[Research Update] Fine-Tuning Backyard Mosquito Control

Nuisance biting mosquitoes and the increased spread of mosquito-vectored diseases have created a new market for pest management professionals. These mosquito control services target outdoor recreational areas, businesses and especially residential backyards. In 2004, we investigated the fundamental question of whether pyrethroid insecticides, applied to dense vegetation and other adult mosquito resting sites, reduced populations and incidence of bites in people’s yards (see "Do Backyard Mosquito Sprays Work?" PCT, May 2005). The study showed that products such as Demand CS (lambda-cyhalothrin) and TalstarOne (bifenthrin) applied at the small spatial scale of a homeowner backyard can indeed reduce overall mosquito numbers and incidence of bites for about four to six weeks post-treatment. Although Aedes and Ochlerotatus mosquito species were greatly reduced, no significant reductions were noted among Culex mosquitoes — the primary vectors of many important diseases, including West Nile virus. Nonetheless, participating homeowners were happy with the treatment since bites were reduced and they could use their yards again.

Since then, we have received many inquiries about the study and its implications. In particular, questions were raised about the lack of Culex reduction observed, and the amount (volume) of insecticide spray per yard needed to achieve satisfactory results. The following study examines these important questions in hopes of providing further insight on this growing industry service.

MOSQUITO BASICS. Mosquitoes are a diverse group consisting of about 2,500 species, of which 150 reside in North America. Kentucky has more than 50 mosquito species and similar diversity is found throughout much of the country. In suburban areas, the most common species are in one of two groups — Aedes/Ochlerotatus and Culex. Aedes/Ochlerotatus mosquitoes prefer to blood feed on mammals, including humans, dogs, cats and horses. The 2004 study showed that this group is responsible for the majority of bites inflicted in a typical Kentucky backyard (other mosquitoes cause bites in other parts of the country). Although we often think of Aedes/Ochlerotatus mosquitoes as "nuisance biters," they can transmit diseases such as dengue fever, LaCrosse encephalitis and canine heartworm. Common species in the United States include the Asian tiger mosquito (Aedes albopictus), the yellow fever mosquito (Aedes aegypti), and the eastern tree hole mosquito (Ochlerotatus triseriatus).

Culex mosquitoes concern homeowners because they are primary vectors of West Nile virus and other encephalatides. Unlike Aedes/Ochlerotatus mosquitoes, most Culex mosquitoes prefer to feed on birds and rarely feed on humans. However, when a bird host is not present, these mosquitoes will feed on mammals. Common species in the eastern half of the United States include the northern house mosquito (Culex pipiens) and the southern house mosquito (Culex quinquefasciatus).

Because Culex mosquitoes preferentially feed on birds, their resting sites throughout much of the country are presumed to be in upper tree canopies. Mammal-feeding Aedes/Ochlerotatus mosquitoes occupy vegetation closer to ground level. This resting site preference may explain why treatments applied in our 2004 study (applied mainly to ground and lower canopy vegetation) had significant impact on Aedes/Ochlerotatus mosquitoes, but no apparent effect on Culex mosquito populations.

THE STUDY. During the summer of 2005, we sought answers to two additional questions. First, we wanted to know whether Culex mosquito populations could be reduced by applying pyrethroid insecticides into tree canopies, where these species are believed to spend much of their time. Second, we wanted to explore what effect spray volume (gallons per yard) had on overall treatment performance.

