EXPLOITATION OF SORGHUM NONPREFERENCE RESISTANCE TO SORGHUM MIDGE (DIPTERA, CECIDOMYIIDAE) USING MIXED PLANTINGS OF RESISTANT AND SUSCEPTIBLE SORGHUM HYBRIDS

The effectiveness of sorghum resistance to sorghum midge, Contarinia s orghicola (Coquillett), was improved in mixed plantings of resistant a nd susceptible sorghum, Sorghum bicolor (L.) Moench, hybrids in 1989 a nd 1992. Mixed plantings drew sorghum midges from the resistant to the susceptible plants, and damage to resistant plants was lessened. Adul t sorghum midge abundance averaged 5.5 and 1.6 sorghum midges per pani cle during the flowering period in 1989 and 1992, respectively. Mean s orghum midge damage ratings for susceptible and resistant plants were 5.9 and 2.4, and 7.7 and 1.6 in 1989 and 1992, respectively. Damage to resistant plants decreased and per panicle yield increased as the com position of susceptible sorghum plants in mixed plantings increased. T otal grain yield (yield of resistant and susceptible plants combined) was highest when the proportion of susceptible plants was from 10 to 3 0% in 1989 and was 20 and 30% in 1992 and declined when percentage of susceptible plants was increased further. Total grain yields were less when all plants were resistant than when 10 to 30% and 20% of the pla nts were susceptible in 1989 and 1992, respectively.