Penetration and Permanence of Permethrin in Four Softwoods

We evaluated the 24-h penetration and 6-mo permanence of a 0.5% (AI) dilution of permethrin into southern yellow pine (SYP) ( Pinus spp.), northern hard pine ( Pinus spp.), spruce ( Picea abies ), and fir ( Abies spp.) after surface application. Three- to 12-mo permanence was evaluated only with SYP. Penetration with xylene and nonxylene solvents was evaluated at 24 h. Insecticide (220 μl) was applied to the surface of wood blocks (4 by 5 by 1 cm). After 24 h in the laboratory, or 3 or 6 mo in an attic or crawl space, the center cubic centimeter was removed and sectioned (50 μm per slice) from the treated surface to a depth of 2 mm. Permethrin concentration in each slice was measured by gas chromatography. Penetration of permethrin to 950 μm was not significantly different in SYP than in the other softwoods. At 1600 μm, penetration was not significantly different in SYP, northern hard pine, and fir, but was significantly less in spruce. At depths in wood below 1,200 μm, only the cis isomer of permethrin was detected. At 1,200-, 1,600-, and 2,000-μm depths, more permethrin was present in SYP blocks stored in the attic than in blocks stored in the crawl space. The amount of permethrin at the surface in SYP decreased after 3 mo, but no further decrease after 6 mo in the attic and crawl space was apparent. After 12 mo, a significant decrease in the amount of permethrin at the surface of SYP occurred. Penetration of permethrin in SYP did not differ among solvents; at 950 μm, a significant difference between one nonxylene and a xylene formulation was observed.