ABSTRACT

The effects of 3 plant species (Cornus florida [dogwood], rhododendron X ‘Chionoides' [rhododendron], and Ilex opaca [American holly]), 4 insecticide treatments (Demand® CS [lambda-cyhalothrin] at 6.25 ml[AI]/liter; Talstar® Professional [bifenthrin] at 7.81 ml[AI]/liter, and Suspend® Polyzone® [deltamethrin] at 11.72 ml[AI]/liter, and water), and 2 physiological states (blood-fed and unfed) were evaluated for knockdown (1 h) and mortality (24 h) against female Aedes albopictus over an 8-wk sampling period. Analyses determined that there was a significant interaction between the tested plant species and the insecticides evaluated. Significant differences were likewise observed between the insecticide treatments for unfed Ae. albopictus females, with Demand CS demonstrating the highest knockdown and mortality rates (from >90% to >10% at wk 8 and >95% to ∼50% at wk 8, respectively), followed by Talstar Professional (from >75% to <10% at wk 2 and >90% to <10% at wk 2, respectively) and Suspend Polyzone (from >20% to <10% at wk 8 and >25% to >50% at wk 8, respectively). All treatments were no longer significant for knockdown or mortality at the end of the 8-wk timeframe. Significant differences were also observed between insecticide treatments for blood-fed Ae. albopictus females; Demand CS showed high knockdown and mortality rates (from 100% to ∼50% at wk 8 and 100% to >60% at wk 8, respectively), Suspend Polyzone rates were similar to Demand CS (from >80% to ∼50% at wk 8 and ∼90% to >65% at wk 8, respectively), and both were followed by Talstar Professional (from 100% to <10% at wk 4 and 100% to <20% at wk 4, respectively). All tested pyrethroid sprays showed a significant increase in effectiveness against recently blood-fed Ae. albopictus females, as compared to the unfed females. These results suggest that Demand CS can be used as an effective barrier spray against Ae. albopictus adults due to the limited impact of target foliage, its long-term efficacy under environmental conditions, and its continued effectiveness regardless of the blood meal status of the target mosquito.

INTRODUCTION

The Asian tiger mosquito, Aedes albopictus (Skuse), is a worldwide nuisance pest capable of vectoring several viruses (Benedict et al. 2007, Kraemer et al. 2015). This highly invasive mosquito is a generalist feeder on mammals but has been shown to aggressively take blood meals from humans in areas where humans are readily accessible (Gomes et al. 2003, Ponlawat and Harrington 2005, Delatte et al. 2009). Due to its preference for feeding on human hosts, Ae. albopictus often causes public complaints in areas of heavy infestation (Farajollahi 2009). Aedes albopictus has become a public health concern across the globe due to the expansion of its habitable range in conjunction with changing climates (Ponlawat and Harrington 2005, Delatte et al. 2009, Kamal et al. 2018). This species is especially difficult to control because it utilizes artificial containers as breeding sites, which can make removal difficult (Barker et al. 2003, Eisen and Moore 2013).

Control of mosquitoes such as Ae. albopictus can take many forms, such as reduction of larval breeding sites or large area sprays, but many key breeding sites for mosquitoes can be cryptic and difficult to properly treat. Another approach is to apply residual pesticides to resting substrates, which has been shown to reduce populations of mosquitoes (Trout et al. 2007, Amoo et al. 2008, Doyle et al. 2009, Richards et al. 2017, 2019). This application method reduces the amount of insecticidal products applied to the environment, which can aid in decreasing the widespread use of chemicals for pest control (Cilek and Hallmon 2006, Cilek 2008). Many active ingredients have been applied in this manner, including lambda-cyhalothrin (Unlu et al. 2017, McMillan et al. 2018), deltamethrin (Cilek and Hallmon 2006, Richards et al. 2017, McMillan et al. 2018), bifenthrin (VanDusen et al. 2016, Richards et al. 2017, McMillan et al. 2018), and permethrin (Cilek and Hallmon 2006, Amoo et al. 2008). Typical retreatment times in the USA range from 21 to 30 days after the initial application when applied by pest control operators using specified label rates. Due to the variance of environmental exposure, many factors can influence the effectiveness of a barrier spray treatment, such as plant species (Doyle et al. 2009, McMillan et al. 2018), type of sprayer (Farooq et al. 2010), and environmental conditions (Allan et al. 2009). Both Doyle et al. (2009) and McMillan et al. (2018) observed significant impacts from plant species but did not explicitly control for the surface area to which the mosquitoes were exposed. Leaf surface area was therefore controlled in the bioassays of this study to more accurately observe differences in insecticide treatment efficacy due to the plant species.

