Location of immature stages of the mushroom insect pest Megaselia halterata in mushroom-growing medium

Experiments were conducted to examine the location of oviposition by the phorid fly Megaselia halterata (Wood) (Diptera: Phoridae) in uncased and cased compost. Clearly, a majority of the gravid females choose oviposition sites directly after entering the top layer of the compost. In uncased compost, 60% of all adults emerged from the top of four compost layers of equal thickness. When the compost was covered by a casing layer which was still uncolonized by Agaricus bisporus, oviposition was further concentrated in the top compost layer. In this situation, 91% of all adults emerged from the top compost layer whereas only 1.5% emerged from the casing. When the casing layer was colonized by mushroom mycelium, 45% of all adults emerged from the casing layer and 53% emerged from the top compost layer. Further concentration in the top compost layer and the casing layer occurred as a result of upward migration of larvae. When compost was cased after oviposition, up to 43% of all adults emerged from the casing layer. We concluded that in the control of phorid infestations with insect pathogenic nematodes, applications in uncased compost can be restricted to the upper compost layer. When compost and casing are filled simultaneously, nematode applications in the casing layer only could be considered.