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JAMES W. MILES and JAMES E. WOEHST Technical Development Laboratories, National Communicable Disease Center, Public Health Service, U. S. Department of Health, Education, and Welfare, Savannah, Ga. 31402 The
organophosphorus
insecticide Abate has a very low
mammalian toxicity, yet it is extremely effective as a mosquito larvicide The
introduction
in concentrations as low as 0.005 p.p.m. of Abate into potable water stored in
cisterns and drums to control Aedes aegypti larvae has been investigated.
To reduce the frequency of application
and
still maintain concentrations in the range 0.005-0.1 p.p.m., new formulations have been developed; the most promising is in a foamed plastic which releases the insecticide at a controlled rate over a long time. The rate of release depends upon toxicant and surfactant concentration merization
conditions
as well as poly-
of the plastic matrix.
Formulations
have produced lethal concentrations to larvae in reservoirs for 20 weeks.
Come of the more troublesome breeding sites for Aedes aegypti (Lin^ neaus), the mosquito vector of yellow fever, include cisterns, metal drums, and other man-made containers which store water for drinking and household uses in tropical areas. Several larvicidal compounds have been investigated for possible use in treating these containers by Jakob (4) and Brooks and Schoof (J). To be considered for such application, the toxicant must have extremely low mammalian toxicity and should be effective in very low concentrations. One of the more promising compounds investigated was Abate (0,0,0\0'-tetr&meUiyl-0,0'-thiodi-pphenylene phosphorothioate, manufactured by the American Cyanamid Co., Princeton, N. J. ) which is reported to have an oral L D to rats and mice greater than 4000 mg./kg. (3) and is lethal to Aedes aegypti larvae in concentrations below 0.005 p.p.m. A concentration of 1.0 p.p.m. or less of Abate has been accepted as a safe level for treating reservoirs of potable water as a control measure. 5 0
183 Van Valkenburg; Pesticidal Formulations Research Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
184
PESTICEDAL FORMULATIONS RESEARCH
In the work described above, the larvicides were applied as emulsifiable concentrates or granules. There are two disadvantages with emulsifiable concentrates: (1) they produce a turbidity in water which is objectionable to the householders, and (2) the total dosage applied is released instantly. This seriously limits the quantity which can be applied at any given time and makes it necessary to re-treat frequently if the concentration is to be kept low. Granules do not impart turbidity to the water but do release one-third to one-half the applied dose within the first few days. Fortunately, Abate hydrolyzes slowly in water, and larvicidal concentrations can be maintained for long periods of time in clean water with granular applications. Abate also tends to deposit on solid surfaces. This decreases its effectiveness because it migrates to the walls of the container or to debris or organic matter if present in the container with a resulting decrease in aqueous concentration. If lethal concentrations of the toxicant are to be maintained over a time, losses to these surfaces must be replaced. It would be desirable, therefore, if a formulation were available that would release small amounts of Abate into the water over a long time. Several years ago Miles et ai. (6) reported on work with wax and plastic formulations of DDVP (dimethyl dichlorovinyl phosphate), which were designed to release the toxicant into air or water over long times. The plastic formulations described were similar to those proposed by Leeds (5). According to Leeds, if poly (vinyl chloride) and a plasticizer are mixed with a third material to form a paste and if the third material is incompatible with the resin product upon polymerization, a microreticulated structure will be formed which will entrap the incompatible ingredient. It is believed that this type of structure is formed when DDVP is formulated in a PVC resin formulation. When the product is exposed to air or water, DDVP slowly passes from the matrix into the environment. Although the physical properties of Abate are quite different from those of DDVP, it was thought that a PVC resin formulation might prove to be a suitable vehicle for introducing Abate into water. Experimental The first experimental plastic formulations were prepared in the same manner as the early DDVP formulations. Typical combinations of ingredients are shown in Table I. The ingredients were mixed thoroughly Table I.
Typical Non-expanded Plastic Formulations, %
PVC Dibutyl phthalate Surfactant (DowFC172) Abate
No.l
No. 2
No. 3
67.5 30.0
67.5 29.8 0.2 2.5
48.75 48.75
— 2.5
— 2.5
Van Valkenburg; Pesticidal Formulations Research Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
14.