Another factor that has been investigated is how a recently ingested blood meal impacts the mosquito's susceptibility to chemical treatments (Hunt et al. 2005, Spillings et al. 2008, Rajatileka et al. 2011, Oliver and Brooke 2014, Machani et al. 2019), as well as how the blood meal impacts the biochemistry of the mosquito (Benoit et al. 2011, Lahondère and Lazzari 2012). In Anopheles gambiae (Giles), it was observed that female mosquitoes were less susceptible to pyrethroid exposure after being allowed to digest a blood meal (Machani et al. 2019), but similar information for Ae. albopictus is not available. Mosquitoes require suitable resting habitats to protect them from extreme temperature and desiccating conditions. This is especially critical for blood fed mosquitoes that require several days to digest the blood meal and mature a batch of eggs. Also, if the mosquito ingested an infected blood meal, an incubation period is necessary before they can transmit the disease pathogen. Because of their importance in population growth and possible disease transmission, recently blood-fed mosquitoes were used in this study to examine the impacts of a blood meal on barrier spray efficacy.

This study compares the knockdown and mortality rates of unfed and recently blood-fed adult Ae. albopictus females over an 8-wk period for 3 commonly used pyrethroid barrier sprays (Demand CS [lambda-cyhalothrin], Suspend Polyzone [deltamethrin], and Talstar Professional [bifenthrin]) when applied to different plant species.

MATERIALS AND METHODS

Plant species and treatment

Three plant species were selected for this study: Cornus florida (L.) (dogwood), rhododendron X ‘Chionoides' (L.) (rhododendron), and Ilex opaca (Aiton) (American holly). Plants of each species of comparable size and age were purchased from the Fast Growing Tree Nursery (Fort Mill, SC, USA). Two plants of each species were selected and assigned to 1 of 4 treatment groups: 3 insecticide treatments or water as the control. The spray treatments were the highest available label rates: Demand CS (lambda-cyhalothrin; AI 7.9%; Syngenta Crop Protection, Greensboro, NC) at 6.25 ml/liter of water; Talstar Professional (bifenthrin; AI 7.9%; FMC Professional Solutions, Philadelphia, PA) at 7.81 ml/liter of water, and Suspend Polyzone (deltamethrin; AI 4.75%; Bayer Crop Science, Research Triangle Park, NC) at 11.72 ml/liter of water. Applications were administered to plants using a backpack mist blower (SR200, Stihl Corp., Virginia Beach, VA) to the point of runoff with complete coverage of both upper and lower surfaces of leaves. Following the spray applications, plants were allowed to dry, grouped according to treatment, and planted evenly in a 2-m2 plot. Plants were exposed to natural environmental conditions and watered at the base as needed. The test plot was located at Virginia Tech's Prices Fork Research Center, Blacksburg, VA.

Mosquitoes

The adult female Ae. albopictus used in the study were from a colony established from mosquitoes locally collected in Blacksburg, VA; this colony has been maintained by the Virginia Tech Medical Entomology lab for approximately 3 yr. The mosquitoes were reared in an insectary at 24°C, 80% RH, and a photoperiod of 16:8 (L:D) h using the procedures described by Munstermann and Wasmuth (1985). Two-to-3-day-old mosquitoes were used for the unfed bioassays. For the blood-fed bioassays, 2-wk-old mosquitoes were fed on a human host 15 min prior to use. To control for variation, the same researcher was used as the host for all the feedings, the feeding took place at the same time of day, and feeding was allowed to continue until repletion. This feeding was performed voluntarily and did not conflict with any Human Use Biosafety Protocols at Virginia Tech.

