Leaf orientation and biomechanical properties do not explain western flower thrips (Franliniella occidentalis) preference for feeding on the undersides of leaves

Traditionally, breeding plants for resistance to phytophagous insects does not require knowing which plant traits convey resistance. However, with advances in modern molecular techniques and the increasing trend to forego the breeding process in favor of transgenic events, there has been a renewed interest to identify specific plant traits that convey resistance to herbivory. Western Flower Thrips (WFT) is a significant pest of early season cotton and commonly feed on the undersides of cotton cotyledons. This study was undertaken to identify the plant trait(s) that govern the feeding decisions of WFT. First, we determined that manipulating the orientation of an excised cotyledon such that the abaxial side is face-up and adaxial side is face-down did not change the preference for feeding on the abaxial side. These results supported the hypothesis that plant traits, rather than negative phototaxis, explain thrips feeding preferences. Next we investigated whether resistance to mandible and/or maxillary stylet penetration affects feeding preference by comparing leaf biomechanical properties (force to fracture) between the adaxial and abaxial surfaces of cotyledons. These properties were measured using a penetrometer and results were correlated with a no-choice feeding bioassay where thrips were forced to feed either on top or bottom only. Thrips feeding only on the top surface fed significantly less compared to those feeding only on the bottom surface. While this feeding pattern was consistent for two different plant genotypes, leaf surface biomechanical properties between genotypes were variable and did not correlate with plant feeding.