Abundance of 24-methylenecholesterol in traditional African rice as an indicator of resistance to the African rice gall midge, Orseolia oryzivora Harris & Gagné

Rice production is of great importance to food security in Nigeria. The African rice gall midge (AfRGM), Orseolia oryzivora Harris & Gagné, is a major pest of lowland rice in Nigeria and many other Africa countries. Current management efforts are directed at integrating host plant resistance with biological agents. The type and abundance of some phytochemicals are known to affect resistance in host plants, so in this study we measured rice culm hardness, rice culm stickiness and the type and quantity of phytosterols in rice cultivars, and investigated the effects of these factors on resistance to the AfRGM. The study was conducted at 24 ± 2°C, 60 ± 5% relative humidity, and a 12 h light : 12 h dark photoperiod. Three rice cultivars were studied: one Oryza glaberimma cultivar, Tog 7442 (resistant), and two O. sativa cultivars, Cisadane (tolerant) and ITA 306 (susceptible). We found no significant differences in culm hardness and stickiness among the cultivars tested. Using gas chromatography (flame ionization detection) analysis, we identified four major phytosterols: campesterol, 24‐methylenecholesterol, stigmasterol and β‐sitosterol. 24‐Methylenecholesterol was the most abundant sterol in the resistant O. glaberimma cultivar (79.20 μg), whereas β‐sitosterol was most abundant in the two susceptible O. sativa cultivars (Cisadane, 71.27 μg; ITA 306, 66.37 μg). The comparatively high concentration of 24‐methylenecholesterol in this cultivar of O. glaberimma may be an indicator of resistance to AfRGM.