Leaf bioassays

Leaves were sampled from plants in each treatment group between June 29 and August 24, 2018, with the first samples collected the day following treatment (i.e., wk 0). Additional leaf samples were collected at wk 2, 4, 6, and 8. At each sampling, 3 leaves were selected from the plants in each treatment group, avoiding visible new growth. Leaves were selected from the same area on all the plants, and all sampling was done by the same individual to minimize variance between samples. The selected leaves were excised, placed into 1-liter Double Zipper storage bags (Walmart, Christiansburg, VA) and transported immediately back to the laboratory. Nitrile gloves were worn during sampling and were changed between insecticide treatments to prevent cross-contamination. After transportation to the lab, excised leaves were trimmed and immediately used in bioassays, as described below.

Ten mosquitoes were added to a 7-dram borosilicate glass shell vial (Thomas Scientific, Swedesboro, NJ) containing a single leaf from a plant from one of the treatment groups and plugged with a cotton ball. Leaves were cut to fit into the glass shell vials and positioned so that mosquitoes could access both sides while in the vial. After a 5-min exposure, mosquitoes were transferred to sterile 220-ml plastic containers (Corning, New York) and covered with mesh netting. Knockdown and mortality were evaluated after 1-h and 24-h, respectively, and for both blood-fed and unfed females. Mosquitoes were considered knocked-down if they were unable to fly within the vial or container and were considered dead if they were unable to stand after slight agitation.

Environmental data

Data on rainfall (mm), temperature (°C), and relative humidity (RH) (Table 1) were retrieved from Weather Underground (Strubles Mill station: KVABLACK63) (Weather Underground 2019). This weather station is located roughly 2 miles from the research plot.

Table 1.

Weekly rainfall (mm), relative humidity (RH) and mean temperature in Blacksburg, VA, from June 29 to August 24, 2018.

Weekly rainfall (mm), relative humidity (RH) and mean temperature in Blacksburg, VA, from June 29 to August 24, 2018.
Weekly rainfall (mm), relative humidity (RH) and mean temperature in Blacksburg, VA, from June 29 to August 24, 2018.

Statistical analysis

Data on the residual toxicity of barrier spray treatments of mosquitoes exposed to treated leaves were analyzed using a linear mixed model for repeated measures ANOVA with a first-order autoregressive and random effect covariance structure (Littell et al. 2000, Feazel-Orr et al. 2016, McMillan et al. 2018). The model examined plant species (dogwood, rhododendron, and American holly), insecticide treatment (Demand CS, Suspend Polyzone, Talstar Professional, and water), mosquito blood meal status (unfed or blood-fed), sampling wk (0, 2, 4, 6, and 8), and their interactions as the fixed effects factors. If the 4-way interaction of plant species × insecticide treatment × mosquito blood meal status × wk was not significant, the model was rerun with this interaction effect excluded. In the presence of a significant 3-way interaction, we analyzed one of the 2-way interactions at each level of the third factor and repeated this process for each of the other 2 factors, as suggested by Ott and Longnecker (2001). Before each analysis, the response variable measurements, proportion knockdown and mortality, were tested for normality and transformed using a Box-Cox transformation (Osborne 2010). Shapiro-Wilk W test and/or the skewness and kurtosis values were used to judge the goodness-of-fit of the transformed data when compared to the normal distribution (Thode 2002, Zar 2010). Post hoc multiple comparison tests were carried out with the Tukey HSD or Student t-test, where appropriate. All statistical analyses were performed on the means of 3 replicates using JMP Pro v14 (SAS 2019) at a significance level of α = 0.05.

RESULTS

Effect of plant species

In this study, analysis showed that there was a significant interaction between plant species and insecticide treatment for both knockdown (F6, 71.7 = 3.0925, P = 0.001) and mortality (F6, 87.3 = 4.0274, P = 0.001). Demand CS did not have significant differences in knockdown or mortality across the examined plant species, Talstar Professional varied significantly in observed mortality, and Suspend Polyzone varied significantly for both knockdown and mortality (Table 2). No significant 2-way interactions were observed between plant species and any other fixed effects, i.e., mosquito blood meal status and week.

Table 2.