M I L E S A N D WOEHST
Release of Abate in Water
185
Figure 1. Photomicrographs of thin sections of Abate-PVC formulions (a) : Non-foamed (b) : Foamed with CO, (c) : Foamed with azodicarbonamide plus foam stabilizer
Van Valkenburg; Pesticidal Formulations Research Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
186
PESTICIDAL FORMULATIONS RESEARCH
and poured into aluminum molds to a depth of about 1/4 inch, then polymerized in an oven for 12 minutes at 130 °C. The product was a firm, tough, plastic solid with a somewhat disagreeable odor of Abate. A photomicrograph of a thin section of the plastic formulation is shown in Figure la. To test the new formulations, approximately 12 grams of each were placed individually in steel drums containing 35 gallons of water. This was equivalent to about 2 p.p.m. of Abate. Water was withdrawn periodically and analyzed for Abate, and bioassays were conducted simultaneously with third instar Ae. aegypti larvae of the Charlotte Amalie strain (DDT-dieldrin resistant). The tests were conducted for 3 weeks after introducing the toxicant, but during this period no larvae kill was observed in any of the drums. The following procedure was used for chemical analysis for Abate. A 500-ml. sample of water was made slightly alkaline with dilute NaOH and extracted three times with 25-ml. portions of CHC1 . The CHC1 extracts were combined in a 250-ml. beaker and evaporated to near dryness on a hot plate. The residue was transferred quantitatively to a 22 X 175-mm. test tube and evaporated to dryness in a water bath. The Abate residue was oxidized by adding 0.3 ml. 60% HC10 and 0.1 ml. HNOa and heating the test tube in an aluminum block at 200°C. for 1 hour. The test tubes were placed in the block cold, and the temperature was raised gradually to prevent spattering. The resulting orthophosphate was determined colorimetrically by the method of Chen et al 12) with modifications (8). The Abate concentration in all cases was found to be less than 0.005 p.p.m., which is about the limit of detection by this method. The results showed that the rate of release of Abate by these formulations was too slow to be practical. To increase the release rate, the formulation was modified in two ways. A blowing agent was added to foam the product to increase the surface area, and varying amounts of surfactant were added to increase the rate of solution of the Abate into water. The basic formula for the expanded formulation is shown in Table II. 3
3
4
Table II. Typical Expanded Plastic Formulations Percent Abate PVC Dibutyl phthalate Triton X-100 Ammonium carbonate Citric acid Water
5.0 40.0-45.0 40.0-45.0 10.0- 0.0 1.0 3.0 1.0
The organic materials were mixed thoroughly in a mortar, and ammonium carbonate was added and ground with a pestle into a smooth paste. Finally, the citric acid was dissolved in the water and stirred into
Van Valkenburg; Pesticidal Formulations Research Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
14.
M I L E S A N D WOEHST
Release of Abate in Water
187
the formulation. The product was poured into aluminum molds to a depth of about 1/4 inch and heated in a forced draft oven for 20 minutes at 130 °C. Upon cooling, the material formed a tough stable foam. To compare the rate of release of the various formulations, 3 grams of each product were placed individually in gallon jarsfilledwith water. In this experiment the total amount of Abate added to the water was equivalent to about 40 p.p.m. Weights were attached to the expanded Table III. Effect of Surfactant and Foaming on the Rate of Release of Abate Abate Concentration, p.p.m. Not Expanded 0% 10% TX-100 TX-100
Days 1 2 3 4 7 11 15
.009 .013 .041 .042 .057
Expanded 0% TX-100 .012 .026
.050 .072 .118 .156 .123
.034 .045 .046 .028
3% TX-100 ;036 .173 .171 .204 .256 .266 .095
6% TX-100
10% TX-100 .083 .372 .364 .375 .430 .499 .413
.085 .272 .278 .333 .265 .320 .236
Table IV. Rate of Release of Abate by Plastic and Granular Formulations in Water Highest Treat Weeks Cone. ment Ob Abate Found, p.p.m. (average) of Formu Level, 100% served, lation p.p.m.1st mo.2nd mo.3rd mo.4th mo.5th mo. Kill p.p.m. 0
A Β A Β Sand (1%) Sand (1%) β ft
1.3 2.6 4.0 8.0
.017 .019 .023 .023
.019 .031 .049 .044
