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Effect of Temperature and Glandular-haired Medicago species on Development of Alfalfa Weevil Larvae¹

We evaluated annual glandular-haired alfalfa diploids (Medicago disciformis DC., M. blancheana Boiss.) and tetraploids [M. rugosa Desr., M. scutellata (L) Mill.] for resistance to alfalfa weevil larvae [Hypera postica (Gyllenhal)] under three temperature regimes in growth chambers. Perennial M. sativa L. control cultivars were ‘Arc’ (tolerant to weevil larvae) and ‘Lahontan’ (susceptible).

The tetraploids, M. scutellata and M. rugosa, were highly resistant over all temperature regimes. Larval mortality was complete during the first instar.

Although the diploids were less resistant than the tetraploid annuals, they caused consistent larval mortality at all three temperatures. Mortality of first instar larvae was higher on M. blancheana than on M. disciformis. However, the mortality of second instar larvae or pupae was highest on M. disciformis. Fewer larvae matured to adults on our diploids than on M. sativa. More larvae matured to adults on M. disciformis than on M. blancheana. The mean weight of adults developing on M. disciformis was less than that on M. blancheana and M. sativa, which did not differ in adult weights.

The rate of weevil development increased with temperature on the resistant diploid annuals and on the alfalfa cultivars Arc and Lahontan. At each temperature, larval development was slower on the glandular-haired diploids than on the alfalfa cultivars. Plant resistance did not alter the length of the pupal stage. However, this stage was shorter at high than at low temperatures.

Key Words: Medicago disciformis DC. • M. blancheana Boiss • M. rugosa Desr. • M. scutellata (L.) Mill • M. sativa L. • Larval mortality • Adult weights • Weevil developmental rates • Growth chamber

² Assistant professor, Texas Agric. Exp. Stn., Texas A&M Univ. System, Weslaco, TX 78596, formerly graduate research assistant, Dep. of Entomology; research agronomist, USDA, SEA. Dep. of Agronomy; and professor, Dep. of Entomology, Kansas State University, Manhattan, KS 66506, respectively.

Received for publication October 24, 1979.