Proportion knockdown and mortality (mean ± SE) of Ae. albopictus for each plant species over 8 wk, sorted by insecticide treatment. Exposure time of 5 min. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each column followed by the same letter are not significantly different. Controls not shown due to no significance. Tukey's HSD (P > 0.05).

Proportion knockdown and mortality (mean ± SE) of Ae. albopictus for each plant species over 8 wk, sorted by insecticide treatment. Exposure time of 5 min. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each column followed by the same letter are not significantly different. Controls not shown due to no significance. Tukey's HSD (P > 0.05).
Proportion knockdown and mortality (mean ± SE) of Ae. albopictus for each plant species over 8 wk, sorted by insecticide treatment. Exposure time of 5 min. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each column followed by the same letter are not significantly different. Controls not shown due to no significance. Tukey's HSD (P > 0.05).

Comparative residual toxicity of barrier spray insecticide bioassays

The results showed that there was a significant effect of insecticide treatment (F3, 71.7 = 249.7380, P < 0.0001), mosquito blood meal status (F1, 71.7 = 77.4261, P < 0.0001), week (F4, 196.2 = 54.7980, P < 0.0001), the interaction of insecticide treatment × week (F12, 212.8 = 19.2568, P <0.0001; Figs. 1A and 2A, 2B), and the interaction of insecticide treatment × mosquito blood meal status (F3, 71.7 = 23.5357, P = 0.0001; Fig. 3A) on mosquito knockdown at 1 h. Overall, Demand CS had the highest proportion knockdown against blood-fed female Ae. albopictus, followed by Suspend Polyzone, Talstar Professional, and the control. For unfed mosquitoes, the Demand CS treatment also had the highest proportion knockdown, with similar knockdown observed with Suspend Polyzone and Talstar Professional.

Fig. 1.

Proportion knockdown (A; mean ± SE) and mortality (B; mean ± SE) of Ae. albopictus by insecticide treatment over time. Exposure time of 5 min. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each graph followed by the same letter are not significantly different. Tukey's HSD test (P > 0.05).

Fig. 1.

Proportion knockdown (A; mean ± SE) and mortality (B; mean ± SE) of Ae. albopictus by insecticide treatment over time. Exposure time of 5 min. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each graph followed by the same letter are not significantly different. Tukey's HSD test (P > 0.05).

Fig. 2.

Proportion knockdown (Top; mean ± SE) and mortality (Bottom; mean ± SE) of unfed (A, C) and blood-fed (B, D) Ae. albopictus over time. Exposure time of 5 mi. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each week followed by the same letter are not significantly different. Tukey's HSD test (P > 0.05).

Fig. 2.

Proportion knockdown (Top; mean ± SE) and mortality (Bottom; mean ± SE) of unfed (A, C) and blood-fed (B, D) Ae. albopictus over time. Exposure time of 5 mi. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each week followed by the same letter are not significantly different. Tukey's HSD test (P > 0.05).

Fig. 3.

Proportion knockdown (A; mean ± SE) and mortality (B; mean ± SE) of Ae. albopictus by insecticide treatment, separated by physiological state. Exposure time of 5 min. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each graph followed by the same letter are not significantly different. Tukey's HSD test (P > 0.05).

Fig. 3.

Proportion knockdown (A; mean ± SE) and mortality (B; mean ± SE) of Ae. albopictus by insecticide treatment, separated by physiological state. Exposure time of 5 min. Knockdown and mortality evaluated at 1 h and 24 h, respectively. Means within each graph followed by the same letter are not significantly different. Tukey's HSD test (P > 0.05).

However, the effects of the insecticides varied over time as shown through the interaction of insecticide treatment × week for blood-fed and unfed female Ae. albopictus (Fig. 2A, 2B). The efficacy of Demand CS declined steadily after wk 2 and that of Talstar Professional decreased rapidly after wk 0. Although the efficacy of Suspend Polyzone was initially lower than the other two insecticides, the proportion knockdown remained relatively steady throughout the study. At wk 8, there was no significant difference in knockdown between Demand CS and Suspend Polyzone. After wk 2, knockdown from Talstar Professional was equivalent to that of the control (Fig. 1A).

