EFFECT OF HOST-PLANT RESISTANCE ON ECONOMIC INJURY LEVELS FOR THE SORGHUM MIDGE, CONTARINIA-SORGHICOLA

Sorghum midge, Contarinia sorghicola Coq. (Cecidomyiidae: Diptera), is the most important pest of grain sorghum worldwide. The effect of mid ge-resistant cultivars on economic injury levels (EILs) for sorghum mi dge was studied on two midge-resistant (ICSV 197 and ICSV 745) and fou r commercial cultivars (CSH 1, CSH 5, ICSV 1 and ICSV 112). There was a linear relationship between midge density and percentage loss of gra in in the midge-susceptible cultivars CSH 1, CSH 5, and ICSV 112. Howe ver, there was only a marginal increase in midge damage with an increa se in midge density in the midge-resistant cultivars ICSV 197 and ICSV 745 while ICSV 1 showed a moderate increase in midge damage. Insect d ensity-damage relationships were better correlated when the cultivars were infested four times with a range of midge densities (5-40 midges/ panicle) than with single infestations. Regression coefficient (b-valu e) and coefficient of determination (R2%) increased with number of inf estations and insect density in the midge-susceptible cultivars. EILs based on four infestations (across infestation levels) were 0.1-0.2 mi dges/panicle for the commercial cultivars ICSV 1, ICSV 112, and CSH 1, and 25 and 33 midges for the midge-resistant cultivars ICSV 197 and I CSV 745, respectively. EILs at 40 midges/panicle (across number of inf estations) were: 0.2 midges/panicle for CSH 1, ICSV 1, and ICSV 112 co mpared with 6.7 midges for ICSV 745, and 100 midges for ICSV 197. Econ omic injury levels for sorghum midge therefore differ with plant resis tance, insect densities, and the number of days for which the panicles are exposed to the midge flies. Panicles infested with a range of ins ect densities, and for three to four days give a reliable estimate of EILs for sorghum midge. It is important to determine EILs for resistan t and susceptible cultivars for appropriate pest management decisions.