Penetration and permanence of an organophosphate and a pyrethroid in structural wood for insect control

Studies were conducted on the penetration and permanence of 1.0% chlorpyrifos [0,0-diethyl 0-(3,5,6-trichloro-2-pyridyl) phosphorothioate], formulated as Dursban® TC (Dow Chemical Co.), in pine (Pinus spp.), spruce (Picea spp.), and fir (Abies spp.). Results indicated the highest degree of penetration in southern yellow pine. Surface roughness and orientation had an effect upon chemical penetration. Longevity studies ranging over time periods from three to 52 months revealed rapid loss of chlorpyrifos from the surface of the wood but relatively stable below-surface concentrations.

Similar studies with 0.5% permethrin [(3-phenoxypheny]) methyl] (+) - cis, trans-3-(2,2-dichloroethenyl)-2,2dimethylcyclopropane-carboxylate] applied to the same wood types revealed comparable penetration differentials between types of wood. Formulation differences had little significant effect on penetration. One-year longevity studies revealed surface losses less drastic than those reported for chlorpyrifos, as well as superior below-surface stability.

Studies employing volumes five, 10, and 25 times less than that required to achieve a runoff application revealed significantly lower chlorpyrifos penetration with reduced volume. Some reduced volumes resulted in below-surface concentrations of chlorpyrifos higher than the LC₉₀ value for Hylotrupes bajulus (Coleoptera: Cerambycidae).

Toxicity testing with chlorpyrifos on first-instar larvae of H. bajulus, the old house borer, revealed contact LC₅₀ and LC₉₀ values of 14 ppm and 78 ppm, respectively. Similar tests with permethrin indicated LC₅₀ and LC₉₀ values of 2 ppm and 8 ppm. Tests with first-instar larvae on wood treated with 1.0% chlorpyrifos and 0.5% permethrin resulted in 100% mortality on wood aged for nine months in the case of chlorpyrifos, and six months in the case of permethrin. Tests utilizing H. bajulus eggs placed on treated wood revealed ovicidal properties of permethrin and lack of ovicidal effect with chlorpyrifos.

Accelerated ageing studies employing an environmental chamber (40°C, 70% RH) at a 1:6 ratio of accelerated to natural weathering indicated no significant difference in surface or below-surface concentrations of permethrin in wood aged artificially for one month and in wood aged naturally for six months. Similar results were obtained with chlorpyrifos-treated wood artificially aged for 10 months and aged naturally for 4-1/3 years. Mathematical models were developed to predict surface and one millimeter concentrations of chlorpyrifos and permethrin after five and 10 years.