Effects of Growing Medium Type and Moisture Level on Predation by Adult Rove Beetle, Dalotia coriaria (Coleoptera: Staphylinidae), on Fungus Gnat, Bradysia sp. nr. coprophila (Diptera: Sciaridae), Larvae under Laboratory and Greenhouse Conditions

Abstract

The fungus gnat, Bradysia sp. nr. coprophila (Lintner) (Diptera: Sciaridae), is an insect pest of greenhouse production systems. The rove beetle, Dalotia coriaria [Kraatz] (Coleoptera: Staphylinidae), is a commercially available predator of certain greenhouse insect pests that reside in growing media, including fungus gnats. There is minimal information discussing how growing medium type and moisture level (watering treatment) impact the interactions between pests and natural enemies. Therefore, we conducted laboratory and greenhouse experiments to investigate the influence of two growing media (Sunshine® LC1 Professional Growing Mix and Fafard® 3B Mix Professional Formula) and two moisture levels (“constantly saturated” and “initially saturated”) on predation by adult D. coriaria on B. sp. nr. coprophila larvae after releasing one or two rove beetle adults. In the laboratory experiment, moisture content or the amount of water retained by the growing medium did not significantly influence the recovery of adult fungus gnats for any of the rove beetle treatments. However, there was a significant difference in the recovery of fungus gnat adults between the two growing media. Fewer fungus gnat adults emerged from the Sunshine® LC1 Professional Growing Mix (0.9 ± 0.2 adults) than the Fafard® 3B Mix Professional Formula (6.0 ± 0.9 adults). Significantly fewer adult fungus gnats were recovered in the treatments where one rove beetle adult was released (2.7 ± 0.7 adults) and two rove beetle adults were released (2.3 ± 0.5 adults) compared with the control without rove beetles (5.4 ± 1.4 adults). However, there was no significant difference between the number of rove beetle adults released. In contrast to the laboratory experiment, moisture content in the greenhouse experiment significantly influenced the recovery of adult fungus gnats. More adult fungus gnats were recovered from the “constantly saturated” treatment (9.9 ± 1.4 adults) than the “initially saturated” treatment (3.8 ± 1.0 adults). Similar to the laboratory experiment, there was a significant difference in the recovery of fungus gnat adults between the two growing media, with fewer adults captured from the Sunshine® LC1 Professional Growing Mix (3.2 ± 0.8 adults) than the Fafard® 3B Mix Professional Formula (10.4 ± 1.4 adults). However, the treatments with rove beetle adults [one rove beetle (6.6 ± 1.8 adults) or two rove beetles (5.3 ± 1.5 adults)] were not significantly different from the control without rove beetles (8.6 ± 1.5 adults), suggesting that the growing media and moisture levels were acting directly on fungus gnat survival. The results of our study demonstrate that survival of fungus gnat larvae that reside in the growing medium and the success of rove beetle adults used to regulate these pests can be influenced by growing media and the moisture content within growing media.