Effect of cuticle surface and external structures of Lepidoptera larvae on entrapment of pesticide droplets

Abstract

An insect's defence mechanisms vary during its different development stages. Certain stages are more susceptible to insecticides than others, with the cuticle that varies in its composition. Insects can also develop resistance to insecticides. The mechanisms of insect resistance include altered cuticles that reduce the penetration of insecticides, reduced sensitivity of the target site, and increased activity or level of detoxification of enzymes. Toxins are absorbed more slowly by insects that evolved penetration resistance compared to susceptible insects. This may be due to the outer cuticle that, for example, may be thicker in resistant insects, provide a barrier to insecticides. These barriers reduce the absorption of harmful chemicals into their bodies. A thickened cuticle layer could further delay penetration of insecticides. Wetting ability of the cuticle of insects is also important for effective control with spray applications of insecticides. Wettability is affected by the cuticle surface structure, as well as protuberances on the cuticle. The aim of this study was to examine the mesonotal cuticle thickness of different instar larvae of Spodoptera frugiperda (J.E Smith) (Lepidoptera: Noctuidae), as well as the cuticle surface and protuberances of S. frugiperda, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) and Busseola fusca (Fuller) (Lepidoptera: Noctuidae) larvae. The role of external cuticle structures in droplet entrapment was also observed. Third to sixth-instar larvae from the respective species were used in this study. Larvae were freeze-dried, after which the head and thorax were removed and prepared to capture micrographs of the pronotal surface appearances and to measure cuticle thickness using a scanning electron microscope (SEM). The cuticular surface topography differed between the species. Protuberances on the cuticle of H. armigera included large, closely spaced conical protuberances. The bumps present on the cuticular surface of S. frugiperda were absent on the cuticles of H. armigera, C. partellus and B. fusca. The cuticle of later instar S. frugiperda larvae (fifth- and sixth-instar) was significantly thicker than the cuticles of third- and fourth-instar larvae, which may contribute, together with other mechanisms, to older larvae being more tolerant to insecticides. The wax-coated spikes and/or bumps on the cuticle surface alter the hydrophobicity or hydrophilicity and allow non-waxy structures to efficiently capture water droplets. Third and sixth-instar larvae from the respective species were sprayed with water, as well as water containing a non-ionic wetting agent. Stereomicroscope images showed that droplets were entrapped by the roughness of the cuticle, as well as by setae on the cuticle surface. Surface hydrophobicity was reduced when a surfactant was added to the spray mixture. It is therefore important to adhere to label recommendations of insecticides and use the correct nozzle to obtain droplets of the recommended size for effective covering of lepidopteran pest larvae. The addition of a wetting agent was also observed to increase wettability and its addition to spray mixtures may improve effective covering of the cuticle of the target insect.