ABSTRACT

A comparison of two formulations of Permethrin was carried out in Cape May County, NJ in 1994. These formulations are designed for use as a ULV spray. Permanone 31-66 is diluted with an oil diluent, and is currently available for use in the United States. Aqua-Permanone, on the other hand, has been created to be mixed with water and is an experimental formulation not registered for use in the U.S. at this time. Applications utilized a truck mounted Beecomist 25HD ULV aerosol sprayer; spray effectiveness was monitored with caged Aedes sollicitans placed at three distances (150, 300, and 600 feet) from the point of application. Both formulations in conjunction with the Beecomist 25HD produced droplets in a range acceptable for ground ULV application. There was no significant difference in mortalities between the two formulations. The Aqua-Permanone/water mix produced coarse air bubbles as a result of sloshing in the mix tank. These bubbles had an effect on the FMI metering pump of the Beecomist unit.

INTRODUCTION

Nearly 36% of Cape May County's geographic area is comprised of coastal wetlands. The salt marsh mosquito Aedes sollicitans is the predominant species produced on these wetlands. The status to Aedes sollicitans as the epidemic vector of eastern equine encephalitis in coastal New Jersey (Crans 1977, Crans et al. 1986) coupled with its extensive flight range forces the Cape May County Mosquito Commission (CMCMC) to annually devote much of its energy and budget to this mosquito's surveillance and control.

Adulticiding is an important facet of the IPM program employed by the CMCMC. ULV applications against Ae. sollicitans take place both aerially utilizing a Hiller UH12E helicopter outfitted with two (2) Beecomist rotary heads, and from the ground using truck mounted Beecomist 25HD aerosol sprayers. While aerial ULV applications annually accounts for the majority of the acreage treated and materials used, ground ULV treatments remain a vital part of our adulticiding program.

For the last several years the synthetic pyrethroid resmethrin (Scourge 18 + 54), has been used almost exclusively when adulticiding with ground equipment, and has performed well. This past season, however, our Commission had the opportunity to assist in the comparison of two different products. The trials described here were conducted as a comparison of ground ULV applications of two formulations of Permethrin: (1) Permanone 31-66 in a conventional oil diluent, and (2) Aqua-Permanone, an experimental formulation diluted with water.

TEST LOCATION

Trials were carried out in an open field adjacent to Rt. 47, one mile south of the CMCMC compound in Cape May County, NJ. The site is located adjacent to the coastal wetlands of the Delaware Bay shore, wetlands that serve as breeding habitat for Ae. sollicitans. The field is approximately 0.5 miles square and bordered by mixed hardwood tree species on all but the eastern side. Grass and other herbaceous and woody vegetation in the field rarely exceeded 3 feet in height. Fields such as the one used for this field trial are important as resting and host seeking habitat for potentially disease important populations of Ae. sollicitans (Crans et al. 1976, Crans and Ebsary 1977).

MATERIALS AND METHODS

A Beecomist 25HD ULV aerosol sprayer equipped with a variable-flow pumping system was used for all trials. Prior to application the pump was calibrated for proper flow rate using an internal calibration standard of 10 mph. Samples of spray droplets were collected on microscope slides for droplet size spectrum determinations. For Permanone in oil, teflon coated slides were used. For Aqua-Permanone, magnesium oxide coated slides were used.

Larvae and pupae of Aedes sollicitans were collected in the field several days prior to the test, and adults were allowed to emerge in screen cages under laboratory conditions. Adults were given 10% sucrose in water as needed. On the day of the trial, 25 adult mosquitoes were transferred with mouth aspirators to 3.5 in. (d) by 5.5 in. (1) cylindrical field exposure cages fabricated from 20 mesh aluminum screen. A cotton ball soaked in 10% sucrose was placed on the top of each cage. Cages were held and transported in styrofoam coolers to the field site.

At the test site, a straight application path was marked perpendicular to wind direction. Bamboo stakes (5' high) were placed in three rows parallel to wind direction and perpendicular to the line of application. The rows were 75 feet apart, each consisting of three stakes placed 150, 300, and 600 feet from the application line. Two additional stakes upwind held control cages.

