Innate Immune Response of Galleria mellonella (Lepidoptera: Pyralidae) Larvae to Lemongrass Essential Oil and Citral

Abstract The innate immune response in insects relies on cellular responses. In most lepidopteran species, challenging the immune system causes an increase in hemocyte numbers and promotes encapsulation and melanization of invading objects, such as parasitoid eggs or artificial objects (e.g., nylon implants). We studied the effects of carbon particles (Sumi ink) on the immune response of Galleria mellonella L. (Lepidoptera: Pyralidae) last-instar larvae. Ink injections increased total hemocyte counts in a dose-dependent manner, with the 40 and 80% concentrations being most effective in eliciting hemocyte response. Hemocyte numbers increased significantly at 4 and 6 h after ink injection. Lemongrass essential oil (0.18, 0.36, and 0.72 µg/larva) and citral (0.12, 0.24, and 0.48 µg/larva) reversed the mobilization of hemocytes by the ink. Melanization of nylon larvae implants started as early as 15 min after implant insertion and reached maximum observed levels within 1 h. The degree of melanization also was higher in larvae challenged with ink 1 h prior to implant removal, which suggests initialization of the immune response. Citral injections inhibited implant melanization both in unchallenged and ink-challenged larvae. In the former case, citral concentrations needed for effective inhibition were less (0.3 µg/larva) than in ink-challenged larvae (1.2 and 2.4 µg/larva).