| Thrips are the major insect pest of onions in South Texas. Host plant resistance to this pest would be very useful in improving the control of thrips and avoiding yield losses. In this project the mechanisms of resistance and heritability of resistance to thrips in the IPA-3 cultivar from Brazil were studied. Three experiments were conducted to achieve these goals. An evaluation was conducted of 'IPA-3' and 'TG1015Y', thrips "resistant" and "susceptible" cultivars, respectively, during two seasons to study mechanisms of resistance and to provide statistical evidence that the two parental cultivars to be used in the inheritance study were significantly different in thrips resistance. The narrow {dollar}(hsp2){dollar} and broad {dollar}(Hsp2){dollar} sense heritabilities were estimated from the variances of the parental, F1, F2, and backcross populations.; There were significant differences in the number of thrips and level of damage between the two cultivars in both seasons, 'IPA-3' being more resistant than 'TG1015Y'. Differences were found between the two cultivars for bulb pungency, brix, sucrose, and scanning electron micrographs (SEMs) of leaf wax structure. These factors could be related to the mechanism of resistance and were studied in the inheritance study. Yield difference was also significant in the second season indicating tolerance was important in 'IPA-3'. Nonpreference and tolerance appeared to be involved in the resistance. Significant yield and thrips differences were also found between the insecticide sprayed and nonsprayed treatments.; The results of the inheritance study revealed very low levels of heritability based upon field thrips counts at Weslaco, TX. The {dollar}hsp2{dollar} and {dollar}Hsp2{dollar} were calculated as being 5.3% and 4.1%, respectively using one method and 4% and 8%, respectively with another method. This indicates that additive genetic variance accounts for most of the gene action responsible for this trait in this study. Correlations of segregating F2 plants were calculated between thrips numbers and pungency, brix, sucrose, and SEMs to identify resistance mechanisms. The results were inconsistent with the parental comparisons; therefore being inconclusive. A family based selection scheme should be used to breed for thrips resistance in onion. |
