Cultivars and IPM Systems for Organic Cotton Production: Distribution of Thrips Resistance in Multiple Generations of Cotton.

Pest management in organic production systems is a serious consideration, as synthetic insecticide use is prohibited by certified organic guidelines.  Thrips (Thysanoptera: Thripidae) are major early-season arthropod pests in cotton (Gossypium hirsutum L.) systems on the Texas High Plains.  Developing cotton varieties with high host-plant thrips resistance would be an important tool for managing these pests in organic systems.  An interspecific cross was made between an elite G. hirsutum line (‘CA 2266’) and a thrips-resistant G. barbadense L. accession (‘TX 110’).  The parental lines, F₁ generation, F₂ generation, and a susceptible check (‘All-Tex Atlas’) were evaluated for resistance to thrips feeding injury in a greenhouse trial under elevated thrips pressure.  Phenotypic data included leaf area reduction (compared to a sprayed control), thrips numbers per plant, and a visual damage rating of thrips feeding injury.  The parental, F₁, and susceptible check lines were evaluated as six-plant experimental units, while approximately 240 F₂ individuals were evaluated as a potential mapping population for molecular marker development.  There were significant differences in leaf area reduction, thrips numbers, and damage ratings among the parent and F₁ lines (P < 0.05).  However, the distribution of resistance among these lines contrasted with what previous research has indicated, likely as a result of insufficient thrips pressure during the experiment.  Leaf area reduction was highly variable in the F₂ population, but a high degree of segregation for leaf area, independent of thrips damage, suggested this method was insufficient for adequately discriminating among individual plants for resistance.  Visual ratings, however, appeared to discriminate most effectively among individual plants.  Thrips counting was also not sensitive enough to distinguish differences between individual plants, but the other data sets for the F₂ population suggest the resistance trait is multigenic.  Further refining of the phenotyping method, with greater emphasis on visual damage ratings, would aid in more effective characterization of thrips resistance for future cultivar development.