Abstract:
A method of controlling mosquitos by adsorbing onto the surface of a body of water suitable for breeding mosquitos a monomolcular or duplex film of an organic material which reduces the surface tension of the body of water to 30 dynes/cm or less.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains generally to mosquito control and in particular to a nonpoisonous mosquito larvae and pupae control. 
     A common approach to mosquito control is to prevent the emergence of the adult from its aquatic breeding site through the application of a water-immiscible, organic chemical which forms a film on the water. It is necessary for these film-forming chemicals and the surface films which they form to have certain physicochemical properties to ensure not only effectiveness against aquatic forms of mosquito, but also ease of application and persistence. The chemicals must be nonionic, nonvolatile, and water immiscible liquids. Furthermore, the chemicals and the resulting films must have a low freezing point, be commercially available at a reasonable cost, and be nontoxic and be noncorrosive. It is also necessary for the surface film to be fluid with rapid and spontaneous spreading and with high respreading potential. 
     Unfortunately, the presently used compounds fail to have one or more of these properties. Petroleum-based oils have, in general, the disadvantages of fouling the shore areas, killing other marine life and vegetation, and extra cost due to a large usage requirement. The nonvolatile hydrocarbon component of these films containing only alkanes and cycloalkanes are slow in their impact on larvae. The more rapid toxic effects are obtained from volatile aromatics and other toxic compounds contained in these films which are drawn into the larvae breathing trachae during respiration. However, these compounds have poor resistance because of their volatility and often are toxic to too many other aquatic creatures. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of this invention to provide a nontoxic and environmentally safe control of mosquitos. 
     Another object of this invention is to provide a method of controlling mosquitos which has little difficulty in application and produces persistent results. 
     A further object of this invention is to provide an economical method of controlling mosquitos. 
     These and other objects are achieved by the adsorption to the surface of the water of a nonionic, autophobic or nearly autophobic, immiscible, nonvolatile fluid which has a density less than that of water and high degrees of fluidity and which is capable of reducing the surface tension of the water to 30 dynes/cm or less, so that the immature forms of mosquito are prevented from attaching themselves to the surface of the water by a persistant biologically inert means which is only one molecule thick or, on the average, greater than one but less than twenty molecules thick. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The monomolecular or duplex films of this invention are adsorbed to the water surface by adding to the water a compound or composition described below at a surface concentration of 80 microliters/square meter of surface or less and preferably 40 microliters/square meter of surface or less. A surface concentration of 40 μl/m 2  is about 20 times greater than that required for monomolecular layer. Consequently, the lower limit to the above ranges would be about two microliters per square meter for a monomolecular film covering the entire water surface. The excess allows the film to be replenished automatically when the need arises. It should be noted that even if the entire surface is no longer convered by the monomolecular film of this invention the film would continue to eradicate the immature forms of mosquitos in the portions of surface which are still covered. 
     The compounds encompassed by the practice of the present invention are nonionic and autophobic or nearly autophobic. An autophobic material is one which does not spread over its own monomolecular films. An excess of the material remains concentrated at the point of addition. The nearly autophobic compounds generally come from the small impurities present in the commercial grade of many of the compounds encompassed by the present invention. The impurities compromise the autophobicity of the compounds with the result that the films varies in thickness from one molecule to several molecules. This latter type of film is referred to as a duplex film. The duplex films behave in the same manner as the monomolecular films. In fact, if the amount of material added to the surface of the water is equal to or less than that required to cover the surface with a monomolecular layer, the film which would otherwise be duplex would be monomolecular. 
     The compounds of this invention have a density of less than that of water, a freezing point of less than 5° C., a boiling point of 170° C. or higher and preferably of 200° C. or higher, an HLB number of 10 or less and preferably of 6 or less, and a bulk viscosity of less than 1000 centistokes at the temperature of use and preferably less than 200 centistokes. The fluidity of the compound solutions is such that the compounds or solutions have a spreading velocity of 10 cm/sec for the first 100 cm and preferably 14 cm/sec for the first 100 cm. Most importantly, the compounds reduce the surface tension of the water to 30 dynes/cm or less. This particular combination provides a means of lowering the surface tension which is persistent enough to be used in many environments. 
     The preferred compounds or compositions for the practice of this invention are: sorbitan monooleate, a solution of 70 volume percent to less than 100 volume percent of sorbitan monooleate and 2-ethyl butanol; saturated, branched chain alcohols with a total carbon from 15 to 19 carbon atoms and one to three oxyethylene groups; unsaturated cis alcohols with 15 to 19 carbon atom chain length; unsaturated ethers with a chain length of 12 to 18 carbon atoms and three to five oxyethylene groups; and oleyl ether with two oxyethylene groups. Combinations of any of the above compounds are useful. The most preferred compounds or compositions are: sorbitan monooleate; a solution of 75 volume percent of sorbitan monoleate; and 25 volume percent of 2-ethyl butanol; isostearyl alcohol with two oxyethylene groups; lauryl ether with four oxyethylene groups; a solution comprising 50 volume percent of sorbitan monooleate and 50 volume percent of lauryl ether with four oxyethylene groups; and oleyl ether with two oxyethylene groups. 
    
