<i>Tyrosine Hydroxylase</i> Is Required for the Larval–Pupal Transformation and Immunity of <i>Plutella xylostella</i>: Potential for Pest Management

<i>Plutella xylostella</i> has developed high levels of resistance to many commonly used insecticides. <i>Tyrosine hydroxylase</i> (<i>TH</i>) is essential for insect survival; thus, we evaluated whether <i>TH</i> could be a potential target for controlling <i>P. xylostella</i>. In this study, <i>PxTH</i> was identified; further qPCR analysis showed that <i>PxTH</i> increased its expression during larval pupation and was highly expressed in the head and epidermis of prepupa in <i>P. xylostella</i>. Subsequently, we found a significant decrease in insect pupation and eclosion rates after injection of ds<i>PxTH</i> or a feeding diet supplemented with 3-iodo-tyrosine (3-IT) as a TH inhibitor in <i>P. xylostella</i>. Moreover, this study suggested that PxTH enzyme activity and dopamine concentrations were significantly decreased, agreeing with the blockage of larval-pupal cuticle tanning, with thinner puparium and less melanization after feeding 3-IT. In addition, expression levels of four antimicrobial peptide genes were significantly inhibited after <i>P. xylostella</i> feeding with 3-IT, and injection of <i>Escherichia coli</i> resulted in 73.3% mortality, indicating that <i>PxTH</i> was required for immune responses. In summary, these results confirmed that <i>PxTH</i> was involved in the development and immunity of <i>P. xylostella</i>, suggesting a critical potential novel insecticide target for RNAi-based pest control.