Cephalonomia tarsalis (Hymenoptera: Bethylidae) for the control of the sawtoothed grain beetle, either alone or in combination with the predatory mite Blattisocius tarsalis

Abstract

The sawtoothed grain beetle Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae) is an important pest of stored cereals. Due to the phasing out of many active compounds registered as insecticides and the occurrence of insect resistance to the remaining active compounds, it is crucial to develop alternative pest control strategies, such as the use of natural enemies. In this study, we evaluated the ability of Cephalonomia tarsalis (Ashmead) (Hymenoptera: Bethylidae), a specialised larval ectoparasitoid, to locate and parasitize O. surinamensis in vertical PVC pipes 20 cm in diameter filled with 7–26 kg of paddy rice. The parasitoid was able to move through the rice and reduce pest emergence up to 60% at depths of 40, 100, and 150 cm, even when larvae were offered simultaneously at all three depths. Since most of the pest populations are found in the upper layer of 1 m of the grain piles, this result suggests that the parasitoid would be able to locate and parasitize the pest within the stored paddy rice piles. We also evaluated the possibility of complementing pest reduction by the parasitoid with the aid of the egg-predatory mite Blattisocius tarsalis (Berlese) (Mesostigmata: Ascidae). When each natural enemy was offered its target pest instar for one week, both were able to reduce the pest similarly by 52–65%. When they were released in 2 kg of paddy rice with 10 O. surinamensis females for 10 weeks, the parasitoid achieved higher pest reduction (96%) than the predatory mite (42%). The pest reduction obtained by combining both natural enemies did not improve the efficacy of the parasitoid, suggesting that the parasitoid alone can be an effective alternative to maintain the beetle populations under control.