Camptothecin weakens the mechanical properties of insect cuticle by inhibiting tanning.

The mode of action of botanical pesticides is often complex. In this study, camptothecin (CPT), a plant-derived compound, was found to alter the mechanical properties of the cuticle in the lepidopteran pest, Ostrinia furnacalis. CPT treatment resulted in a thinner, distorted cuticle with significantly reduced pigmentation. Fourier transformed infrared spectroscopy indicated decreased tyrosine-mediated crosslinking and melanin formation, while metabolomics showed a significant reduction in N-acetyldopamine content, with no reduction in dopamine or related precursors. Despite a slight increase in laccase 2 transcript levels, laccase activity was significantly reduced in CPT-fed larvae, suggesting that CPT may inhibit laccase translation or zymogen activation. This reduction in laccase activity led to impaired sclerotization and pigmentation, producing a disordered cuticle with weakened mechanical properties. These findings provide new insights into the mode of action of botanical pesticides and their potential to regulate insect cuticle formation.