Comparison of economic injury levels for sorghum head bug, Calocoris angustatus, on resistant and susceptible genotypes at different stages of panicle development

Abstract

Sorghum head bug, Calocoris angustatus Leth. (Hemiptera: Miridae) is a major pest of grain sorghum in India. Studies were conducted to estimate economic injury levels (EILs) for the sorghum head bug on resistant (IS 17610 and IS 21443), a moderately susceptible (IS 9692) and a commercial cultivar (CSH 11) using different protection levels at the half-anthesis, complete-anthesis, milk and dough stages of panicle development. Data were recorded on bug numbers and grain yield. Two to four sprays of demeton-S-methyl (Metasystox) controlled the bugs on CSH 11 and IS 9692, whereas one spray was sufficient on IS 17610 and IS 21443. Maximum grain yield was obtained with four insecticide sprays on CSH 11 and IS 9692, whereas one and two sprays resulted in maximum yield on IS 17610 and IS 21443, respectively. Avoidable losses were greatest in IS 9692 (66%), followed by CSH 11 (38.4 and 55.7% during 1988 and 1989, respectively), IS 21443 (29.1%), and IS 17610 (8.2%). Cost: benefit ratios were > 1:3.4 with four insecticide sprays on CSH 11, IS 9692 and IS 21443; however, it was not economical to spray insecticides on the bug-resistant cultivar, IS 17610. Economic injury levels were 0.2–0.9 bugs per panicle for CSH 11 at the half-anthesis and complete-anthesis stages, respectively, compared with 0.8–4.2 bugs for IS 9692, 10–50 bugs for IS 17610 and 0.5–1.3 bugs for IS 21443. Based on multiple regression, the EILs varied from 0.04 to 0.30 bugs for CSH 11, from 0.9 to 6.6 bugs for IS 9692, 2.7 bugs for IS 17610, and 0.3–5.1 bugs for IS 21443 at the half-anthesis stage. EILs vary across stages of panicle development and cultivars. Head bug-resistant cultivars not only decrease the rate of population increase of bugs, but also withstand greater head-bug densities.