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Humidity Response of Adult Male Oryzaephilus surinamensis (Coleoptera: Cucujidae) with Special Reference to the Effect of Carbon Dioxide

Abstract The behavioral response of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.), to moisture gradients has been studied in some detail, but the effect of carbon dioxide (CO2), alone and in combination with low humidity, has never been reported. The effect of CO2 is of some interest because CO2 levels in stored grain are often elevated well above normal atmospheric levels by the metabolism of insects, fungi, and the grain itself. Also, CO2 is sometimes applied to bulk grain to control stored-product insects. The response of adult male O. surinamensis to humidity alternatives of 60 or 100% RH was determined in 2-way choice chambers after various treatments. Beetles removed from laboratory cultures and held at 60% RH with food for 4 h showed a strong avoidance of the higher humidity (hygronegative response). Beetles held without food in a dry atmosphere (<5% RH) with an elevated level of CO2 (39%) for 4 h showed a reversal of the response from hygronegative to hygropositive. The same period of exposure, again without food, to dry air (<5% RH), or to a moist atmosphere (80% RH) with an elevated level of CO2 did not produce a reversal. The failure of CO2 to produce a reversal when exposure occurred in a moist atmosphere suggests that the gas acts indirectly on the humidity response by increasing the rate of moisture loss. Elevated levels of CO2 are known to cause prolonged opening of the spiracular regulatory apparatus that controls moisture loss in insects.