Leaf disc-mediated oral delivery of small molecules in the absence of surfactant to the two-spotted spider mite, Tetranychus urticae.

The two-spotted spider mite (TSSM), Tetranychus urticae, is a chelicerate herbivore with a wide host range and strong ability to develop pesticide resistance. Experimental TSSM populations are easy to maintain, and the recent publication of the complete TSSM genome sequence and development of RNA interference-based reverse genetics protocols make this species an ideal chelicerate model for the study of pesticide resistance and plant-herbivore interactions. In such studies, treated leaf discs are often used for oral delivery of test compounds. When preparing these leaf discs, the organosilicone surfactant Silwet L-77 is used to promote wetting of the leaf surface and distribution of the test compound across the entire leaf surface. Here, we examined the toxicity of Silwet L-77 and found it to be toxic to TSSMs. We then developed a novel means of preparing leaf discs in which a polypropylene sheet rather than Silwet L-77 was used to ensure distribution of a tracer dye across the entire leaf surface. These leaf discs were then successfully used to deliver the tracer dye into the midgut of TSSMs. No significant differences were observed in the survival, fecundity, or feeding activity of TSSMs fed on leaf discs treated with water via our novel method compared with those fed on untreated leaf discs. Thus, our novel method of preparing leaf discs eliminates concerns regarding the bioactivity of surfactants in TSSMs, and we anticipate that it will be useful for improving oral delivery-based bioassays that use TSSMs.