A study was conducted to identify mechanisms of resistance in sunflower to the sunflower midge, Contarinia schulzi Gagné. Infestation ratings, larval counts, and damage ratings were used to quantify midge resistance in seven sunflower hybrids grown at three locations. Six of the seven hybrids possessed midge resistance. Each hybrid was further categorized with respect to infestation resistance, antibiosis, and tolerance. Two bud characteristics, percent open and size, were examined for their contribution to infestation resistance. The percentage of time that buds of each hybrid were open was negatively correlated with infestation (indicating a preference for the open characteristic). Infestations were higher in buds ~2.5 cm in diameter. In artificial infestation trials, damage caused by first and second instars was very low due to high larval mortality. When sunflower buds were artificially infested with midge adults, moderate larval infestations occurred. Artificially infesting plants with adults confirmed the presence of antibiosis in two resistant and one susceptible hybrid.
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