With regard to mosquito mortality, significant effects were observed for insecticide treatment (F3, 87.3 = 416.2152, P < 0.0001), mosquito blood meal status (F1, 87.3 = 113.2793, P < 0.0001), week (F4, 183.6 = 67.0994, P < 0.0001), the interaction of insecticide treatment × wk (F12, 197.6 = 32.4952, P < 0.0001; Figs. 1B and 2C, 2D), and the interaction of insecticide treatment × blood meal status (F3, 87.3 = 15.4379, P = 0.0001; Fig. 3B). Trends in mortality were similar to those observed for knockdown with respect to each of the insecticide treatments. Mortality declined steadily from 100% to ∼55% by wk 8 for Demand CS, remained between ∼55% and ∼70% for Suspend Polyzone, and dropped drastically from ∼100% to the level of the control after wk 2 for Talstar Professional (Fig. 1B).

Effect of blood meal

Blood meal status had a significant effect on the mosquito knockdown observed, with blood-fed mosquitoes being significantly more susceptible when exposed to insecticide treatments (Fig. 3A). Although there were higher knockdown numbers recorded for all insecticide treatments, Suspend Polyzone showed the greatest difference in knockdown between unfed and blood-fed mosquitoes. Unfed mosquitoes showed the greatest proportion knockdown from Demand CS exposure, followed by Suspend Polyzone, and then Talstar Professional (Talstar Professional was still significant from the controls). Demand CS also had the highest observed mosquito knockdown against blood-fed mosquitoes, with Suspend Polyzone showing comparable knockdown proportions and Talstar Professional being significantly less effective than both (but again still significant from the controls). Notably, blood-fed mosquitoes exposed to Talstar Professional experienced significantly lower proportion knockdown than unfed mosquitoes exposed to Demand CS and were statistically similar to unfed mosquitoes exposed to Suspend Polyzone (Fig. 3A).

Blood meal status also had significant effect on observed mosquito mortality, again with blood-fed mosquitoes being significantly more susceptible to insecticide treatments than unfed mosquitoes (Fig. 3B). Demand CS and Suspend Polyzone again demonstrated similar mortality rates against blood-fed mosquitoes, but Demand CS caused significantly more mortality against unfed mosquitoes. Talstar Professional induced significantly less mortality than either of the other active ingredients but again was still significant from the controls. Talstar Professional caused lower mortality rates against blood-fed mosquitoes than Suspend Polyzone did against unfed mosquitoes (Fig. 3B).

Environmental data

Mean temperature, mean RH, and total precipitation during the study period were 22.4°C, 80%, and 156.7 mm, respectively (Table 1). This temperature is consistent with the historical average temperature for this time period (∼22.5°C) and is representative of typical average rainfall in the area (∼152 mm), according to climate averages for the Blacksburg, VA, area (weatherspark.com 2020).

DISCUSSION

Barrier sprays have become a popular control option for mosquitoes in the USA, and using residual insecticides in this way has been shown to reduce or prevent adult mosquito pressure from the surrounding environment (Anderson et al. 1991, Cilek and Hallmon 2006, Trout et al. 2007). These residual compounds must remain effective despite environmental complications, such as temperature changes, sunlight degradation, and varying levels of moisture. The best compounds can withstand these changing conditions for long periods and thus remain biologically available and effective for that timeframe, resulting in less frequent retreatment requirements. This study was designed to examine how differences in plant species, insecticide formulations, and mosquito blood meal statuses might interact to influence the effectiveness of a barrier spray treatment against Ae. albopictus. The application rates and the commercial formulations used in this study were chosen based on typical options for applicators in the field.

Previous research suggests that the species of a plant has a significant impact on the efficacy of a barrier spray treatment (Doyle et al. 2009, McMillan et al. 2018). McMillan et al. (2018) suggested that this finding could have been related to differences in available surface area. In this study, we controlled for available surface area by trimming leaves to a uniform size before testing and still found significant differences in efficacy between the insecticide treatments applied to the different plant species (Table 2). It was observed that the interactive effects between plant species and insecticide treatment were significant, with Suspend Polyzone showing significant differences in knockdown and mortality based on the plant species and Talstar Professional showing significant differences in mortality. Demand CS did not show any significant differences in either knockdown or mortality based on the plant species to which it was applied (Table 2). Chowdhury et al. (2001) conducted a study of residual pyrethroid effectiveness against Folsomia candida (Willem), a collembola commonly used to evaluate pyrethroid efficacy, and examined treatments applied to 16 plant species. Their study determined that a clear pattern for plant impact on residual effectiveness of pyrethroid treatments was not obvious but also mentioned that observed differences may have been linked to varying levels of exposure (Chowdhury et al. 2001). Therefore, with the findings of this study and those of Chowdhury et al. (2001), it has been shown that plant species can have an impact on the residual effectiveness of pyrethroid treatments, but additional work on the topic needs to be conducted. Investigation into the influence of leaf cuticular wax may help to explain the differences in effectiveness observed from this and previous studies.

