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Factors Affecting Resistance of Field-Planted Alfalfa Clones to the Alfalfa Weevil¹

Seventy alfalfa, Medicago sativa L., clones representing a range of observed reactions to the alfalfa weevil, Hypera postica (Gyllenhal), were evaluated in a field test for oviposition and larval feeding, and for interrelationships between plant resistance and growth characteristics.

Oviposition preference accounted for 26% of the variation among clones for larval feeding damage. Plants with decumbent growth, wide crown, little growth, and small stems tended to receive fewest eggs and suffer least larval feeding; they had many characteristics of Medicago falcata. Although weevils preferred to oviposit in plants with large stems, oviposition differences among clones were still significant after adjustment for stem size by covariance analysis. Weevils oviposited only sparingly in several of the clones with medium large stems. The following correlation coefficients were positive and significant: Number of egg masses and eggs per stem, stem diameter and eggs per stem, larval feeding and egg masses per stem, larval damage scores in different years, and number of egg masses in different years. Further study is needed to determine the kinds of resistance mechanisms involved and whether oviposition preference mechanisms reported here offer suitable control of alfalfa weevil damage.

Key Words: Medicago sativa L. • M. falcata • Hypera postica (gyllenhal)

Acknowledgment for assistance in preparation of this manuscript is made to D. K. Barnes, Crops Research Division, ARS, USDA, Beltsville, Maryland

² Formerly Research Agronomist (now Manager, Norwood Farms, McBee, South Carolina), formerly Research Entomologist (now Entomologist, Pesticides Regulation Division), Research Agronomist, and Research Entomologist, respectively.

Received for publication April 4, 1966.