Plant growth stage effects on the feeding and growth responses of the bertha armyworm, Mamestra configurata to canola and mustard foliage

Mamestra configurata (Walker) (Lep., Noctuidae) larvae were fed excised Brassica juncea (commercial brown mustard) or B. rapa cv. Tobin (Canola) foliage of three plant growth stages ‐ rosette (stage 2), stem elongation (stage 3) and flowering (stage 4). Relative consumption rates (RCRi) were not significantly different between the plant species. Within the B. juncea treatments, there were no significant growth stage differences in RCRi. However, within B. rapa, RCRi increased with advancing plant growth stage. Larvae fed B. juncea foliage had significantly reduced relative growth rates (RGRi) compared to larvae fed B. rapa foliage. Within the B. juncea treatments, RGRi decreased with advancing plant growth stage. There were no significant growth stage differences in RGRi in the B. rapa treatments. RGRi was inversely proportional to the levels of isothiocyanate‐releasing glucosinolates in the B. juncea treatments. RCRi was inversely proportional to the levels of indolyl glucosinolates in the B. rapa treatments. Levels of total phenols and catechols in B. juncea did not show any trend which could be related to growth stage effects in the insect nutritional indices. In B. rapa, levels of phenols and catechols in stage 3 and 4 foliage were lower than that of stage 2 foliage. Analyses of total nitrogen in field‐grown plants showed reductions in percent nitrogen from rosette to flowering stage foliage. The response of M. configurata to different growth stages of its host plants are discussed in relation to changing levels of allelochemicals and nitrogen.