Comparative Efficacy of Fumigants Against Hatchling Whitefringed Beetle (Coleoptera: Curculionidae) Larvae and Their Sorption by Soil

Abstract

The toxicity of the fumigants methyl isothiocyanate, carbonyl sulfide, carbon bisulfide, and methyl bromide (control) was assessed against 1st instars of the soil-dwelling pest whitefringed beetle, Graphognathus leucoloma (Boheman), by injection into sealed glass jars containing the insects. Methyl isothiocyanate was the most toxic compound, with a steep dose-response relationship, followed by methyl bromide, carbonyl sulfide, and carbon bisulfide, all of which had flatter response lines. Sorption of the fumigants by 3 contrasting soil types was determined by injecting them into the headspace of sealed glass jars containing soil at different temperatures and moisture contents, and sequentially analyzing subsequent headspace concentrations over 24 h by using gas chromatography. Methyl isothiocyanate was rapidly and strongly sorbed by all soils, with ≈1% of the starting concentration remaining after 2 h. Carbonyl sulfide was the next most sorbed fumigant (its sorption increased with time), whereas methyl bromide was sorbed least. Soil that was rich in organic matter was most sorptive. Except for high sorption of methyl bromide by dry soil, we detected little effect of temperature and moisture content on the extent of sorption. Despite the high potency against whitefringed beetle of methyl isothiocyanate, the primary bioactive degradation product of metham sodium, its rapid and high sorption precludes field use of low quantities of this costly chemical.