Phthalide-based host-plant resistance toSpodoptera exigua andTrichoplusia ni inApium graveolens

A chemical basis for the difference in suitability between two celeriac (Apium graveolens var.rapaceum) cultivars for the survival and growth ofSpodoptera exigua (Hübner) andTrichoplusia ni (Hübner) was identified as sedanenolide (3-n-butyl-4,5-dihydro-isobenzofuranone). Sedanenolide was isolated using a bioassay-driven extraction and purification procedure and was identified using several spectrometric methods. Foliar concentrations of sedanenolide were negatively correlated with larval performance and were significantly higher in the cultivar less suitable for larval survival and growth. Sedanenolide andBacillus thuringiensis Berliner acted additively in reducing larval growth when combined in artificial diets, a result that is consistent with previous studies in which the combined effect of host plant cultivar andB. thuringiensis on larval survival and growth were additive.