Pesticide Misting System Enhances Residual Pesticide Treatment of HESCO Geotextile

We have demonstrated the efficacy of residual pesticide treatment of US military materials such as camouflage netting and HESCO blast wall geotextile material to reduce biting pressure from a range of natural populations of disease-vector and nuisance insects across multiple field environments (Britch et al. 2010, 2011, 2018). This method of passive control may be leveraged as a key component of the Department of Defense integrated vector management (IVM) system (Kitchen et al. 2009) to protect personnel within perimeters of treated materials in the field. Although the initial residual pesticide treatment may be applied as a factory treatment or by units prior to deployment, the efficacy of the treatment will eventually wane and require retreatment in the field. However, in most US military field scenarios where camouflage and blast wall materials are deployed, retreatment may be difficult or impossible due to restrictions of safety and/or resources and not timely, thus creating periods of increased vector-human contact and risk.

One solution is to introduce a complementary component such as a pesticide misting system (Aldridge et al. 2018) into the IVM scheme. A pesticide misting system mounted around a perimeter of US military material could potentially contribute two factors to the IVM scheme: 1) misting on its own could reduce biting or nuisance pressure that targets insects in flight, producing an additive control effect with the residual treatment already on the perimeter that targets insects that land, and 2) misting could deposit pesticide on perimeter materials that persists as a residual, either enhancing the existing residual treatment or restoring a residual treatment that is waning. In this study we combined a pesticide misting system with small HESCO blast wall enclosures to investigate whether misting could enhance an existing residual pesticide treatment in a hot-arid desert environment.

On August 5, 2013, we set up 5 experimental enclosures constructed of brown geotextile HESCO MIL barriers (Szabó et al. 2011) following the methods described in Britch et al. (2018) in a hot-arid desert site in the Coachella Valley, CA (Britch et al. 2009). We oriented the 5 enclosures in an approximate north-south line among sparse vegetation, each separated from the next by approximately 25 m. We treated the northernmost 2 HESCO enclosures and the southernmost 2 enclosures with λ-cyhalothrin as described in Britch et al. (2018), leaving the middle enclosure as an untreated control unit.

We installed a pesticide misting system (MistAway Gen 1.3; MistAway Systems, Inc., Houston, TX) on the southernmost 2 HESCO enclosures following the modifications described in Aldridge et al. (2018), mounting a 40 ml/min flow rate nozzle at each outside corner around the midline of each of the 2 enclosures (Fig. 1A). We conducted periodic misting at the 2 southernmost HESCO enclosures from August 5 to 8 with a combination of two misting formulations, EcoExempt with EcoEmulsifier (rosemary oil 18%, cinnamon oil 2%, lemongrass oil 2%; EcoSMART Technologies, Inc., Franklin, TN) and SECTOR Misting Concentrate (10% permethrin, 10% piperonyl butoxide; McLaughlin Gormley King Co., Minneapolis, MN), applied at maximum label rates in water. We used 2 different misting formulations in support of a separate but simultaneous effort to investigate effects of pyrethroids versus botanical misting agents on collections of natural populations of nuisance flies (Aldridge et al. 2015). Misting activity resulted in a total of 0.13 liter of EcoExempt and 0.16 liter of Sector applied across the 2 southernmost HESCO enclosures. With this arrangement we were able to measure 1) the efficacy of a residual treatment of geotextile on its own and 2) the combined efficacy of residual treatment and pesticide misting.

To measure efficacy over time we conducted laboratory bioassays on a year-long series of periodic 13 cm × 15 cm cuttings of incrementally weathered geotextile with 5 species of colony reared insects at 24-h exposures using the methods described in Aldridge et al. (2013). The colony-reared insects consisted of Culex quinquefasciatus Say, Musca domestica L., Stomoxys calcitrans (L.), and Phlebotomus papatasi Scopoli as target organisms and Hippodamia convergens Guérin-Méneville as nontarget organism. All insect colonies were kept in 30 cm Bug Dorms (MegaView; Taiwan) or custom cages in incubators maintained at 27°C; 30–80% RH, and 12:12 L:D cycle. We obtained H. convergens specimens through a commercial supplier (Arbico Organics, Oro Valley, AZ) in batches of 1200 every week for 3 wk in July 2014. Colony histories of the remaining colony organisms are described in Britch et al. (2018).

