5 EFFICACY

THE BIOCIDAL effect of a fumigant depends not only on the concentration that reaches the specific insect, but also on the time that the fumigant is present and the physiological response of the species. Certain stages of insect development, such as the egg, have slow metabolic processes, which require a longer exposure time (Kenaga 1957). Thus, most efficacy studies report the integrated concentration (concentration � time) required to kill a given species at a specific developmental stage. Temperature and humidity also affect the efficacy of a fumigant. At several exposure lengths and temperatures, sulfuryl fluoride was found to be more toxic than the same concentration of methyl bromide for beetles (Kenaga 1957).

Efficacy studies have determined the concentrations of Vikane required to kill at least 95% (LD95) of the adults for various species in a 16-hour period (table 4). The LD95 on all the life stages of the insects tested, except for the egg stage, are less than Dow's recommended dosages, which are calculated based on the conditions for the fumigation (Dow Chemical Co. 1982). Thus, the recommended concentrations for fumigation in a contained space should be more than sufficient to kill most pests. In fact, much lower concentrations may be used to kill some insects and therefore minimize the amount of Vikane available for deleterious reactions to materials. The University of Florida has conducted efficacy measurements to determine the minimum dosage required for several beetle species, commonly found in museums, at each stage of their development (Su 1990). In addition to the pests listed in table 4, Vikane has been used successfully on postembryonic forms of the following species: Oriental and brown-banded cockroaches, death watch and powder post beetles, clothes moths, old house borers, fleas, rodents, ants, bees, wasps, spiders, millipedes, bedbugs, silverfish, springtails, earwigs, fire brats, and book lice (Dow Chemical Co. 1982); southern armyworms and Mexican bean beetles(Kenaga, 1957); drywood termites (Bess and Asher 1960); and brown dog ticks (Roth 1973).

Table 4 Efficacy Studies on Vikane for Selected Insects

Beetle eggs can require as much as 76 g Vikane/m3 air in order to kill them (Kenaga 1957). One study found that it took at least 24 hours for the fumigant to penetrate the shell of some termite eggs (Outram 1967a). Outram (1967b) suggested that this resistance is due mainly to the impermeability of the egg shell to the fumigant, with most of it being chemically held by the outer protein layer of the egg shell as well as the embryonic membranes.

Sulfuryl fluoride may have multiple biochemical effects on a target species. One study, on insect eggs of the desert locust and yellow mealworms, indicated that Vikane gas reduced the egg's oxygen uptake in addition to disturbing its phosphate balance and partially suppressing the hydrolysis of several fatty acids (Outram 1970). Another study indicated that fluoride was the primary toxin and that the glycolysis process was blocked, which inhibited the insect's mechanism to maintain a sufficient source of energy (Meikle, Stewart, and Globus 1963).

Like other fumigants, Vikane has no residual pest control effects. Care must be taken that the collection is not reinfested.

The microbiological effectiveness of Vikane has not previously been examined. Current work indicates that the presence of Vikane decreases the activity of some bacteria and fungi (tested on Aspergillus niger, Aspergillus flavus, Actinomyces sp., Penicillium sp., and Bacillus subtilis, cultured from proteinaceous materials), but that it does not kill the spores when applied at the concentration levels recommended for insects (Valentin 1950). In this respect, Vikane is less effective than ethylene oxide as a sterilizing agent.