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Insecticidal activity of Brassica juncea seed meal to the fungus gnat Bradysia impatiens Johannsen (Diptera:Sciaridae)

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Abstract

Fungus gnats (Bradysia impatiens) can be a serious pest especially to plants grown in confined areas, and although various methods of control are available, safer and more effective control measures are desirable. Mustard seed meal, a by-product remaining after oil removal for use as a biodiesel feedstock, contains compounds called glucosinolates that hydrolyse to insecticidal 2-propenyl isothiocyanate. Our objective was to produce a dose-response curve for making recommendations of Brassica juncea seed meal applications that will result in fungus gnat larvae control. Twenty colony-raised fungus gnat larvae were added to 20 g (226 per cm3) of potting media, and adult emergence monitored during 2 weeks using yellow sticky cards. Treatments included without meal, detoxified meal and 19 doses ranging from 0.05 to 3.0 g seed meal. A logistic model was used to predict an LC50 of 0.18 and an LC90 of 0.38 g seed meal for the 20-g pot. The amounts of seed meal required to produce the observed LC50 and LC90 were predicted to produce 0.08 and 0.17 μmol 2-propenyl isothiocyanate per cm3 potting medium, respectively. B. juncea seed meal has potential utility for the control of B. impatiens, thus warranting additional studies to determine the seed meal's chronic impact on fungus gnats, phytotoxicity and plant fertility benefits.

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