Tree Canopy Trial. To initially study the first question, we treated lines of mature trees bordering a field on the University of Kentucky Spindletop Research Farm in Lexington, Ky. Twelve tree lines, each 100 linear feet in length, were randomly assigned one of two treatments; either 1) Demand CS (lambda-cyhalothrin) applied at 0.8 ounces per gallon of water (0.06 percent suspension) or 2) water alone. Working under our supervision, the service manager of a local pest control firm applied treatments July 18, 2005. Both treatments were applied one time using a truck-mounted, gasoline-powered spray rig equipped with a JD-9 gun. Adjusting output of the cone nozzle enabled the applicator to treat tree canopies up to a height of about 25 to 30 feet. Three different trapping devices were utilized for assessing treatment effects: a modified CDC light trap provisioned with dry ice as a carbon dioxide (CO2) source installed near ground level; a CDC gravid trap also installed near ground level; and another modified CDC light trap hoisted up into the tree canopy about 25 feet off of the ground. (The CO2-baited light traps capture adult female mosquitoes seeking a blood meal, whereas gravid traps catch females that have already blood fed and are seeking water to lay eggs.) All 12 treated plots were sampled weekly, beginning two weeks before treatment and continuing eight weeks post-treatment.

Results. The 2005 season was characterized by a severe drought, beginning about the time of our application, which greatly reduced the naturally occurring mosquito populations. In spite of this, about eight times as many mosquitoes (8,892, most of which were Culex spp.), were captured in traps up in the tree canopy vs. at ground level (1,263). This reinforced our assumption that the majority of Culex mosquitoes indeed resided in tree canopies, whereas Aedes/Ochlerotatus species generally reside closer to the ground. Despite the drought-induced natural decline in populations, at one, four and six-weeks post-treatment, the Demand CS treatment reduced Culex populations by 82 percent, 72 percent, and 67 percent respectively, relative to treatment with water alone. (See Figure 1 on page 112.)

Residential Study. As a follow up to the tree canopy study, we wanted to see if similar reductions in Culex populations could be achieved by spraying tree canopies in residential backyards. We also investigated the effects of higher vs. lower spray volumes on overall product performance. For this, we solicited 24 homeowner volunteers (three per neighborhood, eight neighborhoods in all) in Lexington, Ky. Within each neighborhood, each yard received one of three treatments: Demand CS (0.06 percent) applied at a relatively high spray volume; Demand CS (0.06 percent) applied at low volume; or water alone. The high and low volume applications approximated the upper and lower spray volumes recommended on the product label (2 to 5 gallons of water per 1,000 square feet).

Treatments were applied Aug. 1-3, 2005, as a single application using a backpack mist blower (Stihl, model SR420), operated at full throttle with a nozzle setting of "3." For both high- and low-volume applications, upper and lower vegetation surfaces were treated. In addition, both applications’ spray mist was directed into upper tree canopies where Culex mosquitoes tend to be most abundant. High volume treatments were intense and thorough, and included the inner portions of shrubs, bushes and other dense vegetation. Reduced volume applications were less thorough, targeting mainly the outer surface of foliage, and less time and effort were spent applying into tree canopies. For high-volume treatments, an average of 2.6 gallons of spray was applied to each property, while reduced volume treatments averaged 1.3 gallons per property. Care was taken to minimize spray drift into adjoining yards, and treatments were suspended when wind was noticeable. Homeowners were not charged for the applications, but were unaware of which treatment they received.

The mosquito population at each property was monitored for two weeks before treatment and four weeks post-treatment (six weeks total). Four different trapping methods were used: modified CDC light traps installed both at ground level and within tree canopies, gravid traps and ovi-traps (blacked containers provisioned with stagnant water and strips of paper from which mosquito eggs are collected and species determined by rearing the immature to the adult). In addition, percent mortality was determined by isolating 20 laboratory reared Aedes albopictus mosquitoes with a randomly treated leaf from each property for 24 hours.

Participating homeowners also completed pre- and post-treatment attitudinal surveys and a weekly log to measure their sensitivity toward mosquitoes and satisfaction level with the treatments.

Results. Significant reductions in Aedes/Ochlerotatus mosquitoes resulted from applying Demand CS at either high or low spray volumes, relative to treatment with water alone. (See Figure 2 on page 114.) Reductions in these species were numerically greater with the higher volume applications, although the difference vs. reduced volume sprays was not statistically significant based on mosquito monitoring. However, laboratory bioassays with mosquitoes confined on treated leaves indicated the high-volume treatment had a statistically higher mortality rate than either the low-volume or the water-only treatments, which were statistically indistinguishable. Moreover, only the high-volume applications significantly reduced backyard populations of Culex mosquitoes — presumably because more time was spent directing the spray into tree canopies where these mosquitoes reside.

