Phytosterol unsuitability as a factor mediating food aversion learning in the grasshopper Schistocerca americana

Sixth stadium nymphs of the grasshopper Schistocerca americana (Drury) (Orthoptera: Acrididae) were presented with spinach with or without added cholesterol or β‐sitosterol, and the lengths of the first four meals were monitored. Spinach alone evoked a typical aversion learning pattern in which successive meals were of decreasing duration; however, when cholesterol or β‐sitosterol was added the spinach remained acceptable. Nymphs fed spinach for 24h preferred glass‐fibre discs with added cholesterol in a choice test, but nymphs fed wheat (an acceptable food) did not prefer cholesterol‐treated or control discs. Nymphs pretreated with spinach did not distinguish between discs treated with stigmasterol (an unsuitable phytosterol for grasshoppers) and control discs. In no‐choice tests, nymphs pretreated with spinach also fed significantly longer on discs with added 10% cholesterol or 1%β‐sitosterol, compared to meals on discs with stigmasterol or sucrose controls. Detection of appropriate sterols is likely to involve a post‐ingestive feedback mechanism, as significantly longer meals on discs treated with only sucrose resulted when spinach‐fed nymphs, were force‐fed gelatin capsules packed with β‐sitosterol or cholesterol, compared with stigmasterol. Contact chemoreception is unlikely to play a role in this behaviour, as discs (lacking sucrose) treated with cholesterol, β‐sitosterol, stigmasterol, or a chloroform control, all required similar numbers of contacts before feeding was initiated.