4 PHYSICAL PROPERTIES

FUMIGANTS ARE used to control insect infestations in inaccessible places such as within wood, walls, or other hiding places. In order to kill insects, an adequate concentration of the fumigant must penetrate all types of materials rapidly and remain in the presence of the insect life stages for a sufficient amount of time, ideally without being absorbed by the material. Vikane is effective in this regard, exhibiting high penetration rates and low absorption characteristics (table 3).

Table 3 Physical Properties of Sulfuryl Fluoride

At room temperature, sulfuryl fluoride has a vapor pressure of 13,400 mm Hg, more than eight times greater than that of methyl bromide. The high vapor pressure, low substrate reactivity, and high diffusion rate of Vikane provide rapid penetration of toxic amounts into the pest sites. Using kill efficiency studies Kenaga (1957) demonstrated that Vikane penetrates materials and soil much better than methyl bromide. (“Kill efficiency” describes the percent mortality of the insects being examined, and it is used to determine the effectiveness of the dose of fumigant that actually reaches the insect.) In his study, Vikane had 100% kill for black carpet beetle larvae and confused flour beetle adults at a depth of 9 inches of hardwood flour. Methyl bromide, at the same depth, had only 0% and 6% kill for the larvae and adults, respectively. Vikane also had a very low affinity for soil, clay, and sand; due to the low water-solubility of sulfuryl fluoride, it penetrated dry soil much better than wet soil.

A recent study looked at the desorption rates of Vikane from typical foodstuffs. Osbrink et al. (1987) found vegetable oil desorbed the most; apple, dog food, cake, and flour fell in the middle; and milk, beef, and acetaminophen desorbed the least. Comparison of the desorption amounts to the residual fluoride and sulfate found in Scheffrahn et al. (1989) indicates materials that react with Vikane have correspondingly lower desorption amounts. However, neither of these studies measured the amount of Vikane that was absorbed in a material initially; thus, a substance, such as acetaminophen, can have low desorption and low residues because of its low physical and chemical reactivity, respectively.

Another study on Vikane desorption rates for household objects found measurable amounts (>0.1 ppb) of Vikane desorbing 26 hours after fumigation (at 36 g/m3 for 20 hours, approximately 10 times the dosage used for drywood termites) in concrete blocks, gypsum/cardboard drywall, fiberglass insulation, polystyrene insulation, wood, topsoil, carpet padding, polyester cushion fiber, wool fabric, cotton fabric, baseball glove, latex baby bottle nipples, and a toy soldier (Scheffrahn et al. 1987). Forty days after treatment, ppb levels of Vikane were still desorbing from the polyester cushion, the polystyrene insulation, the wool fabric, and the toy.