Differences Among Medicago Species in Resistance to Oviposition by the Alfalfa Weevil1

Resistance of alfalfa to the alfalfa weevil, Hypera postica (Gyllenhal), was sought first among commercial alfalfa varieties and North Carolina synthetics in small field plots. All commercial varieties tested were susceptible to the weevil. A search for weevil resistance among species of Medicago was then initiated. These investigations were conducted in the greenhouse and laboratory where adult preference for feeding and oviposition could be critically evaluated in the absence of larval damage. Plants and excised stems were exposed to an adult weevil population far larger than normal fall populations. Medicago sativa var. gaetula Urb. (accession 59–7) a round-stemmed variety of alfalfa, was found to be nearly immune to weevil injury and oviposition in a laboratory study conducted in 1960–61. The resistance of Gaetula to egg laying has been repeatedly confirmed in greenhouse tests since 1961. Another accession of Gaetula and also M. falcata L. exhibited a high degree of resistance in these same tests when the commercial alfalfa variety Atlantic (M. sativa L.) was used as the susceptible check. The total number of eggs recovered from the stems of Gaetula and M. falcata accession averaged 4 to 18% as many as from the stems of the Atlantic check. Reduction in total eggs was correlated with reduction in number and size of egg masses and associated with a decrease in stem pith or an increase in stem solidness. Since the alfalfa weevil is known to be developing resistance to the chlorinated insecticides, the transmission of Gaetula or M. falcata resistance to an agronomically desirable alfalfa would be especially important.