Evaluation of compounds for insecticidal activity on adult mosquitos. 1. Responses of adult mosquitos to some new insecticides.

Bull. Org. mond. Santt Bull. Wld Hlth Org. 1970, 42, 353-368 Evaluation of Compounds for Insecticidal Activity on Adult Mosquitos* 1. Responses of Adult Mosquitos to some New Insecticides A. B. HADAWAY, F. BARLOW, J. E. H. GROSE, C. R. TURNER & L. S. FLOWER Responses shown by female mosquitos to topical applications of solutions, to deposits from water-dispersible-powderformulations, or to exposure to the vapour from such deposits, are described for a number of compounds that have shown promise as residual contact insecticides. The behaviour of Anopheles stephensi females in laboratory tests indicates that departures ofmosquitos from treated buildings, and their survival rates, are likely to be increased as a result of irritation and consequent activity after only a briefperiod of contact with phthalthrin (OMS-1011) and other synthetic pyrethroids but not with dieldrin (OMS-18) and lindane (OMS-17) or the carbamates and organophosphorus compounds tested. Mosquitos were not disturbed by exposure to the vapour from deposits of any of the compounds. The rates of action of the different compounds following their application in solution directly to the insect cuticle or contact of the mosquitos with deposits on plywood and plaster emphasize the importance of the lipid-solubility and partitioning properties of an insecticide in determining its passage from a surface deposit into, and through, the cuticle to the site of action. Evaluation of the insecticidal activity of new compounds in screening programmes is generally based upon the mortality of test insects in suitably designed bioassays. In our tests, which form part of the collaborative programme conducted by the World Health Organization for the evaluation of insecticides for vector control, additional information is obtained which may be of value in the prediction of the probable effects of house treatments on mosquito populations or in the interpretation of results obtained in experimental window-trap huts. Such information on the rate of action, as well as the toxicity, of some promising new insecticides, the behaviour of mosquitos in contact with them, and their possible fumigant effect, is assembled in this report. MATERIALS Test insects Mosquitos (Anopheles stephensi List. and Aedes aegypti L.) were reared by standard methods at 25°C and a relative humidity of 70°/-80%. * From the Ministry of Overseas Development, Tropical Pesticides Research Unit, Porton Down, England. 2476 Insecticides The compounds tested are shown in the following list. OMS-1: malathion, diethyl mercaptosuccinate S-ester OMS-2: OMS-14: OMS-15: OMS-17: OMS-18: with O,O-dimethyl phosphorothioate, and its oxygen analogue malaoxon fenthion, O,O-dimethyl 0-[4-(methylthio)m-tolyl] phosphorothioate, and its oxygen analogue fenoxon dichlorvos, 2,2-dichlorovinyl dimethyl phosphate m-isopropylphenyl methylcarbamate lindane, 1,2,3,4,5,6-hexachlorocyclohexane, gamma isomer dieldrin, 1,2,3,4,10,1O-hexachloro-6,7-epoxy- 1,4,4a,5,6,7,8a-octohydro-1,4-endo-exo-5,8OMS-29: OMS-33: OMS-43: OMS-93: dimethanonaphthalene carbaryl, l-naphthyl methylcarbamate o-isopropoxyphenyl methylcarbamate1 fenitrothion, O,O-dimethyl 0-(4-nitro-mtolyl) phosphorothioate methiocarb, 4-(methylthio)-3,5-xylyl methylcarbamate 1 The name "propoxur" has been recommended by the British Standards Institution for this compound. -353 - 354 A. B. HADAWAY AND OTHERS OMS-123: fenchlorphos, O,O-dimethyl 0-2,4,5-trichlorophenyl phosphorothioate OMS-597: Landrin, 3,4,5-trimethylphenyl methylcarbamate OMS-658: bromophos, 0-(4-bromo-2,5-dichlorophenyl) O,O-dimethyl phosphorothioate OMS-666: o-allyloxyphenyl methylcarbamate OMS-708: Mobam, 4-benzothienyl methylcarbamate OMS-712: 2-chloro-1-(2,4-dichlorophenyl)vinyl dimethyl phosphate OMS-716: promecar6' ni-cym-5-yl methylcarbamate OMS-775: 0,2-chloro-1-(2,4-dichlorophenyl)vinyl 0,0dimethyl phosphorothioate OMS-971: Dursban,, O,O-diethyl 0-(3,5,6-trichloro-2pyridyl) phosphorothioate OMS-1011: phthalthrin, ' 2,2-dimethyl-3-(2-methylpropenyl)cyclopropane carboxylic ester with N- (hydroxyniethyl)-l-cyclohexene-1,2-diacarboximide OMS-1075: Cidial, ethyl mercaptophenylacetate ester with O,0-dimethyl phosphorodithioate OMS-1094: 2,3-dihydro-2-methylfuran-7-yl methylcarbamate OMS-1 21 1: iodofenophos, 0-(2,5-dichloro4-iodophenyl) O,O-dimethyl phosphorothioate METHODS Topical applications,of insecticides to adult mosquitos were made from a microburette similar to that described by Kerr (1954). It consists of a 15-cm length of precision-bore glass capillary tubing with a bore diameter of 0.2 mm, drawn out and bent at right angles at one end so that the bore diameter at the ground tip is about 50 ,t. The microburette is filled with a solution of insecticide in di-isobutyl ketone and is mounted on the mechanical stage of a microscope so that the image of the meniscus of the solution in the capillary is thrown on to a graduated scale adjacent to the tip of the microburette by means of a camera lucida arrangement with a magnification of x25. When an insect is held in contact with the tip of the microburette, solution flows from the capillary; the volume applied is determined from the movement of the image of the meniscus on the graduated scale. Unfed female mosquitos, 1-2 days old, were lightly anaesthetized with carbon dioxide and a known volume of solution was applied from the microburette to the dorsal surface of the thorax of each insect. Two batches of 20 mosquitos were treated at each of 4 dosages obtained by varying the volume applied from 0.01 [LI to 0.025 [lI per mosquito. After treatment, each batch of mosquitos was placed in an unwaxed paper cup with a gauze top, provided with sugar solution, and stored at 25°C and a relative humidity of 70% for 24 hours before a mortality count was made. The median lethal dose of the insecticide was determined by plotting the dosage-mortality data. The rate of action of the insecticide was also determined by inspection of the various batches of mosquitos at regular intervals of time after treatment. Contact toxicity tests were carried out at 25°C and a relative humidity of 50 %7-55 % with deposits from aqueous suspensions of water-dispersible-powder formulations sprayed on to plywood or plaster of Paris panels in a modified Potter tower at a standard dosage of 1 g of active ingredient per Mi2. Plywood panels were cut from sheets of 6-mm Thames Marine, makore-faced plywood, B.S.S. 1088. Plaster panels were prepared by casting a slurry of plaster of Paris (BDH) in moulds on a glass plate, levelling the surface and allowing the plaster to set. The panels, measuring 8.5 cm square by 0.5 cm thick, were then easily removed from the moulds and were stored at 25°C. The surface sprayed was that which had been in contact with the glass plate and was smooth and shiny. The treated panels dried quickly and, unless otherwise stated, were ready for testing about 3 hours after spraying. Mosquitos were expose (...truncated)