Specialist/generalist divergence: phylogenetic constraints on the correlation between allelochemical tolerance spectra and host ranges of herbivorous insects

Specialist insect herbivores are projected to be better adapted to one or several plant defense allelochemicals in their specialized host plants than their generalist counterparts but less tolerant to a broad range of other allelochemicals potentially experienced by the generalist herbivores. It is unclear whether the projected specialist/generalist differences are universally true across different taxonomic ranks or constrained by their phylogenetic relationships. To address this issue, we used diet incorporation to compare the tolerance spectrum of the neonates of one specialist ( Helicoverpa assulta ) and two generalists ( H. armigera and Spodoptera frugiperda ) to 12 naturally occurring allelochemicals. We found that the specialist H. assulta and its within-genus generalist counterpart H. armigera were equally tolerant to 4 (indole-3-carbinol, 2-tridecanone, chlorogenic acid and ferulic acid) of the 12 allelochemicals . Meanwhile, H. assulta was less tolerant than H. armigera to seven (coumarin, capsaicin, quercetin, gossypol, rutin, xanthotoxin and flavone) of the 12 allelochemicals, but was more tolerant than H. armigera only to nicotine present in its specialized host plant tobacco. In contrast, H. assulta, like its within-genus generalist H. armigera , was more tolerant to 8 (coumarin, indole-3-carbinol, capsaicin, 2-tridecanone, gossypol, xanthotoxin, nicotine and flavone) of the 12 allelochemicals, but less tolerant only to 3 (quercetin, chlorogenic acid and rutin) of the 12 allelochemicals than its across-genera generalist S. frugiperda. These results demonstrate that the predicted specialist/generalist divergence in allelochemical tolerance profiles only applies to corresponding species within a genus, implying that the projected positive correlation between allelochemical tolerance spectra and host ranges of insect herbivores is phylogenetically constrained, at least at the family level of Noctuidae.