Method for quantitative sampling of fungus gnat larvae in soilless growing media

A simple method is described for separating fungus gnat larvae from soilless growing media. Samples are first fractionated by water flotation with an inverted flask procedure and then the sediment is degassed under reduced air pressure and fractionated in magnesium sulfate (MgSO4) solution (density of 1.12 g cm(-3)). Fungus gnat larvae with only a small amount of contaminating debris are recovered from the surface of the MgSO4 solution for immediate counting or for preservation in alcohol. In evaluations of different commercial soilless growing media with a range of components, two repetitions of the water flotation step eliminated 20-40% of the dry weight of samples and virtually all of the perlite from further processing. Repeating both the water and MgSO4 flotations a third time only marginally improved the recovery of larval fungus gnats, Bradysia sp. nr. coprophila, added to pasteurized media. Extraction efficiency differed between instars and, to a lesser extent, between different types of media. Across three commercial soilless media tested, recovery was 24-33% for first, 68-85% for second, 85-95% for third, and 98-100% for fourth instars. Within combinations of media and instar, recovery was consistent. With this method, a 400-cm3 sample can be processed and be ready for counting in 1-1.5 h; samples can also be processed in batches or in assembly-line manner to process many samples per day. The method may also prove useful for quantitative recovery of shore fly larvae, thrips pupae, and other arthropods from soilless growing media.