Studies on plant resistance in sorghum to the chinch bug, Blissus leucopterus leucopterus (Say) (Hemiptera: Lygaeidae)

A comparison of screening techniques to identify sorghum lines with chinch bug resistance indicated that single-seedling no-choice tests are a good starting point in discerning resistance/susceptibility. Multiple choice tests may not be very effective in identifying specific antixenotic properties. Dual-choice single seedling tests helps in the pairwise comparison of lines with a resistant or susceptible check. This technique is very sensitive and can identify very low levels of resistance/susceptibility. Seedling tolerance tests that study the ability of the line to compensate for early damage by chinch bugs are of more value because they help to identify sorghum lines with good field tolerance to the pest. Sorghum plant volatiles extracted as steam distillates from the resistant line KS94 possessed chinch bug repelling qualities when used in high concentrations (2000 and 4000 ppm). The extracts of the susceptible line Double Dwarf Yellow Milo (DDYM) did not possess chinch bug attracting attributes. Chinch bug resistant lines contained significantly higher levels of total phenolics and tannin when compared to susceptible lines. Phenolic acid concentration ranged from 14.21 $\mu$g/10 mg of plant sample in DDYM at 2 weeks after planting to 82.61 $\mu$g/10 mg of plant sample in KS94 at 12 weeks after planting. Tannin content ranged from 41.04 $\mu$g/ml of tannin extract at 2 weeks after planting in DDYM to 230.95 $\mu$g/ml of tannin extract in KS94 at 12 weeks after planting. HPLC separation of phenolic acids indicated that the resistant lines KS94 and KS95 had significantly higher concentrations of p-hydroxybenzoic, ferulic and protocatechuic acids. Bioassays with the phenolics showed that p-hydroxybenzoic acid produced the highest mortality (LD$\sb{50}$ = 3.98 mM) in chinch bugs and caffeic acid produced the lowest mortality (LD$\sb{50}$ = 30.08 mM). The resistant lines possessed more of the phenolic acids that were detrimental to chinch bug survival. There appears to be a strong association between sorghum phenolics and plant resistance to chinch bugs. Chinch bug salivary secretions and whole body extracts were found to contain pectinases, enzymes that degrade pectin, a structural component of cell wall and middle lamella of higher plants. The enzymes apparently help the chinch bugs in the feeding process