Effect of a Physical Barrier on Adult Emergence and Egg Survival Associated with the Fungus Gnat, Bradysia sp. nr. coprophila (Diptera: Sciaridae), under Laboratory Conditions

This study was conducted to assess the direct and indirect effects of Growstones aggregates, which are made from recycled glass, on fungus gnat, Bradysia sp. nr. coprophila (Diptera: Sciaridae), adult emergence, female egg-laying capacity, and egg survival. A series of experiments were performed under laboratory conditions to evaluate the effect of different sizes (2.0 to 10.0 mm) of Growstones aggregates, layer thicknesses (0.63 to 3.18 cm), and the use of the biological control agent, the rove beetle, Dalotia coriaria, along with different thicknesses (1.27 and 3.18 cm) of small Growstones aggregates on fungus gnat adult emergence. For each experiment, Growstones aggregates were applied to the surface of the growing medium in 473-mL polypropylene deli containers. This study demonstrated that the thickest (3.18 cm) layer of small (2.0 mm) Growstones aggregates significantly reduced or delayed the emergence of fungus gnat adults. In addition, the thickest layer of small Growstones aggregates may have indirectly affected egg survival. However, the use of Growstones along with rove beetle adults did not significantly reduce fungus gnat adult emergence. Fungus gnats are a commonly encountered insect pest of greenhouses (Dennis, 1978; Hamlen and Mead, 1979) and are typically a problem in moist environments such as those that occur in propagation or plug production (Cloyd, 2000). Adults cause minimal direct damage to plants; however, a single female Bradysia sp. nr. coprophila Lintner (Diptera: Sciaridae) can lay 96 eggs on the surface of the growing medium (Meers and Cloyd, 2005). After eggs hatch, larvae feed on plant roots causing plant stunting, wilting, and possibly plant death (Jarvis et al., 1993; Springer, 1995; Wilkinson and Daugherty, 1970). In addition, indirect damage by larvae is associated with transmission of certain soilborne plant pathogens (Gardiner et al., 1990; Gillespie and Menzies, 1993; Jarvis et al., 1993). Currently, insecticides applied as drenches to the growing medium are an effective management strategy against fungus gnat larvae (Hamlen and Mead, 1979; Lindquist, 1994; Lindquist et al., 1985). However, as a result of the limited number of insecticides registered for use in greenhouses, and the costs associated with development and registration of new insecticides (Lewis, 1977), it is important to judiciously use the currently available compounds (Lindquist et al., 1985). Biological control agents such as the rove beetle, Dalotia coriaria Kraatz (Coleoptera: Staphylinidae), have been shown to be an effective predator against fungus gnat larvae (Carney et al., 2002; Echegaray, 2012). However, a number of insecticides are directly and indirectly harmful to rove beetles (Cloyd et al., 2009). Consequently, physical barriers may be a long-term management strategy that could also be used with biological control agents (Banks, 1976). Physical barriers are used to alter the environment so as to make it inaccessible to insect pests (Banks, 1976). There are a number of benefits associated with physical barriers such as compatibility with other insect pest management strategies including biological control and minimal impact on the environment (Boiteau, 2002). Nonetheless, physical barriers do not reduce an insect population immediately, which typically happens with most insecticides (Boiteau, 2002). A physical barrier could be placed on the surface of the growing medium to inhibit adult fungus gnat emergence or possibly reduce adult female egg-laying or egg survival (Cloyd et al., 2007a), whereas rove beetles feed on fungus gnat larvae in the growing medium. Physical barriers have been evaluated as a pest management strategy against insect pests, specifically the fungus gnat, Bradysia sp. nr. coprophila (Cloyd et al., 2007a). Previous research by Cloyd et al. (2007a) evaluated the effect of dry and moist diatomaceous earth and sand as a physical barrier against fungus gnats. Diatomaceous earth affects insects either by removing or absorbing oils and waxes on the outer insect cuticle or by abrading the cuticle causing extensive moisture loss leading to desiccation. However, when diatomaceous earth becomes moist, it loses any abrasive properties (Korunic, 1998). Cloyd et al. (2007a) determined that both dry and moist diatomaceous earth and sand formed cracks or air pockets over time, which decreased their ability to negatively affect fungus gnats. This suggests that diatomaceous earth and sand may not be effective physical barriers. Despite this, there are other materials that may act as physical barriers against fungus gnats such as Growstones (Growstone, Inc., Albuquerque, NM). Growstones aggregates are processed from 100% recycled glass and are primarily used as a hydroponic growing substrate in which plants are inserted into containers containing Growstones aggregates. A nutrient solution is applied to the container. Growstones aggregates are lightweight and can be manufactured into different particle sizes (Evans, 2011). It is possible that Growstones aggregates may serve as a physical barrier against fungus gnats in greenhouses. Therefore, the objectives of this study were 3-fold: 1) to determine the direct effect of Growstones aggregates applied to the growing medium surface on fungus gnat adult emergence; 2) to assess the indirect effect of Growstones aggregates placed on the growing medium surface on female egglaying and/or egg survival and subsequent adult emergence; and 3) to determine if using rove beetle, Dalotia coriaria, adults in conjunction with Growstones is synergistic in affecting fungus gnat adult emergence. Materials and Methods Fungus gnat colony. Laboratory-reared colonies of the fungus gnat, Bradysia sp. nr. coprophila, were located in the Department of Entomology at Kansas State University (Manhattan, KS) and maintained in 8.0-L plastic containers with tight-sealing lids. Openings ( 15 cm square) were cut in the lids and then insect screening [50 · 24 (0.2 · 0.8 mm; Greentek, Edgerton, WI)] was hotglued to the lids to allow for ventilation. The Received for publication 27 Mar. 2014. Accepted for publication 28 Apr. 2014. We thank Growstones, Inc. (Albuquerque, New Mexico) for providing the funds necessary to conduct this research. In addition, we thank Paula Costa of Growstones, Inc. (Albuquerque, NM) and Dr. Mary Beth Kirkham in the Department of Agronomy at Kansas State University (Manhattan, KS) for their valuable feedback associated with previous drafts of the manuscript. To whom reprint requests should be addressed; e-mail rcloyd@ksu.edu. HORTSCIENCE VOL. 49(7) JULY 2014 905 DISEASE AND PEST MANAGEMENT