Innate immune response of Galleria mellonella (Lepidoptera: Pyralidae) larvae to lemongrass essential oil and citral

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 (Lepidoptera:Pyralidae) last larval instar. Ink injections increased total hemocyte counts in a dose-dependent manner. At 40% and 80%, ink concentrations were the most effective. Hemocyte numbers increased significantly at 4h and 6h after ink injection. Lemongrass essential oil (at 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 minutes after implant insertion and reached maximum values within 1h. The degree of melanization was also higher in larvae challenged with ink 1h prior to implant removal, which suggests priming 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).