Significant differences in residual effectiveness between the three insecticide treatments were observed in this study, with the longest effect lasting 8 wk (Fig. 1). This residual effectiveness is similar to previous research on the topic (Li et al. 2010, McMillan et al. 2018) and showed that Demand CS can be used effectively as a barrier spray for approximately 2 months. Additional research on lambda-cyhalothrin could determine if this result is unique to the formulation of Demand CS or an aspect of the active ingredient. Differences observed between the pyrethroid insecticides tested in this study were most likely due to different active ingredients and formulations. Bifenthrin is a type I pyrethroid, whereas lambda-cyhalothrin and deltamethrin are both type II pyrethroids. Also, Talstar Professional (bifenthrin) is not a formulation that protects the active ingredient from environmental degradation, such as the microencapsulation present in Demand CS (lambda-cyhalothrin) or the microscopic polymer film from Suspend Polyzone (deltamethrin). These aspects could influence the effectiveness of the active ingredient when used as a residual barrier spray, and additional investigation of these effects would clarify these differences further. Doyle et al. (2009) reported that bifenthrin (Talstar® One) demonstrated a significant decrease in efficacy after 1 wk, and our results support those observations. Against unfed female Ae. albopictus, Demand CS showed the greatest knockdown/mortality, Suspend Polyzone showed low knockdown/mortality numbers throughout the study, Talstar Professional was similar to Demand CS on wk 0 for both knockdown and mortality but was not significant for the remainder of the study. This information suggests that Demand CS lasts roughly 3 times as long as Talstar Professional when applied as a barrier spray in the field, and that Suspend Polyzone, due to its low efficacy throughout the study, may not provide significant levels of control in field applications. Additional observations of the deltamethrin formulation used in this study (Suspend Polyzone) may be warranted to corroborate the results described here.

The physiological state of Ae. albopictus has been shown to influence their circadian rhythms (Lima-Camara et al. 2014), with mated, blood-fed females demonstrated to have the only significant reduction in locomotion. This finding justifies a short exposure time in the leaf bioassays conducted in this study and the inclusion of blood-fed females explores the impact of 2 different physiological states. In all insecticide treatment groups, blood-fed female Ae. albopictus showed greater susceptibility to the treatments, represented as both higher knockdown and mortality numbers (Fig. 3). Knockdown measurements for Demand CS declined steadily from ∼100% at wk 4 down to ∼50% at wk 8, Talstar Professional knockdown dropped sharply from ∼100% at wk 0 down to <10% at wk 4 and showed no significant effect afterward, and Suspend Polyzone maintained between 50% and 80% knockdown for the duration of the study (Fig. 2B). For mortality, Demand CS again declined steadily from ∼100% at wk 4 down to ∼50% at wk 8, Talstar Professional dropped sharply from 100% at wk 0 down to <20% at wk 4 and was no longer effective from wk 4 onwards, and Suspend Polyzone declined slightly from ∼90% at wk 0 down to >60% at wk 8 (Fig. 2D). Results from Doyle et al. (2009) showed that recently blood-fed Ae. albopictus were susceptible to Talstar One (AI bifenthrin 7.9%) residual treatments for less than 14 days, using an exposure time of 1 h, and those findings are expanded upon by the findings of this experiment.

