Indiscrimination of Manduca sexta larvae to overexpressed and underexpressed levels of phenylalanine ammonia-lyase in tobacco leaves

Manduca sexta (L.) larvae were unable to discriminate between transgenic tobacco leaves expressing high or low levels of phenylalanine ammonia-lyase (PAL) in dual choice arenas. Tobacco leaves overexpressing PAL had up to ten times more chlorogenic acid and up to twice as much rutin in their leaves than leaves containing the same sense-suppressed gene. Caterpillars reared previously on artificial diet or tobacco leaves had no preference for either leaves over- or underexpressing PAL. Application of exogenous chlorogenic acid to PAL-suppressed leaves at levels similar to the overexpressed leaves did not affect M. sexta's choice. When applied at higher rates, treated leaves were preferred by caterpillars reared on tobacco leaves but not by diet-reared larvae. Our results with leaves confirm earlier studies with M. sexta using simpler substrates and mixtures of test compounds and provides further evidence that leaf phenolics, such as chlorogenic acid, do not act as feeding deterrents for larval M. sexta.