A comparison between preventative and curative soil treatments for control of fungus gnats

Abstract

Preventative and curative insecticide treatments were assessed for efficacy against fungus gnats (Bradesia paupera) in potting soil. Insecticide solutions (20 ml per 250 ml of growing medium per 100 mm diameter pot) were either mixed into the potting medium before planting tobacco seedlings (preventative treatment) or watered into the pots immediately after planting into medium infested with fungus gnat larvae (curative treatment). Preventative treatments of fipronil (0.1, 0.2 or 0.4 g/litre) and diflubenzuron (0.25 g/litre) were equally effective over 4 weeks at preventing plant loss, maintaining plant vigour (measured by the length of the longest leaf) and keeping fungus gnat larval numbers low (<1 per 25 ml medium versus 18 per 25 ml from watertreated medium). In the curative trial, 70% of the water-treated plants died, compared to 60% of the diflubenzuron and <7% of the fiproniltreated plants. Vigour of the surviving plants was similarly affected, but larval numbers had declined in all treatments from day 0 to14 days after treatment. Measuring the effect of treatments on a susceptible test plant and on larvae in the growing medium were both important for determining the efficacy of treatments for fungus gnat control.