Nematicidal Properties of Iodinated Hydrocarbons

A.G. Appel and R. Rodríguez-Kábana, Department of Entomology and Plant Pathology, Auburn University, Auburn, Alabama 36849.

Methyl bromide has been used as a fumigant against a variety of pest species that attack crops, perishable commodities, stored grains, wood products, and structures. Unfortunately, methyl bromide has a number of undesirable characteristics including destruction of the Earth's ozone layer and will eventually be removed from general use. The need for effective fumigants, however, remains. We have investigated the insecticidal and nematicidal properties of several low molecular weight [C1–C8] iodinated hydrocarbons to determine their potential as soil and structural fumigants. Iodinated hydrocarbons are volatile, toxic, and do not affect ozone.

We examined the insecticidal properties of iodinated hydrocarbons in laboratory experiments with American, Periplaneta americana and German, Blattella germanica cockroaches. Groups of cockroaches were confined in 0.95-liter glass jars with a small cotton ball. Exactly 50 µl of a test chemical was applied to the cotton ball and the jar was sealed. Knockdown, defined as the inability to walk in a coordinated manner, and mortality were assessed every 15 minutes for the first hour and hourly thereafter for 8 hours and again after 24 hours. Mono-iodo compounds [C1–C3] were generally more effective than di-iodo compounds against both species. Iodomethane, iodoethane, and 1-iodopropane knocked down 100 percent of cockroaches within 1 hour after treatment and caused 100 percent mortality within 2 hours. Di-iodomethane knocked down 40 percent of cockroaches within 1 hour and killed 100 percent of cockroaches by 4 hours. American cockroaches were more sensitive to 1,6-diiodohexane and 1,5-diiodopentane and iodoform than German cockroaches. Diiodomethane and 1,4-diiodobutane were repellent to German cockroaches. Several diiodinated compounds did not cause mortality even after 24 hours of continuous exposure.

We also assessed the nematicidal properties of iodinated hydrocarbons in greenhouse experiments with field soil infested with Meloidogyne arenaria and Heterodera glycines. Chemicals were added directly to infested soil at rates <100 mg/Kg soil. Treated soil was kept moist (approximately 60 percent field capacity) and uncovered for 7–10 days when samples were collected and the pots were planted with 'Brim' soybean, Glycine max. Diiodo compounds were generally more effective in controlling nematodes and reducing root gall formation by M. arenaria than the monoiodo hydrocarbons; however, there were significant differences in nematicidal properties among the diiodo compounds. 1,2-diiodoethane (C2), 1,3-diiodopropane (C3), 1,4-diiodobutane (C4), 1,5-diiodopentane (C5), 1,6-diiodohexane (C6), and 1,8-diiodobutane were applied to soil at rates of 2.5, 5.0, 7.5, and 10 mg/Kg soil. All rates of C2 and C8 were ineffective in reducing M. arenaria juvenile (J2) populations in pre-plant samples. This was also true for C2 and J2 populations of H. glycines; however, C2 rates >5.0 mg suppressed J2 populations of H. glycines. C3, C4, C5, and C6 applied at rates of >5.0 mg virtually eliminated J2 populations of M. arenaria and H. glycines in pre-plant samples. H. glycines J2 numbers in final soil samples were generally inversely related to rates for C3–C8. This pattern of suppression in J2 numbers was also applicable to M. arenaria in the final soil samples, but only for C3–C6. C2 applications resulted in either no change in H. glycines J2 numbers or in increased numbers. Final numbers of M. arenaria J2's in soil increased directly with C2 dosage, but did not change significantly in response to C8 rates. Applications of C3–C6 to soil increased shoot weights in a manner directly related to dosages, but in a curvilinear pattern. Number of galls/g root and root-knot index values declined proportionately to increasing rates of C3–C6, but the opposite was observed for C2 and C8. Results showed that C3–C6 were the most nematicidal of the iodinated hydrocarbons tested.

The toxicity of iodinated hydrocarbons differs somewhat between insects and nematodes, indicating possible different modes of action of iodinated hydrocarbon compounds between phyla. Because of their chemical properties, most notably their solubility, low molecular weight mono- and di-iodinated hydrocarbons have the potential to be mixed to make an effective insecticidal and nematicidal formulation that performs as well as methyl bromide against insects and nematodes.

Last Updated: July 24, 2000