Pretreatment attitudinal surveys indicated that nearly all (96 percent) of the homeowners in our study said they were bothered by mosquitoes. More than three-fourths (79 percent) believed there was a mosquito problem in their neighborhood, and two-thirds (67 percent) felt the problem had worsened in the past five years. These responses were similar to those from our previous study conducted in 2004.

In our previous study ("Do Backyard Mosquito Sprays Work?" PCT, May 2005), post-treatment surveys indicated that a majority of homeowners were able to detect a difference between properties treated with insecticide and those treated with a "placebo" (water alone). Post-treatment surveys this time, however, indicated that respondents were less able to make such a distinction. When homeowners were asked: "Do you believe the treatment reduced mosquito populations to your satisfaction?" no apparent correlation between their level of satisfaction and mosquito reduction was apparent. Interestingly, the majority of homeowners receiving the full volume treatment were equally satisfied to those receiving the water only application. In response to another question, most homeowners receiving the high-volume pyrethroid treatment or water alone said they were able to use their yards again, whereas property owners receiving reduced volume pyrethroid applications said they had to avoid their yard or wear repellent.

ADDITIONAL THOUGHTS. It was enlightening to see that Culex mosquito populations could indeed be reduced by applying pyrethroid insecticides into tree canopies. These are the most serious disease vectors throughout much of the United States, and such an approach holds promise for public health efforts where disease transmission is a serious concern. For the pest management industry, however, the decision to target Culex species with tree canopy sprays in residential areas should be made with care. Spray drift into neighboring properties and non-target areas is a serious concern for our industry, and the risks are elevated when performing such applications. Considering that most mosquito bites suffered by homeowners are probably due to Aedes/Ochlerotatus mosquitoes, it may be prudent to limit applications to lower vegetation areas where these nuisance species tend to occur.

Preliminary evaluation of the effects of spray volume suggests that higher volumes (gallons per yard) afford better coverage than lower volumes, and a greater reduction in mosquito numbers. The extent to which the performance difference will be discernable to homeowners, under a range of conditions, is not yet known. In the previous study (2004), high-volume treatments were applied to residential backyards during a year with heavy rainfall resulting in a large mosquito population. The current study was conducted during a drought season resulting in a smaller mosquito population. Seasonal weather conditions also should be considered when fine-tuning mosquito control treatments since mosquito populations are dependent upon rainfall.

Another interesting finding from the current study was homeowners’ inability to differentiate levels of mosquito control. Last year under heavy mosquito pressure, homeowner responses were good indicators of the level of mosquito reduction achieved by our treatments. This year drought suppressed mosquito populations to levels where homeowners were less able to distinguish between treatments and those receiving only water were almost as satisfied as those whose yards were sprayed with insecticide. Customer satisfaction alone may not always be a reliable indicator of mosquito population suppression on their property. Verification would require supplemental monitoring using one or more traps like those used in this study. Some pest managers may even want to consider offering periodic mosquito monitoring to selected clients (perhaps as a maintenance contract) as is offered for other pests.

This study evaluated only the effect of pyrethroid sprays as a treatment tool. Comprehensive mosquito management services also consider the removal of standing water, strategic larvaciding and client education, including advice on personal protection. Instituting such a program will maximize the chance for homeowners to regain the use of their yards.

All photos are courtesy of Michael F. Potter.

Drs. Grayson Brown and Michael F. Potter are professors at the University of Kentucky. Rebecca Trout is a former graduate student and senior laboratory technician at the same institution. Partial funding for the study was provided by the National Pest Management Association through its Pest Management Research Foundation. The authors would like to thank All-Rite Pest Control, Lexington, Ky., for assistance in performing treatments.