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Rejection of wheat germ diet by cabbage-reared Pieris rapae larvae is explained by the fact that the diet contains strong feeding deterrents to the larvae. These deterrents were present mainly in hexane and butanol (BuOH) fractions of diet extracts, but the post-BuOH water fraction also showed some deterrent activity. Although minor diet components such as sorbic acid had a weak deterrent effect, wheat germ was primarily responsible for the activity. Seven compounds isolated from the BuOH extract of wheat germ were deterrent to the larvae. UV spectra suggested that most of these compounds are apigenin-based flavones. Fractionation of the hexane extracts showed that most of the active compounds were methanol-soluble, and HPLC of the MeOH fractions revealed the presence of active compounds that were cross habituationabsent in the BuOH extract of wheat germ, as well as other compounds that were common to both fractions. Acceptance of nasturtium by P. rapae larvae reared on wheat germ diet is explained by a type of of the larvae to feeding deterrents. When larvae were reared on cabbage leaves treated with chemical fractions from wheat germ diet, they readily accepted nasturtium, which is normally refused by cabbage-reared P. rapae larvae, due to the presence of strong feeding deterrents in the plant. A high percentage of diet-reared larvae also fed on Erysimum cheiranthoides and Iberis amara, which are normally rejected. The chemical constituents encountered in wheat germ can almost completely account for the larval acceptance of nasturtium. The suitability of wheat germ diet for rearing phytophagous insects for behavioral assays must be questioned.
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