Results of bioassays with 13 periodic posttreatment samples of geotextile materials against the 5 colony-reared organisms are shown in Fig. 1B–F. Each chart in Fig. 1 shows mean 24-h percent mortality with 95% confidence interval (CI) bands generated in R (ver. x64, 3.5.1) by the geomsmooth(lowess) function in the ggplot2 package. Separate curves are plotted in each chart for the mean mortality from exposure to material from the residual treatment on its own (solid lines, blue CI) and for mean mortality from the residual and misting combined (dotted lines, pink CI). We normalized all mortality data based on exposure of colony organisms to geotextile from the untreated HESCO using Abbott's formula (Abbott 1925).

The threshold of high efficacy of treated material, i.e., the number of days posttreatment before mortality fell below 90% (Britch et al. 2018), is shown by the horizontal dashed line in each chart. The duration above high efficacy varied across the 5 bioassay organisms and in many cases varied within organisms depending on whether misting was present (Fig. 1B–F). Minimum 90% efficacy of the residual treatment without misting lasted the longest against stable flies (67 days), with less longevity against sand flies (42 days) and mosquitoes (37 days). High efficacy against house flies was not observed even in the very earliest sample when the residual treatment had just been applied. High residual efficacy was observed against the nontarget ladybird beetles for up to 12 days posttreatment.

Bioassay data from the HESCO enclosures with residual treatment combined with misting revealed a different pattern (Fig. 1B–F). For mosquitoes, the threshold 90% high efficacy was extended for nearly 2 wk in residual-treated HESCO enclosures that had been misted. Similarly, threshold high efficacy was extended for more than 1 wk for sand flies exposed to misted geotextile. The threshold for house flies was unaffected by misting and still did not meet or exceed 90% efficacy. The effect of the presence of misting on stable flies was unexpected and initially actually reduced the duration of high efficacy by more than 1 wk. Nevertheless, the enhanced effect of misting on stable fly mortality could be seen in later samples (days 112–363, Fig. 1C). For the nontarget ladybird beetles, the combined residual and misting effect extended the high efficacy 90% threshold for 7 days (Fig. 1D).

In this study we demonstrated that a pesticide misting system can additively enhance the performance of a residual pesticide treatment on a prominent US military material currently used throughout the world to protect troops in the field. The enhancement of performance in residual-treated HESCOs exposed to misting was shown in the form of either higher efficacy (for instance, in the case of sand flies; Fig. 1F), longer duration of efficacy (stable flies; Fig. 1C), or both (mosquitoes; Fig. 1B). This finding supports immediate use of pesticide misting systems along residual-pesticide treated perimeters to improve the Department of Defense IVM scheme. This combination of active and passive control creates a more failsafe system than either component on its own and could reduce or eliminate the need to retreat perimeters. Pesticide misting systems can be timed to activate during peak biting hours and adjusted as nuisance and vector threats change throughout the year.

Future investigations should examine the capability of misters combined with residual treatments to reduce biting pressure (field collections) of natural nuisance and disease-vector populations within perimeters. Misting in the present study was conducted for only a short period following the application of the residual at the beginning of the weathering period, yet the enhanced effect caused by misting persisted for months. Future studies should investigate periodic misting over a longer period to determine whether the high efficacy 90% threshold can be maintained indefinitely. Also, we layered misting formulations, and it is not possible to separate their effects on the bioassay results; future work should evaluate botanical and synthetic misting formulations separately as well as mixed. If botanical misting formulations show comparable efficacy to pyrethroids for both misting and residual functions, botanicals could reduce or replace use of pyrethroids to allow greater intensity of misting and residual deposition with lower risk to human health and the environment.

We thank the expert staff at the Coachella Valley Mosquito and Vector Control District for providing key support. For expert production of colony insect specimens we thank H. Brown, T. Carney, K. Kern, B. Smith, C. Swain, and J. Urban (mosquitoes), H. Furlong and R. TenBroeck (stable flies), and W. Delaney, D. Johnson, C. Taylor, and R. White (house flies) at the USDA-ARS-CMAVE insectaries. This research was supported by the USDA–ARS and the US Department of Defense (DOD) Deployed War-Fighter Protection Program (DWFP). Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by USDA, CVMVCD, or the DOD. Data in this study have been added to the Mobile Pesticide App operational entomology decision support system database (https://ars.usda.gov/saa/cmave/PesticideApp; Britch et al. 2014). The USDA is an equal opportunity provider and employer.