Researchers in the US have investigated the effect of steam on dust treatments, with the results giving some helpful pointers on how best to carry out bed bug treatments.
Cimex lectularius, the common bed bug, is undoubtedly one of the most challenging urban pests to control. Although many non-chemical treatment options exist, such as vacuuming, interceptor traps, mattress casings, and steam applications, these options are unable to provide residual control. An insecticide spray or dust treatment can be a cost-effective control method, but numerous studies have documented the rise in pyrethroid resistance which can impact the efficacy of such treatments.
Permethrin dust is particularly ineffective against resistant bed bug strains, but a range of other dust formulations are available with alternative insecticides such as bendiocarb and physical insecticides such as silica and diatomaceous earth.
Although dusts are an important treatment option for certain areas of a room where liquids are inappropriate (e.g. electrical outlets), as we know the physical properties of dusts can be impacted by humidity. The question is, does this impact their performance on bed bugs?
In a study published in the Journal of Economic Entomology in August 2020, researchers from The State University of New Jersey in the US evaluated the efficacy of selected insecticide dusts under different moisture conditions by applying steam and by ageing the treated tiles in chambers with relative humidity ranging from 52-100%. In this study, the researchers evaluated the effect of moisture on the efficacy of three commercially available insecticide dusts against C. lectularius: Cimexa (92.1% amorphous silica gel), Alpine (0.25% dinotefuran, 95% diatomaceous earth), and Tempo (1% cyfluthrin). These insecticides are among the most commonly used products by professionals for bed bug treatments in the US.
In the steam treatment experiment, each insecticide was applied to a number of vinyl tiles. The tiles were then exposed to constant, low-pressure steam for 90 seconds before being removed and stored. A number of bed bugs were then exposed to the tiles and their mortality was recorded daily at 1-7, 10, 14, and 21 days after exposure. The efficacy of the steam-treated tiles was compared against the efficacy of tiles without steam exposure that had been aged for the same duration.
All three insecticide dusts were shown to have reduced efficacy when compared to their ‘fresh’ non-steamed performance. From day 1 to day 10, dry dust caused significantly higher mortality to bed bugs than the wet product for each insecticide except for Alpine during days 1-3, which was perhaps to be expected as Alpine has a slow mode of action.
Both Alpine (0.25% dinotefuran, 95% diatomaceous earth) and Tempo (1% cyfluthrin) achieved relatively low levels of control after 10 days when dry, achieving mortality of 69% and 48% respectively. When treated with steam, bed bug mortality dropped to 43% and 30% respectively.
Cimexa (silica gel) performed significantly better than the others. On day 10, the dry Cimexa tiles caused 100% mortality while the steam-treated tiles caused 76% mortality. In short, Cimexa, the amorphous silica gel dust, widely outperformed the other two insecticide dusts.
In the second experiment, the researchers wanted to look at the efficacy of the insecticides under varying humidity conditions, over a longer period of time i.e. looking at how effective a dust application would be in a typical bedroom environment in the days and weeks after treatment. Due to the poor performance of both Alpine and Tempo in the first experiment, the researchers decided to test only Cimexa.
As previously, the insecticide was applied to vinyl tiles but this time the tiles were aged under different levels of relative humidity, at 52%, 75% and 100%. The tiles were subjected to the humid conditions for one month and two months, then were exposed to bed bugs and their behaviours were observed.
When Cimexa was freshly applied to a tile, the bed bug mortality rate was 99% at day 10. As expected the humidity impacted performance, with the level of mortality decreasing with increasing humidity. At one month the performance of the 52%, 75% and 100% treatments had dropped to 81%, 79% and 56% respectively. After two months, the efficacy had dropped further with mortality rates of 60%, 60% and 39% respectively.
In conclusion, two factors have been shown to degrade an insecticide dust treatment: firstly, a short period of exposure to steam, and secondly, long-period ageing in a moist environment.
For pest managers, this means steam should not be applied to areas where insecticide dusts have already been applied. If both steam and dust are to be used to treat the same area, steam should be used first and allowed to dry prior to applying dust for residual control. Also, it is better to keep the areas where dust has been applied dry while bed bugs are still present. The findings also explain why pest managers might see good insecticide performance in hotel rooms with air conditioning (low humidity) and residential homes that have less than 50% relative humidity for at least some periods of the year. Worse performance would likely be seen in humid environments without air conditioning, with the residual performance dropping off quicker in humid environments.
Overall, one thing that this latest research makes clear is that pest managers must consider moisture when applying insecticide dusts for controlling bed bug infestations.
Further reading: Sabita Ranabhat, Changlu Wang, Effect of Moisture on Efficacy of Selected Insecticide Dusts Against the Common Bed Bug, Cimex lectularius (Hemiptera: Cimicidae), Journal of Economic Entomology, Volume 113, Issue 4, August 2020, Pages 1933–1939, https://doi. org/10.1093/jee/toaa122