New ‘stimuli-enriched’ laboratory bioassay used to identify improved botanical repellent treatment, Lem-ocimum, to control the stored-grain pest Tribolium castaneum

•More effective repellent plant bioassay was developed to protect grain.•Tribolium castaneum more responsive in the new bioassay than in the standard assays.•Lemongrass and sweet basil were the most repellent plants in the bioassay.•Combination of repellent plants reduced number of beetles infesting bags of grain.

A laboratory study of Tribolium castaneum, a major pest of stored grain, was conducted to develop a more efficient and effective ‘choice’ bioassay for identification of new repellent botanical treatments. Standard bioassays to test the repellency of candidate plants include pit-fall traps and open arena choice tests, environments lacking in some of the most important natural stimuli that guide the movement of food-searching beetles, e.g., 1) materials they can burrow through, which stimulate ‘positive thigmotaxis’, 2) a range of light and dark areas, which stimulate ‘negative phototaxis’ and 3) three-dimensional habitats, which stimulate ‘positive geotaxis.’ The lack of these stimuli can lead to two common problems; ‘low efficiency’ (high proportion of beetles remain in the area that surrounds treatments without making a ‘choice’), and ‘low efficacy’ (high variability in proportions found in control and treated samples). The new ‘stimuli-enriched’ bioassay, which included all three of the above stimuli, was significantly more efficient (P < 0.0001) and effective than three standard bioassays. The stimuli-enriched bioassay was used to compare the repellency of four candidate plants; Ocimum basilicum (Sweet Basil) and Cymbopogon nardus (Lemongrass) were significantly more repellent than Vernonia amygdalina or Nauclea diderrichii (Tukey Contrasts; P < 0.01). A novel method of applying repellent material (a paste of repellent plant is applied between the layers of double bagged grain) was tested on the most promising repellent plants materials; a combination of C. nardus and O. basilicum (‘Lem-ocimum’) at 0.5% w/w of each was significantly more effective than O. basilicum on its own (Tukey Contrasts; P < 0.05). These results show that the stimuli-enriched bioassay provides more consistent and accurate assessments than the standard bioassays of the repellency of candidate botanicals, and that Lem-ocimum treated double-bags are a promising new method of protecting sorghum from T. castaneum.