    
     In order to demonstrate the practical utility of this invention, the following examples are given. It is understood that these examples are given by way of illustration and are not intended to limit this disclosure. 
     EXPERIMENT I 
     Distilled water was covered with a film of liquid in equilibrium with a small excess lens. Surface tensions were measured at 25° C. by the ring method using a Kruss du Nouy tensiometer. Surface tensions of the film-covered water remained essentially constant over a period of 24 hours for the liquid compounds and formulations used in this research shown in Table 1. 
     
                       Table 1______________________________________Monomolecular Organic Surface Films andSurface Tensions                    SurfaceFilm                     TensionLiquid Composition            N/m     dyn/cm______________________________________A     Diethylene glycol monolaurate                        0.0276  27.6B     Sorbitan monooleate, 75%; 2-ethyl                        0.0289  28.9 butanol, 25%C     Sorbitan monooleate, 37.5% lauryl ether containing 4 oxyethylene groups, 50% 2-ethyl butanol, 12.5% 0.0283  28.3D     Isostearyl alcohol containing 2 oxyethylene groups   0.0282  28.2E     Lauryl ether containing 4 oxyethylene groups   0.0271  27.1______________________________________ 
    
     EXPERIMENTS II &amp; III 
     In both laboratory and field experiments the films were applied at a surface concentration of 40 μl/m 2 . This value is about 20 times that required for a single monomolecular layer, and was used to provide an excess of chemical to resupply losses from the film. The excess material did not spread over its own film, but remained as an unspread patch or liquid lens which acted as a reservior to maintain complete coverage of the water surface. In order to test the breadth of effective of the compounds of Table 1, Anopheles quadrimaculatus and Aedes aegypti were treated. These two species of mosquitos are two of the most deadly species of mosquitos. The Anopheles quadrimaculatus is the major carrier of malaria and the Aedes aegypti is the major carrier of yellow fever. 
     II 
     Influence of Surface Films on Anopheles quadrimaculatus 
     Films of liquids, A,B,C, and D were spread over water in polyethylene-lined trays (25×51 cm) in the laboratory and onto the surface of four similar ponds in a natural paludal setting. An additional control tray and pond were maintained in both laboratory and field experiments, with the exception of the terminal studies where the control organisms were treated with a film of isostearyl alcohol (2 oxyethylene groups) to provide additional data. Fourth-instar larvae of Anopheles quadrimaculatus had been placed into the trays and ponds several hours prior to application of the film-forming liquids to allow time for the larvae to adjust to the new environment. The field ponds had a surface area of about 4 m 2  and were created in a drainage ditch with wooden barriers to segregate the five equal areas. The ponds were lined with polyethylene sheet to prevent water and chemical exchange between them and to facilitate larval mortality counts. The results are given in Tables 2, 3, and 4. 
     
                                           Table 2__________________________________________________________________________Laboratory Data, Effecton Monomolecular Films onImmature Stages ofAnopheles Quadrimaculatus                   Cumulative Mortality             Number                   After Indicated Hours             Larvae                   of Posttreatment                               PercentFilm              Pupae Exposure    totalLiquid    Composition   Used  Hours                       Larvae                           Pupae                               Mortality__________________________________________________________________________A   Diethylene glycol mono-             322 Larvae                   24  247 0   76.7    laurate             48  264 1   82.3*                   72  269 1   83.8B   Sorbitan monooleate, 75%             100 Larvae                   24  95  0   95.0    2-ethyl butanol, 25%                   48  95  0   95.0                   72  95  0   95.0B   Sorbitan monooleate, 75%             93 Pupae                   24      13  56.5    2-ethyl butanol, 25%                   28      13  56.5.sup.+C   Sorbitan monooleate, 37.5%             435 Larvae                   24  435 0   100.0    Lauryl ether (4 oxyethy-    lene groups) 50%    2-ethyl butanol, 12.5%D   Isostearyl alcohol             462 Larvae                   24  462 0   100.0    (2 oxyethylene groups)D   Isostearyl alcohol             25 Pupae                   24      25  100.0    (2 oxyethylene groups)    Controls      462 Larvae                   24-72                       0   0   0.0__________________________________________________________________________ *one(1) Adult successfully eclosed after 48 hrs. .sup.+ Two(2) (2) and eight(8) adults successfully eclosed after 24 and 2 hrs, respectively Pupae has been added to SMO 75/2EB used in previous study with 100 larvae; film was 20 hrs old at time of emplacement. 
    
