside at the level of the phloem sieve element, prohibiting the aphid from successfully exploiting phloem, and as a consequence the aphid abandons the Dowling plant or dies of inadequate nutritional intake.