Effect of host‐plant resistance on economic injury levels for the sorghum midge,Contarinia sorghicola

Abstract

Abstract Sorghum midge, Contarinia sorghicola Coq. (Cecidomyiidae: Diptera), is the most important pest of grain sorghum worldwide. The effect of midge‐resistant cultivars on economic injury levels (EILs) for sorghum midge was studied on two midgeresistant (ICSV 197 and ICSV 745) and four commercial cultivars (CSH 1, CSH 5, ICSV 1 and ICSV 112). There was a linear relationship between midge density and percentage loss of grain in the midge‐susceptible cultivars CSH 1, CSH 5, and ICSV 112. However, there was only a marginal increase in midge damage with an increase in midge density in the midge‐resistant cultivars ICSV 197 and ICSV 745 while ICSV 1 showed a moderate increase in midge damage. Insect density‐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‐value) and coefficient of determination (R2%) increased with number of infestations and insect density in the midge‐susceptible cultivars. EILs based on four infestations (across Infestation levels) were 0.1–0.2 midges/ 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 ICSV 745, respectively. EILs at 40 midges/panicle (across number of infestations) were: 0.2 midges/panicle for CSH 1, ICSV 1, and ICSV 112 compared with 6.7 midges for ICSV 745, and 100 midges for ICSV 197. Economic injury levels for sorghum midge therefore differ with plant resistance, insect densities, and the number of days for which the panicles are exposed to the midge flies. Panicles infested with a range of insect densities, and for three to four days give a reliable estimate of EILs for sorghum midge. It is important to determine EILs for resistant and susceptible cultivars for appropriate pest management decisions.