Soybean leaf trichomes under drought and herbivory have differential effects on soybean looper (Chrysodeixis includens Walker)

Leaf trichomes have been extensively studied as physical defensive structures against abiotic and biotic stress in plants. Climate change-driven stresses such as drought and/or herbivory can aggravate crop morpho-physiology as well as physical and chemical defenses. However, the interactions of such stresses on trichomes, as well as their impact on herbivores, remain poorly understood. To study the effects of drought and herbivory (D×H) on leaf trichomes, we introduced soybean looper (SBL) (Chrysodeixis includens Walker), a polyphagous herbivore, to drought stressed soybeans. After assessing trichome density, the trichomes were extracted and incorporated into artificial diets to examine SBL growth traits under various leaf-diet concentrations (10% and 20%) and trichome diets (high and low). Following that, a time-to-feed experiment was performed to evaluate the effects of trichomes on SBL feeding initiation. We found that SBL started feeding faster on shaved leaves with trichomes removed than on unshaved leaves that had intact trichomes. The results also revealed that D×H induced trichomes in new leaves, which led to cascading effects on SBL growth traits. Although SBL showed a positive mass gain under the leaf diet from stressed plants, they had early pupation and eclosion under D×H leaf diet. Post-ingestion effects were evident from frass pellets under trichome diets, since the caterpillars that fed on high trichome diet also had a higher number of trichomes in their frass. In conclusion, trichomes are substantially induced by D×H and have cascading effects on SBL growth and development, exhibiting both pre- and post-ingestive effects. The highly inducible nature of trichomes in response to abiotic and biotic stresses provides the potential for future investigation in defensive priming and integration into resistance-breeding programs.