Differences in observed susceptibility of Ae. albopictus females to various pyrethroid barrier sprays may be linked to the different stressors the mosquito experiences during the digestion of its blood meal. All 3 of the examined pyrethroids experienced a significant increase in knockdown and mortality against blood-fed Ae. albopictus females, as compared to unfed females (Fig. 3). Both Demand CS and Talstar Professional showed roughly a 2-wk extension in efficacy for knockdown and mortality before they began to lose effectiveness on blood-fed females, and Suspend Polyzone showed up to 300% increases in knockdown and mortality rates throughout the study (Fig. 2). The prolonged increase in Suspend Polyzone effectiveness against blood-fed Ae. albopictus females was significantly different from the other pyrethroids examined, and the knockdown and mortality rates for Suspend Polyzone were similar to Demand CS for the blood-fed trials (Fig. 3). It remains to be seen if this extended increase is unique to deltamethrin or if there are other aspects of the formulation that are producing this result. Additional investigation of how the presence of a blood meal increases the susceptibility of a female mosquito to insecticide treatments should be conducted to better understand this mechanism. Research understanding stress associated with digestion of a blood meal has been conducted for mosquitoes, but little information is available about the interplay between an insecticide treatment and a recently taken blood meal. Blood-fed Ae. aegypti (L.) that had been allowed to digest blood meals had been shown to be less susceptible to ultra-low-volume aerosol treatments of synergized resmethrin, but the earliest time those mosquitoes were observed was 24 h postfeeding (Reiter et al. 1990). Barrier spray treatments are primarily implemented to prevent mosquitoes from feeding inside the treated area and therefore should aim to be reliably effective on unfed mosquitoes in addition to recently blood-fed ones. Further research into the impact of different application rates for the examined barrier treatments is also encouraged. Previous findings by McMillan et al. (2018) suggested rates of degradation differed across available application rates for Demand CS barrier treatments, which is relevant information for pest control operators that may not have the budget to consistently apply the maximum label rate for their treatments.

The results of this study provide further support for the use of a microencapsulated formulation of lambda-cyhalothrin as a barrier spray for the control of Ae. albopictus (Trout et al. 2007, McMillan et al. 2018). Also, the effectiveness of pyrethroids as barrier sprays does appear to be affected by the target foliage, thus care should be taken to ensure all surfaces are evenly coated. In this study, approximately 15% more precipitation fell over the course of the study than was observed by McMillan et al. (2018) in 2016 (Table 1). Despite the differences in environmental conditions between the 2 studies, the results of McMillan et al. (2018) and this study both suggest that barrier spray treatments using Demand CS remain effective against Ae. albopictus females for at least 8 wk. The microencapsulation technology used in this formulation of lambda-cyhalothrin protects the AI from degradation by the environment (Wege et al. 1999) and is a probable reason for the longevity of the treatment. Demand CS treatments outperformed the other insecticide treatments in this study, regardless of plant substrate or blood meal status, but Suspend Polyzone efficacy was equivalent in the trials using blood-fed mosquitoes. Due to this, applications utilizing Demand CS may require less frequent reapplications, resulting in lower costs for applicators and less product applied into the environment. Using a conservative mosquito season length of 3 months, Demand CS applied as a barrier spray could reduce the number of treatments required for population control from three total treatments down to only 2, based on reapplication requirements of 4 wk and 6 wk, respectively. This 50% reduction in reapplication requirements could enable pest management companies to focus efforts on treating new areas instead of retreating previous areas and would reduce the overall volume of insecticidal products applied to the environment. When used in conjunction with mosquito surveillance data and an integrated pest management practice, barrier sprays can serve as an additional treatment option for suppressing mosquito species like Ae. albopictus. When choosing to apply a pyrethroid in this way, however, performance can be influenced by the AI and formulation chosen.

ACKNOWLEDGMENTS

This study was funded by Syngenta Crop Protection, LLC. Syngenta Crop Protection reviewed the results shown here and made no changes to the data presented prior to the submission for publication. This manuscript is published in concurrence with Syngenta Crop Protection.

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Author notes

1

Department of Entomology, Virginia Tech, Price Hall, Virginia Tech, 170 Drillfield Drive, Blacksburg, VA 24061.

2

Department of Entomology, Clemson Poole Agricultural Center, 130 McGinty Court, Clemson, SC 29634.

3

Syngenta Crop Protection, LLC, Greensboro, NC 27409.