Table 1. Material Specifications

A theoretical swath width of 300 feet is assumed in the calculation of rate per acre. Immediately prior to each trial, screen cages were hung horizontally from the top of all stakes. The existence of any dead subjects was recorded on each cage just prior to treatments. Application of insecticide was begun 150 feet before the first row of stakes and continued for 150 feet beyond the last row encountered. Wind speed and direction, temperature and humidity were recorded during each run. Cages were removed from the stakes immediately after the spray cloud had passed. Control cages were handled and stored separately from the treated cages.

A total of four spray runs were made; two each of Permanone 31-66 and Aqua-Permanone 20-20. The ULV sprayer was flushed and re-calibrated for proper flow rate between materials. Within 10 minutes of exposure, mosquitoes were transferred to clean cages using an electric powered aspiration device. The clean cages with exposed Ae. sollicitans were then returned to the laboratory, and 10% sucrose on cotton balls was supplied to any cages containing live mosquitoes. Cages were held under controlled conditions (27'C, 80% RH, LD 14: 10). Mortality was determined in each cage at 3, 12, and 24 hours post-exposure.

Exposed mosquitoes were quickly knocked down, but most were capable of varying degrees of movement. The ability to beat wings was determined to be an easily visible all-or-none criterion. Individuals not capable of beating wings were considered dead. An artifact of this criterion is that a small percentage of subjects "revived" between post-exposure counts, resulting in lower raw mortality in later readings than in earlier ones. This occurred only in a few holding cages and was not considered significant.

Raw treatment mortality for each post-treatment interval was corrected for control mortality occurring at each respective interval using Abbott's Formula. Analysis of Variance was used to determine significance of differences between treatments at each distance and post-treatment time interval.

RESULTS

Knockdown was apparent within minutes of application with both materials. Most mortality occurred within 3 hours of treatment. An increase in control mortality between 3 and 12 hours probably masks a real increase in treatment mortality at 12 and 24 hours.

Table 2. Corrected mean percent mortality, of caged Aedes sollicitans

Post-Treatment Interval

There was no significant difference in response between conventional Permanone and Aqua-Permanone at any distance/post-treatment interval combination(a=0.5). The differences at 300 feet approached significance, e.g. P = .065 at 12 hours.

The VMD of spray droplets produced at a flow rate of 5 oz/min was 10.06 um for Permanone 31-66 in oil, and 15.4Azm for Aqua-Permanone in water. Both are acceptable for ULV space spraying. The difference may be real and due to the differences in the formulation, or the necessity of using M902 for sampling the water-based spray may have introduced error, e.g. due to improper spread factor. A recognized spread factor of 0.86 was used on the M902 slides, but

the correctness of this was not determined.

Only one problem was encountered in handling the material. When Aqua-Pennanone was mixed with water and carried in a vehicle on any but the smoothest roads, it formed course air bubbles due to sloshing inside the mix tank. The FMI metering pump used on the Beecomist 25HD (and on many other makes of sprayers) tended to act erratically when the bubbles were pulled into the insecticide line. Calibration is likely to be affected.

REFERENCES CITED

Crans, W.J., 1977. The status of Aedes sollicitans as an epidemic vector of eastern equine encephalitis in New Jersey. Mosq. News 37: 85-89

Crans, W.J., J. Downing and M. Staff, 1976. Behavioral changes in the salt marsh mosquito, Aedes sollicitans as a result of increased physiological age. Mosq. News 36: 437-445

Crans, W.J., J. McNelly, T.L. Schulze and A. Main, 1986. Isolation of eastern equine encephalitis virus from Aedes sollicitans during an epizootic in southern New jersey.

J. Am. Mosq. Control Assoc. 2: 68-72

Ebsary, B.A. and W.J. Crans., 1977. The physiological age structure of an Aedes sollicitans population in New Jersey. Mosq. News 37: 647-653