The diamondback moth, Plutella xylostella (L.) (Lep.: Plutellidae), is a key pest of cruciferous vegetables, throughout the world including Iran. In this study, the effects of vermicompost and humic acid fertilizers on the population growth of P. xylostella were investigated and, furthermore, root system development in the host plants, canola, white cabbage, and red cabbage, was evaluated under the same experimental conditions. Our results showed that brassicaceous plants fertilized by these fertilizers significantly reduced the immature survival rate of P. xylostella. Reared adults from larvae on the treated plants also showed statistically significant reductions in the longevity, oviposition period, and fecundity compared with the control. On canola, white cabbage and red cabbage, the values of R₀ (35.4, 10.2 and 6.2 offspring/individual, respectively), r (0.151, 0.098 and 0.071 day^-1, respectively), and λ (1.163, 1.103 and 1.074 day^-1, respectively) on the plants fertilized with both humic acid and vermicompost were significantly lower than control plants. We also found that on the plants fertilized with humic acid and vermicompost, the number of eggs laid was lower than control plants. The tested fertilizers improved the development of root systems and subsequently enhanced the plant resistance to P. xylostella. Our results show that humic acid and vermicompost root application induces systemic defenses in canola, white cabbage, and red cabbage that have negative effects on the performance of P. xylostella. These findings will help to develop our knowledge regarding the effects of humic acid and vermicompost fertilizers on the resistance of plants to P. xylostella.