PII: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA SO%l-2194(98)00060-X zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM C ‘ro p f’ ro rrc fw~ Vul. Il. No 6, pp. 547-553, 1998 C 1998 Elcr~w Swnc r Ltd . A ll rig hts re se rve d Printe d in G rra l Brita in 026 I-2 194/ 9X/ $ I9.00+0.00 ELSEVIER Effect of hydrogen peroxide on algal growth, cucumber seedlings and the reproduction of shore flies (Scatella stagnalis) in rockwool Irene Vgnninen* and Heini Koskula Agricultural Research Centre (MTT), Jokioinen, Finland institute of f/ant Production, P/ant Protection, 37600 A range of hydrogen peroxide concentrations was tested to reduce the growth of green algae on rockwool with cucumber seedlings as the crop. A single application of 1 dl of 125 ppm hydrogen peroxide to l-week-old seedlings or daily applications over 3 weeks of 1 dl of 100 ppm peroxide reduced algal growth by 40-60% for 1 week, resulting in 73-92% reduction in shore fly zyxwvutsrqpon (Scatella stagnalis, Diptera: Ephydridae) emergence. Both treatments reduced the seedlings’ dry weight, but the effect of the 125 ppm dose disappeared by the end of the second post-treatment week, The appearance of Aphaereta debilitata (Hymenoptera: Braconidae, Alysiinae) in one of the experiments suggested that the species has potential as a biocontrol agent of shore flies. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: greenhouse; algicide; integrated pest management; Aphaereta debilitata introduction Shore flies (Scatella stugnalis Fall&) sometimes occur in large numbers in greenhouses, where they are considered as nuisance and cosmetic pests. They are principally algal feeders in both the larval and adult stages (Zack and Foote, 1978; Connell and Scheiring, 1981; Blair and Foote, 1984), reproducing quickly in short term crops with a rapid turnover of the substrate units. This results in continuous availability of fresh algae for feeding. A single efficient method for controlling shore flies is still lacking (Gouge, 1994; Lindquist and Casey, 1994a,b; Lindquist et al., 1994; Koskula, 1996). The elimination of algae is often proposed as a solution to the problem (McHugh, 1991), but the phytotoxicity of algicides often creates a problem. Shore flies constituted a serious nuisance in the greenhouse operation of a Finnish producer of cucumber (Cucumis sativus) seedlings. Before being sold, the seedlings were grown in the greenhouse for 23-24 days, giving two fly generations time to emerge in the greenhouse. The production system was then changed, and the seedlings were grown in small sowing cubes in a separate room for 1 week and then transplanted to rockwool cubes in the actual produc- tion greenhouse (rockwool is an inert fibrous *Corresponding author. Tel. +358-3-418 8580; Fax +358-3-418 8584. Email: irene.vanninen@mlt.fi inorganic growth substrate produced by burning a mixture of coke, basalt, limestone and possibly slag from iron production (see Nelson, 1991). This change shortened the exposure of the substrate to algal growth and thus the egg-laying period of flies from 3 to 2 weeks, resulting in only one fly generation emerging in the room. Combining this procedure with the use of large yellow sticky traps on the walls and strict hygienic measures achieved a considerable reduction in the population size of shore flies (Vtinninen and Koskula, 1996). A further decline in fly numbers was desirable, however, to reduce the number of pupae still in the substrate units when the seedlings were sold. A product containing hydrogen peroxide named Reciclean@ (Kemira Agro Rozenburg B.V., the Netherlands), is available in Finland for disinfecting water pipes and drip irrigation systems in green- houses. The product is not specifically registered for controlling algae, but due to its mode of use, algae growing on the substrate would be controlled if it turned out that the product also has algicidal proper- ties. Therefore, we wanted to test whether hydrogen peroxide could be used to inhibit algal growth on rockwool. Several hydrogen peroxide concentrations were screened in five experiments in 1995 and 1996 to find a concentration that would inhibit algal growth and prevent or markedly reduce egg-laying of shore flies for at least 1 week without being phyto- toxic to cucumber seedlings. Crop Protection 1998 Volume 17 Number 6 547