Comparison of Pod-Wall Characteristics with Seed Damage and Resistance to the Alfalfa Seed Chalcid (Hymenoptera: Eurytomidae) in Medicago Species

Abstract

The alfalfa seed chalcid, Bruchophagus roddi (Gussakovsky), is the most destructive insect to alfalfa, Medicago sativa L., seed. Because chemicals are ineffective in its control, plant resistance offers the most viable method of reducing seed losses. The objective of our study was to compare pod-wall characteristics with seed damage and resistance to the alfalfa seed chalcid in eight Medicago species and three interspecific hybrids. The percentage of chalcid-damaged seed, based on seed numbers, averaged 0.0% for annual species, 18.4% for perennial glandular-haired species, and 20.4% for perennial control cultivars. The percentage of other seed losses averaged 8.4% for perennial control cultivars, 21.3% for annual species, and 35.5% for perennial glandular-haired species. Pod-wall thickness averaged: perennial glandular-haired species, 131.9μm; annual species, 139.4μm; and perennial control cultivars, 144.0 μm. Increased pod-wall thickness reduced other seed losses not attributed to the chalcid. Phenotypic variation in pod-wall thickness suggests that selection for this trait should be effective, assuming there is significant heritable variation. Our findings suggest also that pod-wall lignification may reduce seed losses due to chalcid damage. The highest levels of resistance to the alfalfa seed chalcid were found in the annual species, which also had highly lignified pod-walls. The annual species, however, are cross-incompatible with M. sativa . Thus, breeding for increased pod-wall lignification may reduce seed losses due to damage by the chalcid, if the mechanisms that control lignin production and accumulation are heritable and in cross-compatible Medicago genotypes.