Effect of Monitoring Technique in Determining the Presence of Fungus Gnat, Bradysia spp. (Diptera: Sciaridae), Larvae in Growing Medium1

Monitoring tools for detecting the presence of fungus gnats (Bradysia spp.) are a critical part of effective IPM programs for these pests in greenhouse production systems. Research has demonstrated the effectiveness of yellow sticky cards for monitoring adult fungus gnats; similarly, potato disks placed on the surface of the growing media in pots have been shown to be effective monitoring tools for the damaging larval stages. Recently, carrot disks have been suggested as an alternative for monitoring fungus gnat larvae, but this practice has not been verified in experiments. Similarly, extension recommendations for using the potato disk method suggest monitoring durations (the time that potato disks are left on the soil surface before examination) varying from 24 to 48 h, but there are no experimental data of the effect of duration on efficiency. The objective of this study was to compare the sampling efficiency of potato and carrot disks and to determine the effect of monitoring duration on efficiency. In a laboratory experiment with 20 third-instar larvae added to peat-based growing media in 250-ml containers without plants, potato and carrot disks did not differ in efficiency, but a greater proportion of larvae were recovered after 48 h (59%) than after 24 h (34%). In a greenhouse experiment with 20 third-instar larvae added to peat-based growing media in 155-mm standard pots with Lantana camara plugs, only 4% of added larvae were recovered, with no difference in efficiency between either potato and carrot disks or 24and 48-h monitoring durations. When this experiment was repeated using 50 third-instar larvae, the proportion of larvae recovered increased from 16.6% at 24 h to 30.5% at 48 h. With a monitoring duration of 48 h, potato disks recovered significantly more larvae (38.2%) than carrot disks (22.8%). These results suggest that potato disks are more efficient than carrot disks as a monitoring device to detect fungus gnat larvae and also that a monitoring duration of 48 h was more efficient in recovering larvae than 24 h.