     
                       Table 3______________________________________Laboratory Data, Rapid Effect on Monomolecular Filmson Larval Stages of Anopheles quadrimaculatus Film                Time     LiveLiquid Composition         (Min)    Larvae______________________________________C     Sorbitan monooleate, 37.5%                      0       25 Lauryl ether (4 oxyethylene                      7       10 groups), 50%        13        4 2-Ethyl butanol, 12.5%                     15       1(3rd                              instar)B     Sorbitan monooleate, 75%                      0       100 2-Ethyl butanol, 25%                     16       16                     26        7                     41       5(3rd                              instar)D     Isostearyl Alcohol   0       462 (2 oxyethylene groups)                     15       20                     20       3 (3rd                              instar)                     35        3                     45        1                     24 hrs    0______________________________________ 
    
     
                                           Table 4__________________________________________________________________________Field Anopheles Effect of Monomolecular Filmson Larval Stages of Anopheles quadrimaculatus*            Hours of                 Cumulative Larval                           Percent TotalLiquid    Film Composition            Exposure                 Mortality Mortality__________________________________________________________________________A   Diethylene glycol            24   3554      71.1    monolaruate  48   3786      75.7            72   4065      81.3            120  4942(*)   98.8B   Sorbitan monooleate,            24   5000      100.0    75%    2-ethyl butanol, 25%C   Sorbitan monooleate,            24   3850      77.0    37.5%    Lauryl ether (4 oxyethy            48   4156      83.1    lene groups), 50%            72   4365      87.3    2-ethyl butanol, 12.5%            120  4842      96.8D   Isostearyl alcohol            24   10,000.sup.+                           100.0    (2 oxyethylene groups)    Controls     24   0         0.0            48   0         0.0            72   55        1.1__________________________________________________________________________ *Observed mortality after 72 hrs may be partially due to unknown causes. Water striders were active on pools 48 hrs after treatment. No controls available for comparison after 72 hrs. .sup.+ Reflects the results of two(2) replicates 5,000 larvae each. 
    
     III 
     Surface Film Effects on Aedes aegypti 
     The experimental procedure of Example II was repeated with this species of mosquito. 
     The larvae, pupae, and emerging adults were adversely affected by monomolecular films of the tested compounds, but not to the same degree as the Anopheles quadrimaculatus was. The results are summarized in the following table. 
     
                                           Table 5__________________________________________________________________________Influence of Monomolecular Surface Filmson Pupae and Emerging Adults of Aedes aegypti      Number           24 Hour      48 Hour      of   Mortality    MortalitySurface Film      Pupae           Pupae               Adults                   Percent*                        Pupae                            Adults                                Percent*__________________________________________________________________________Isostearyl alcohol      96   78  0   81.2 93  3   100containing twooxyethylene groupsSorbitan monooleate,      124  88  7   76.6 89  31  96.8.sup.+75%2-ethyl butanol, 25%Lauryl ether contain      80   80  0   100  80  0   100ing four oxyethy-lene groupsSorbitan monooleate      106  81  10  85.8 81  25  100Control    50    0  0   0     1  0    2__________________________________________________________________________ *Percent total mortality (pupae and adults). .sup.+ Four successful eclosions. 
    
     As the experimental results prove, the organic films of the present invention produce a highly effective means for eradicating immature forms of mosquitos and thus mosquitos themselves. Since the compounds of the present invention do kill the immature forms of mosquitos by reducing the surface tension of the water so that the larvae and pupae sink and drown, rather than by suffocation or poisoning, relatively small amounts are needed and little or no harm occurs to the other forms of aquatic life. Consequently, the compound provides the most environmently safe means for eradicating mosquitos. 
     The present invention was first reported in detail in Garrett, W. D. Mosquito Control in the Aquatic Environment with Monomolecular Organic Surface Films. NRL Report 8020. NRL, July 16, 1976. This report is herein incorporated by reference. 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.