622 Toxicity of Indian Mustard (Brassica juncea) and Allyl Isothio-cyanate to Masked Chaffer Beetle Larvae (Cyclocephala sp.)

The development of alternative methods for control of soilborne pathogens is imperative since the U.S. Clean Air Act bans the use of methyl bromide after 2005. One possibility is to exploit the pesticidal properties of compounds released by macerated Brassica tissues. In this study, masked chaffer beetle larvae were placed in sealed 473-mL jars with 335 g of soil amended with 1%, 2%, 4%, or 8% (g·g–1) Brassica tissue. The most prevalent volatile toxic compound of Brassica juncea (PI 458934) is allyl isothiocyanate (AITC). AITC production was measured in the jars at 0.25, 4, 8, 24, and 48 h using a solid-phase microextraction device (SPME) and gas chromatography. After 7 days, larvae mortality was determined. Control treatments included untreated soil, soil amended with 8% tomato plant tissue, soil amended with pure AITC, and untreated soil with an atmosphere of ≈20% O2 and 0% CO2 changing over 48 h to 2% O2 and 20% CO2. AITC levels were positively correlated to larvae mortality. The estimated lethal concentration for 50% kill (LC50) was 3.6 μg AITC/L soil atmosphere. AITC levels may be influenced by Brassica mass added, soil bulk density, and environmental factors including temperature and moisture. B. juncea has a high tissue AITC concentration. However, the mass of Brassica tissue required for insecticidal application against Cyclocephala sp. is also high, between 4% and 8% of soil mass. Development and selection of Brassica species that produce higher concentrations of isothiocyanate would increase the effectiveness of Brassica biofumigation as an alternative to methyl bromide for controlling soilborne insects.