203 Improving Control of Duponchelia fovealis (Lepidoptera: Pyralidae) by Rooting Media Related Strategies C. Blok and G.J. Messelink Wageningen UR Greenhouse Horticulture Bleiswijk The Netherlands Keywords: soil-dwelling predators, Hypoaspis miles, compost, degradation rate, oxygen uptake rate, water content, cork mulch Abstract Soil-dwelling predatory mites can be very effective as biological control agents against larvae of the lepidopteral pest Duponchelia fovealis. Some growing media were reported to have natural high level and stable populations of predatory mite. The objective of this experiment was to define conditions to establish stable predatory mite populations in the rooting medium and to assess the direct effect of the rooting media on pest development. Eight rooting media were prepared, including a range of degradabilities as measured with the Oxygen Uptake Rate method (OUR). The OUR range was created by mixing peat products, coir dust, bark, perlite, compost and wood fiber. Each treatment was split: half with and half without a commercially used mulch to create a drier top layer. Kalanchoës were grown on these rooting media. After one week soil-dwelling predatory mites (Hypoaspis miles) were added. Adults of the pest Duponchelia fovealis were released during a number of weeks. Both populations were counted. Results show that the OUR range was successfully achieved. The commercial mulch, a cork based fine granulate, reduced the numbers of Duponchelia by 32%. The number of predatory mites was related to the oxygen uptake of the rooting media (R 2 =0.87). The predatory mite reduced the numbers of Duponchelia larvae on average by 58%. Thus, biological control by soil-dwelling predatory mites can be improved by offering rooting media with an increased degradability as measured by the oxygen uptake rate. The combined effects of using predatory mite and mulch layers are discussed. INTRODUCTION A number of pests in greenhouse crops are related to soil or rooting media. One of those is the relatively new, but widespread pest Duponchelia fovealis Zeller (Lepidoptera: Pyralidae) (Romeijn, 1992). The larvae of this moth prefer to live in a moist soil layer where they feed on either plant parts or organic matter. Biological control of this pest has proven to be very effective with soil-dwelling predatory mites. The commercially available species Hypoaspis miles (Berlese) was the most successful predator of the egg stages of D. fovealis (Messelink and van Wensveen, 2003). The success of controlling D. fovealis by these predatory mites however is variable. It was found that population levels of these mites vary between the different rooting media used (Messelink, pers. obs.). Soil- dwelling predatory mites are on the top of complex food webs (Coleman and Crossley, 1996) and their establishment is consequently related to the number of soil micro- arthropods that are suitable as prey organisms. In theory it might therefor be possible to upgrade levels of predatory mites and improve biological control by stimulating soil microarthropods. In this study it was hypothesised that a rich soil fauna is related to the turnover of organic material into carbohydrates and nutrients. Organic materials with a relatively high degradation rate such as composts and wood fiber might, as a result of a richer soil micro life, support larger predatory mite populations than slower degrading rooting media as peat and coir. To examine this hypothesis, rooting media mixes were prepared, ranging from easily decomposable material to very stable material. It was recognized that by using increasing amounts of compost, the water content, salt level and possibly pH would Proc. IS on Growing Media 2007 Eds.: W.R. Carlile et al. Acta Hort. 819, ISHS 2009