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 1-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 (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. none