Inhibition of oenocytoid lysis of the common cutworm Spodoptera litura (Lepidoptera: Noctuidae) by carbon dioxide gas

Among the hemocytes of Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae), oenocytoids are lysed very quickly after hemolymph collection. We clarified that the lysis of oenocytoids can be greatly inhibited by collecting hemolymph under the introduction of carbon dioxide (CO₂) gas. Since nitrogen and helium gasses were ineffective in inhibiting oenocytoid lysis, it was concluded that CO₂ itself was necessary to inhibit lysis. Insect anesthetization with CO₂ gas before bleeding was not necessary to prevent oenocytoid lysis, but CO₂ gas was required at and after the time of bleeding. When the hemolymph was collected in a Petri dish under CO₂ gas and sealed, oenocytoids remained present without lysis even after an hour at 25 °C. Oenocytoids were not irreversibly fixed since lysis occurred when this hemolymph was exposed to air. When the hemolymph was collected in air, granulocytes adhered to the bottom surface of the Petri dish, and plasmatocytes spread out on the bottom surface. However, when collected under CO₂ gas, both types of hemocytes remained spherical, and few cells were observed to spread out on the bottom surface. Thus, it was suggested that oenocytoid lysis inhibition by CO₂ gas is associated with the suppression of defensive reactions to foreign materials by hemocytes. This hemolymph collection method can be easily performed without special pretreatment of the hemolymph. Accordingly, this method is considered to be advantageous for studying the possible role of oenocytoids associated with host defense mechanisms in insects.