Aboveground thrips herbivory alters root morphology more in non-Bt cotton than in Bt cotton (Mpp51Aa2)

Tobacco thrips ( Frankliniella fusca Hinds) is a major pest of cotton seedlings, leading to substantial economic losses in the United States. Aboveground thrips herbivory has been reported to indirectly reduce cotton root length, yet the relationship between thrips infestation levels and cotton root development remains poorly understood. Bt cotton expressing the Mpp51Aa2 toxin (ThryvOn) can greatly reduce aboveground thrips injury. However, it is unclear whether the root growth responses of ThryvOn cotton differ from those of non-ThryvOn cotton under thrips herbivory. To address these knowledge gaps, greenhouse experiments testing the effects of thrips herbivory on ThryvOn and non-ThryvOn cotton were conducted. Seedlings were infested with 5, 10, or 15 adult female F. fusca , with a non-infested control. After 10 days, several root parameters were measured using a WinRHIZO root-scanner and associated software. Belowground seedling attributes were compared to aboveground seedling growth. We found that root development was impaired as the density of thrips infestation increased. Root traits in non-ThryvOn cotton, including belowground biomass, projected area, root volume, surface area, and total dry mass were reduced when compared to ThryvOn cotton. The root-to-shoot ratio and root mass fraction decreased in non-ThryvOn cotton relative to ThryvOn cotton. Specific root area increased in non-ThryvOn cotton, while root tissue density decreased in ThryvOn cotton. Specific root length did not follow a linear trend and tended to increase at high infestation levels. These findings demonstrate ThryvOn’s ability to tolerate early-season thrips herbivory by maintaining root development, reinforcing that insecticide inputs may be unnecessary. • Early thrips injury reduces cotton root growth and development • Cotton varieties show distinct root economics spectrum strategies • ThryvOn exhibits a conservative resource-use strategy under thrips herbivory • Under thrips herbivory, non-ThryvOn adopts a more acquisitive strategy • Managing early-season insect pests can protect root system development