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Raymond A. Cloyd and Amy Dickinson

Fungus gnats (Bradysia spp.) are major insect pests in greenhouses. The adult stage is primarily a nuisance whereas the larval stage is directly responsible for plant injury by feeding on plant roots or tunneling into stems. Insecticides are used to deal with fungus gnat larvae in growing medium, although sometimes with limited success. This study evaluated the potential of using a soil amendment—diatomaceous earth (DE) incorporated into growing media—for controlling the fungus gnat Bradysia sp. nr. coprophila. Two experiments were conducted by testing a series of growing media containing various concentrations of diatomaceous earth, and several without diatomaceous earth. The effects of the growing media containing diatomaceous earth on both the 2nd and 3rd instars of fungus gnat larvae were determined by recording the number of adults captured on yellow sticky cards (2.5 × 2.5 cm). Based on the results obtained from both experiments, the addition of DE to growing medium, at the concentrations tested, did not negatively affect or increase efficacy against both the 2nd and 3rd instars. This suggests that incorporating DE into commercially available growing medium may not be beneficial to greenhouse producers. However, further research is needed to assess whether differential larval susceptibility and moisture content influence the ability of DE to control soil-dwelling arthropods.

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Raymond A. Cloyd and Amy Dickinson

Fungus gnats, Bradysia spp., are major insect pests in greenhouses and interiorscapes. Management typically involves the use of either insecticides or biological control agents such as entomopathogenic nematodes. Efficacy trials provide information to greenhouse producers on the effectiveness of these management options. However, a simple procedure that rapidly evaluates the performance of control products against fungus gnat larvae is needed. Because fungus gnat larvae inhabit the growing medium, excess or deficient growing medium moisture may reduce adult fungus gnat emergence, thus confounding effects from efficacy trial treatments. Therefore, it is important to determine the amount of moisture and moisture content that results in the highest recovery of fungus gnat adults. We conducted two replicated experiments in a completely randomized design using a range of initial water volumes (treatments) and two larval stages (second and third) of the fungus gnat, Bradysia sp. nr. coprophila. The success of the procedure was based on the number of fungus gnat adults that emerged per treatment. In the first experiment, initial water volumes of 0, 25, 50, 75, 100, 150, and 200 mL were applied to 300 mL of a soilless growing medium consisting of 50% composted pine bark, 20% Canadian sphagnum peatmoss, 10% perlite, and 20% medium coarse vermiculite (SB300 Universal Mix). In general, the highest mean numbers (range, 11.2 to 14.6) of fungus gnat adults were recovered from growing medium treated with 50, 75, and 100 mL of water. In the second experiment, initial water volumes (treatments) of 50, 55, 60, 65, 70, and 75 mL were applied to 300 mL of the soilless growing medium (SB300 Universal Mix). There were no significant differences in the mean number of fungus gnat adults recovered regardless of the larval instar (second instar: 15.8 to 17.7; third instar: 14.4 to 17.4). The final percent moisture content ranged from 65% to 68% for the second instars and 56% to 66% for third instars. This study demonstrates that the highest number of fungus gnat adults may be recovered from soilless growing medium (SB300 Universal Mix) treated with between 50 and 75 mL of water, thus enhancing the confidence in any data set generated when evaluating insecticides or biological control agents for control of fungus gnats.

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Raymond A. Cloyd, Amy Dickinson, Richard A. Larson and Karen A. Marley

Multiple-choice experimental arenas, with sample compartments, were used to assess the response of fungus gnat, Bradysia sp. nr. coprophila (Lintner) (Diptera: Sciaridae), adults to varying light intensities in environmentally controlled walk-in chambers. Each sample compartment contained a yellow sticky card (2.5 × 2.5 cm) to capture fungus gnat adults. Under conditions of darkness, fungus gnat adults migrated randomly with no significant differences among the six sample compartments. Fungus gnat adults were observed to positively respond to light intensities less than 0.08374 μmol·m−2·s−1. In addition, adults responded to light intensities that were below the detection threshold of a photosynthetically active radiation light sensor. A higher percentage of fungus gnat adults (22% to 39%) were captured on yellow sticky cards in the sample compartments that were closest to a directional light source compared with sample compartments that were located further away from the light source (2% to 9%). Fungus gnat adults exhibited a significant response when exposed to two distinct ranges of light intensities (0.12 to 0.26 versus 0.87 to 1.02 μmol·m−2·s−1) with adults significantly more attracted to the highest light intensities (0.87 to 1.02 μmol·m−2·s−1). The results obtained in this study indicate that fungus gnat adults are positively phototactic, and as light intensity increases, they display a preference for those higher light intensities. It is possible that modifications in light intensity may be a feasible management strategy for alleviating problems with fungus gnats in greenhouses.