ASSESSMENT OF TWO POSSIBLE PRETREATMENT METHODS OF PREVENTING SORPTION OF INSECTICIDE RESIDUES BY DRIED MUD.

PRETREATMENT FOR PREVENTING INSECTICIDE SORPTION BY DRIED MUD position and interaction of the substituent groups, and the rates of breakdown or detoxification and penetration to the site of action. * * * We are grateful to Coalite and Chemical Products Ltd, Midland Tar Distillers Ltd, William Pearson Ltd, 585 Imperial Chemical Industries Ltd, and Cocker Chemical Co. Ltd for their generosity in supplying samples of phenols and to California Chemical Co., Farbenfabriken Bayer A.-G., Hercules Powder Co., Hooker Chemical Co., Schering A.G., Shell Development Co., Upjohn Chemical Co., and Union Carbide Co. for samples of carbamates. Assessment of Two Possible Pretreatment Methods of Preventing Sorption of Insecticide Residues by Dried Mud by A. B. HADAWAY, Officer-in-Charge, and F. BARLOW, Chemist, Tropical Pesticides Research Unit, Porton Down, Salisbury, Wilts., England Sorption of particulate insecticides from the surface of dried muds used in house construction in many tropical countries is accompanied by a reduction in their effectiveness against adult mosquitos resting on the surface, and the biological activity of the sorbed insecticide is influenced considerably by atmospheric humidity.a Many alkyl phenyl N-methylcarbamates that have a potential use as residual contact insecticides for adult mosquito control are sorbed more readily on a given soil than the chlorinated hydrocarbons DDT and dieldrin, and the influence of humidity on the availability of the sorbed carbamates is also greater.b Several attempts have been made to inhibit sorption of chlorinated hydrocarbons and increase the persistence of surface deposits by altering the insecticide formulation. For instance, Gerolt c prepared new wettable powders based on ground, solidified melts of dieldrin and an adjuvant such as coumarone resin, a synthetic resin (Aroclor), or gilsonite, and laboratory tests showed that the persistence of dieldrin on a sorptive mud surface was greatly extended. The superiority of the dieldrin/resin formulations over the standard dieldrin wettable powder disappeared, however, in field trials at Arusha, Tanzania, when contact periods were unrestricted and the average relative humidity was comparatively high.d a Barlow, F. & Hadaway, A. B. (1958) Bull. ent. Res., 49, 315, 333. b Hadaway, A. B. & Barlow, F. (1963) Bull. ent. Res., 54, 329. c Gerolt, P. (1961) Bull. Wld Hith Org., 24, 577. d Tiel, N. van (1961) Bull. Wld Hlth Org., 24, 593. 1591B It would appear that with an insecticide such as dieldrin, with a high intrinsic toxicity to adult mosquitos and no irritant action, sorption is of little or no significance in practice provided that atmospheric humidity is high. However, there are situations where sorption of insecticides applied to dried mud walls may be an important factor in the failure to obtain adequate control of adult mosquitos and where the authorities are prepared to consider pretreatment of walls in preference to the expensive alternatives of larviciding and mass drug administration. It is appreciated that pretreatment of walls is a laborious procedure to be avoided if possible, but it was considered worth while to re-examine possible methods of preventing or delaying sorption. Materials and methods Plywood panels or dried mud bricks made from our standard sorptive red Babati soil, or similar test surfaces pretreated as described later, were sprayed in a modified Potter tower with aqueous suspensions of wettable powder formulations at a dosage of 1 g active ingredient per M2. They were stored and tested at 25°C and 50%-55Y% relative humidity. Mosquitos (Anopheles stephensi List.) were reared by standard methods at 25°C and 70 %-80 % relative humidity. Batches of 12-15 females, 2-3 days old and blood-fed, were exposed on treated panels or dried mud bricks for known times by the method described previously b and were then transferred to paper-cup cages and stored for 24 hours before mortality counts were made. 586 NOTES Pretreatment with whitewash It is customary in some areas to coat the dried mud walls of houses with " whitewash ", and, if it could be shown that such a practice considerably extended the effective life of an insecticide, then consideration could be given to the promotion of this practice in other areas before spraying programmes are embarked upon. " Whitewash " is a general term, and the chemical nature of the deposit varies. Either calcium hydroxide or calcium carbonate may be the material first applied to walls. If the hydroxide is used, reaction with carbon dioxide in the air results in a slow conversion to the carbonate, and the stability of an alkali-labile insecticide deposited on the surface is likely to vary according to the time elapsed since the hydroxide was applied. In this report, " limewash " will be used to describe deposits of calcium hydroxide, or slaked lime, and " whitewash" to describe those of calcium carbonate. The chlorinated hydrocarbons DDT and dieldrin are stable on limewash, but the substituted phenyl N-methylcarbamates so far examined decompose rapidly when applied to fresh limewash and lose their toxicity to adult mosquitos.e It was necessary, therefore, to check on the stability of such compounds on the less alkaline deposits of whitewash before investigating their persistence on whitewashed mud bricks. Plywood panels were coated with whitewash or whitewash/size. The calcium carbonate used was " whiting ", as defined in the specification of the British Whiting Federation. It was mixed with water in the ratio of 1.5 :1 and the thick cream spread over a row of plywood panels with a spreader as used in thin-layer chromatography. The dried layer was about 0.25 mm thick and the average weight about 600 g/m2. Where size was required, the whiting was mixed with a 2% w/v animal size solution instead of water. The pretreated panels, together with untreated ones, were sprayed with aqueous suspensions of wettable powders at a dosage of 1 g active ingredient per M2. All mosquitos died after an exposure of two minutes on all panels sprayed with a wettable powder of 6-chloro-3,4-xylyl N-methylcarbamate at 0, 1, 2, 4, and 8 weeks after treatment, indicating at least no rapid decomposition of this persistent insecticide. e Hadaway, A. B. & Barlow, F. (1963) Bull. Wld Hlth Org., 28, 63. TABLE 1 CONTACT TOXICITY TO ANOPHELES STEPHENSI OF 3-ISOPROPYLPHENYL N-METHYLCARBAMATE AT A DOSAGE OF 1 g/m2 ON TREATED AND UNTREATED PLYWOOD PANELS Mean kill (%) after contact of: of Test surface Plywood deposit (weeks) 2 min 0 100 2 100 4 54 8 min 1 15 min 100 15 96 5 0 12 2 100 100 4 68 0 72 100 58 8 6 60 min 61 0 16 Plywood/ whitewash 300 min 12 16 100 54 32 100 84 Plywood/ whitewash/ size 2 100 100 4 40 0 8 12 16 92 23 100 ~~~15 89 ~~~~~488 The percentage kills of mosquitos exposed on panels sprayed with a wettable powder containing 50% 3-isopropylphenyl N-methylcarbamate are given in Tab (...truncated)