COMPARISON OF ECONOMIC INJURY LEVELS FOR SORGHUM HEAD BUG, CALOCORIS-ANGUSTATUS, ON RESISTANT AND SUSCEPTIBLE GENOTYPES AT DIFFERENT STAGESOF PANICLE DEVELOPMENT

Sorghum head bug, Calocoris angustatus Leth. (Hemiptera: Miridae) is a major pest of grain sorghum in India. Studies were conducted to estim ate economic injury levels (EILs) for the sorghum head bug on resistan t (IS 17610 and IS 21443), a moderately susceptible (IS 9692) and a co mmercial cultivar (CSH11) using different protection levels at the hal f-anthesis, complete-anthesis, milk and dough stages of panicle develo pment. Data were recorded on bug numbers and grain yield. Two to four sprays of demeton-S-methyl (Metasystox) controlled the bugs on CSH11 a nd IS 9692. whereas one spray was sufficient on IS 17610 and IS 21443. Maximum grain yield was obtained with four insecticide sprays on CSH1 1 and IS 9692, whereas one and two sprays resulted in maximum yield on IS 17610 and IS 21443, respectively. Avoidable losses were greatest i n IS 9692 (66%). followed by CSH 11 (38.4 and 55.7% during 1988 and 19 89, respectively), IS 21443 (29.1%), and IS 17610 (8.2%). Cost:benefit ratios were > 1:3.4 with four insecticide sprays on CSH11, IS 9692 an d IS 21443; however, it was not economical to spray insecticides on th e bug-resistant cultivar, IS 17610. Economic injury levels were 0.24). 9 bugs per panicle for CSH11 at the half-anthesis and complete-anthesi s 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 reg ression, the EILs varied from 0.04 to 0.30 bugs for CSH11, 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 d evelopment and cultivars. Head bug-resistant cultivars not only decrea se the rate of population increase of bugs, but also withstand greater head-bug densities.