Variation in aphid alarm pheromone content among glandular and eglandular-hairedMedicago accessions

Abstract

Pea (Acyrthosiphon pisum Harris) and blue alfalfa aphid (A. kondoi Shinji) deterrency in alfalfa (Medicago saliva L.) may result from incorporating higher levels of the aphid alarm pheromone,(E)-β-farnesene relative to(E)-β-caryophyllene. We evaluated five eglandular and two glandular-haired alfalfa accessions for differences in(E)-β-farnesene and(E)-β-caryophyllene content under glasshouse conditions using supercritical fluid extraction and gas chromatography. In addition, pea and blue alfalfa aphid olfactory behavioral tests were conducted uponMedicago species containing different ratios of(E)-β-famesene relative to(E)-β-caryophyllene. No differences in(E)-β-caryophyllene content were observed among the seven entries (μ=0.42 ng/g plant material). Significant differences (P ≤ 0.05) among entries were observed for(E)-β-famesene content, with KS94GH6 exhibiting the highest (1.18 ng/g), and CUF 101 the lowest levels (0.35 ng/g). Elite tetraploid sources possessed significantly lower levels (μ=0.42 ng/g) of(E)-β-farnesene than did wild and cultivated diploid accessions (μ=0.83 ng/g). Olfactory behavioral tests for both the pea and blue alfalfa aphids demonstrated KS94GH6 repelled aphids while cultivated alfalfa types attracted aphids in each case. Previously demonstrated aphid resistance in diploid KS94GH6 may result from superior(E)-β- farnesene levels, but(E)-β-farnesene is probably not a factor in cultivated alfalfa resistance. Finally, accession KS94GH6 could act as an excellent germplasm resource for the incorporation of higher(E)-β-farnesene levels into cultivated alfalfa.