While ‘real-world’ data is limited, the results of a recent study look promising.
Over the years in working with spiders, the question has occasionally arisen: Is fumigation an effective control measure? A few pest control operators were of the opinion that spiders did not always succumb to fumigation; however, this may have been based on one or two anecdotal observations and not on monitored fumigations in which sufficient dosages were applied. Additionally, homeowners with brown recluse infestations periodically asked whether fumigation would rid their home of this pest spider.
Recently, a few articles in the media and pest control trade journals have discussed the use of fumigation for brown recluse spider control. Data on spider fumigation overall is sparse in the literature; publications on brown recluse spiders do not mention fumigation as a potential control measure. In part, the lack of this data is due to the fact that if a structure is being fumigated, it would seem highly unlikely to get permission from the owner to allow the purposeful introduction of caged brown recluses into the building for experimental reasons.
However, good fortune presented itself in that a research building to which we had access was being fumigated for termites. Having a temporary excess of lab-reared brown recluse spiders in a quarantine facility allowed exposure of the spiders to a commercial fumigation to try to generate data that would be of use or interest to the pest control industry. First though, a review of the known information on laboratory spider fumigation is beneficial.
In 1994, Drs. Ellen Thoms of Dow AgroSciences and Rudi Scheffrahn of the University of Florida summarized several decades of extensive laboratory trials evaluating the efficacy of sulfuryl fluoride for control of 39 species of insects, one tick species and two spider species (brown recluse and southern black widow) in test chambers.
Where feasible, researchers also tested eggs or egg sacs of the same species. The measure that researchers calculated was the Lethal Accumulated Dosage (LAD99), which is the quantity of fumigant necessary to kill 99 percent of the animals tested. Dr. Scheffrahn conducted the research on spiders. At 81°F, the LAD99 for the southern black widow was 82 ounce-hours per 1,000 ft3 and for the brown recluse the value was similar, 77 ounce-hours per 1,000 ft3.
At this temperature, these dosages convert to about 1.7x the rate for controlling drywood termites. One consistent result from their extensive review was that eggs (almost all from insects) compared to other life stages required a higher fumigant dosage for control because the egg shell and membranes reduce the penetration by sulfuryl fluoride. With the southern black widow at 81°F, the LAD99 was 300 ounce-hours per 1,000 ft3 for eggs, which is 3.6 times higher than the dosage required to kill the spider. Brown recluse egg sacs were not tested due to the extreme difficulty in getting a sufficient number of egg sacs of the same age in order to test.
A Unique Opportunity.
Fast forwarding to the present and an opportunity arose to expose brown recluses to a commercial fumigation. Special fumigation cages were made with household aluminum window screen hot glued to the bottom and lid of a plastic vial. These vials were to be placed in a plastic shoe box that had a rectangular hole cut in the lid which also also was covered with the same window screening, hot glued to the lid, leaving no gaps for potential escape. Brown recluses were introduced, one spider per vial, with four vials (two males and two females) per shoe box. In addition to the spiders being enclosed in vials, as multiple precautions against escape, the inner top edge of the shoe box was coated with a layer of petroleum jelly, and the lid of the shoe box was secured with duct tape, ensuring that escape was highly unlikely.
Shoe boxes were placed in three rooms in the building at low, medium and high heights. This placement was done to validate that the fumigant does not stratify within rooms. For untreated controls, a shoe box with four brown recluses was placed in each of the three rooms and double-wrapped in Nylofume bags. Nylofume bags are used to contain any material that should be excluded from the fumigated space. Another 12 brown recluses were maintained in the quarantine facility as additional untreated controls. (The spiders in quarantine were just a check in case any spider died of natural causes during the test.) In total, 36 brown recluses were exposed to fumigant and 24 were untreated.
In addition, 15 brown widow spiders were placed in plastic cups with screen lids and placed in the three rooms exposed to fumigation, five spiders per room, with five additional brown widows double-wrapped in Nylofume bags and five others placed in the quarantine facility. Although brown widows were not likely to require fumigation for control because they are almost always found outside a home, these were tested because they were available and provided data on a second spider species.
A small vial with 10 drywood termites also was placed in one of the fumigated rooms with another vial of 10 termites left in the quarantine facility. The spiders and termites were placed in the building without the knowledge of the fumigation company so that the procedure would be similar to what would be expected during a typical fumigation for drywood termite control.
The building was stucco-faced with two floors consisting of 5,140 ft2 of total area and about 134,000 ft3 of volume. Following application of chloropicrin, Vikane fumigant (99.8 percent sulfuryl fluoride) was introduced at a dose of 9.8 ounces per 1,000 ft3 for a 25-hour exposure with a target accumulated dosage of 117 ounce-hours per 1,000 ft3. The required dosage was based on the target pest and a soil temperature of 71°F for the crawlspace of the building. The fumigation was monitored using an RDA Fumiscope, made by Key Chemical and Equipment Company. The actual accumulated dosage was slightly more than 160 ounce-hours per 1,000 ft3, which translated into 1.7x the drywood termite dosage.
Spiders were checked for survival about 30 hours after the building was aerated using the California Aeration Plan and clearance testing was conducted. Of the 36 brown recluses exposed to sulfuryl fluoride, all 18 males and 13 of 18 females were dead with the remaining five females being moribund. These five were dead within a week. Delay in mortality following fumigation with sulfuryl fluoride also has been observed in insects. All brown widow spiders and termites were dead upon the first check. All untreated spiders survived whether they were double-wrapped in bags or in the quarantine facility. No spiders escaped their vials.
This study provided several bits of information. First, it showed that a normal commercial fumigation using 1.7x the drywood termite dosage rate is sufficient to kill brown recluse and brown widow spiders. Second, the 1.7x rate was also the dosage rate that was calculated to kill brown recluses and southern black widows in laboratory tests, which corroborates the lab results obtained by Dr. Scheffrahn. Third, it also corroborated the results that sulfuryl fluoride reaches all levels of a room and does not stratify because spiders at all levels were killed.
However, as useful as this information might be, it needs to be accompanied by a cautionary warning. Although this commercial fumigation did provide control of brown recluse spiders, the unknown factor is how much fumigant is necessary to kill the contents of egg sacs. Considering that eggs require higher levels of fumigant, it would be very unsatisfying to eliminate the adult brown recluse spider population in a structure only to have the eggs survive and eventually repopulate the building.
Because of this uncertainty and the lack of scientific data on egg susceptibility, when a Florida house infested with Chilean recluse spiders was fumigated, the fumigators used the legal maximum limit of fumigant (10x the drywood termite dosage rate) to eliminate all life stages of the pest. So, the keystone for successful recluse spider fumigation may rely on controlling the egg stage. To ensure fumigation firms are successful in eradicating brown recluse infestations, additional studies are planned to determine if less than a 10x dosage rate will control brown recluse eggs.
The author is a retired staff research associate with the University of California, Riverside, and one of the country’s leading authorities on spiders.
Thoms, E. M., and R. H. Scheffrahn. 1994. Control of pests by fumigation with Vikane gas fumigant (sulfuryl fluoride). Down to Earth 49(2): 23-30.
Vetter, R. S., M. S. Hoddle, D.-H. Choe and E. Thoms. 2014. Exposure of brown recluse and brown widow spiders (Araneae: Sicariidae, Theridiidae) to a commercial sulfuryl fluoride fumigation. Journal of Economic Entomology 107: 1813-1817.