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Spikelet Characteristics and Midge Predation of Hermaphroditic Genotypes of Big Bluestem

Dep. of Plant Science, South Dakota State Univ., Brookings, SD 57007

^* Corresponding author.

Seed yields of big bluestem (Andropogon gerardii Vitman) can be limited by poor caryopses set. The purpose of this study was to determine if sex expression had an effect on caryopses set, caryopses weight, and mature spikelet predation by the bluestem seed midge (Contarinia wattsi Gagné). Two hermaphroditic and two andromonoecious genotypes of big bluestem from an eastern South Dakota population were evaluated for 3 yr in a space-plant nursery at Brookings, SD. Andromonoecious genotypes exhibited 11% higher caryopses set in sessile spikelets. Sessile and pedicellate spikelet caryopses sets of hermaphroditic genotypes were 60.8% and 57.4%, respectively. Caryopses from sessile spikelets were 46% heavier than those from pedicellate spikelets of hermaphroditic genotypes. Weights of caryopses from sessile spikelets of hermaphroditic and andromonoecious genotypes were similar. Frequencies of sessile spikelets containing diapausing larvae of the bluestem seed midge were greater for hermaphroditic than andromonoecious genotypes. Mature sessile spikelets of hermaphroditic genotypes were more frequently infested by larvae than pedicellate spikelets, and no larvae were found in pedicellate spikelets of andromonoecious genotypes. The bisexual pedicellate spikelet trait increased total spikelet caryopses set in the presence of the bluestem seed midge and thus may have potential for improving seed yields.

Received for publication October 24, 1988.