Evaluation of compounds for insecticidal activity on adult mosquitos*

Bull. Org. mond. Sante 1970,42,381-385Bull. Wid Hkth Org. Evaluation of Compounds for Insecticidal Activity on Adult Mosquitos* 4. Insecticidal Activity and Physical Properties of Some Halogenated Organophosphorus Esters A. B. HADAWAY, F. BARLOW, J. E. H. GROSE, C. R. TURNER & L. S. FLOWER The effects of changes in structure upon toxicity to adult mosquitos and upon physical properties which are important in determining contact effectiveness and residual activity are described for a group of OO-dialkyl O-halogenated-phenyl phosphates, phosphoro- thionates and phosphamido(thio)ates. This group includes such established insecticides as fenchlorphos, bromophos and iodofenphos. For some years the Tropical Pesticides Research Unit, Ministry of Overseas Development, England, has been evaluating the toxicities to adult mosquitos of a wide range of candidate insecticides. Results can be selected from the testing programme and arranged to illustrate the effects of changes in structure within a group ofclosely related compounds upon toxicity and upon physical properties which are important in determining contact efficiency and residual action. An interesting group comprises the O,O-dialkyl O-halogenated-phenyl phosphates, phos- phorothioates and phosphoamido(thio)ates, some of which (e.g., fenchlorphos and bromophos) are established insecticides. The properties of 13 phos- phoramidates of this type have already been de- scribed (Hadaway & Barlow, 1965). MATERIALS Those compounds which were solids were puri- fied by crystallization. The two liquids, OMS-659 and OMS-1301, were not specially purified but exam- ination by gas-liquid chromatography and determi- nation ofthe esterified phenol contents both indicated high purity and the compounds were therefore con- sidered suitable for comparisons of toxicity. Their purities were certainly adequate for the physical property measurements since both were miscible with the solvent chosen for solubility determinations and specific methods of analysis were used in vola- tility measurements. * From the Ministry of Overseas Development, Tropical Pesticides Research Unit, Porton Down, England. BIOLOGICAL AND PHYSICAL PROPERTIES A measure of the intrinsic toxicity of each com- pound to adult female mosquitos of the species Anopheles stephensi and Aedes aegypti was obtained by topical application of solutions in di-isobutyl ketone, and of the contact toxicity of deposits to An. stephensi by the method described by Hadaway & Barlow (1963). Solubilities in n-hexane were obtained by the pro- cedure given in Hadaway & Barlow (1966), and volatilities from residues on glass-fibre filter-papers.1 All the compounds were determined in extracts of residues by the 4-amino-antipyrine (4-aminophena- zone) method for phenols produced on alkaline hydrolysis. RESULTS AND DISCUSSION Some physical properties of the compounds, melt- ing point, solubility in n-hexane and volatility from glass-fibre filter-papers, are listed in Table 1, the median lethal doses determined by topical applica- tion in Table 2, the initial and residual contact toxicities in Table 3 and the stability of some of the compounds on a tropical soil in Table 4. The most extensive bioassay results are those for intrinsic toxicity as measured by topical application 1 Methods described in Evaluation of insecticides for vector control. 2 (WHO working document VBC/68.66a). A limited number of copies of this document is available to persons officially or professionally interested on request to Distribution and Sales, World Health Organization, 1211 Geneva, Switzerland. 2479 -381 4 A. B. HADAWAY AND OTHERS TABLE I SOME PHYSICAL PROPERTIES OF THE COMPOUNDS TESTED Melting Solubility Compound point in Volatilityb Stabilityc(OC) n-hexane a OMS-1283 (0,0,-dimethyl 0-2,5-dichloro-4-iodophenyl phosphate) 64-65 1.4 0.036 315 OMS-1301 (0,0-dimethyl 0-2-chloro-4-bromophenyl phosphorothioate) - d miscible 0.50 - OMS-123 (0,0-dimethyl 0-2,4,5-trichlorophenyl phosphorothioate) 41-42 62 0.30 8.2 OMS-B58 (0,0-dimethyl 0-2,5-dichloro-4-bromophenyl phosphorothioate) 54-58 12 0.15 8.7 OMS-1211 (0,0-dimethyl 0-2,5-dichloro-4-iodophenyl phosphorothioate) 74-75 3.0 0.0055 8.2 OMS-1210 (0,0-diethyl 0-2,5-dichloro-4-iodophenyl phosphorothioate) 49-50 20 0.0055 1.7 OMS-59 (0,0-diethyl 0-2,5-dichloro-4-bromophenyl phosphorothioate) - d miscible 0.070 - OMS-1285 (0-ethyl 0-methyl 0-2,5-dichloro4-lodophenyl phosphorothioate) 44-46 25 0.018 - OMS-10 (0-methyl 0-2,4,5-trichlorophenyl ethyl-phosphoramidate) 68-69 2.5 0.0070 693 OMS-1302 (0-methyl O-2,5-dichloro-4-bromophenyl n-propylphosphoramidate) 58-59 8.9 0.0025 6 600 OMS-1287 (0-methyl 0-2,5-dichloro-4-iodophenyl isopropylphosphoramidate) 109-110 0.26 0.010 4 620 OMS-5 (0-methyl 0-2,4,5-trichlorophenylethyl phosphoramidothioate) 42-43 approx. 100 0.029 41 OMS-1290 (0-methyl 0-2,5-dichloro-4-iodophenylethyl phosphoramidothioate) 66-67 2.2 0.0010 38 a Percentage w/v saturated solution at 25°C.b Rate of loss (g/ml/day) from an Initial deposit applied at the rate of Ig/m' on glass-fibre fllter-paper. c Pseudo-first-order rate constants, k min-" x 10', in 38 % ethanol + 62 % N sodium hydroxide by volume d Liquid at room temperature (20°C-25°C). TABLE 2 TOXICITIES OF VARIOUS COMPOUNDS TO FEMALES OF 2 SPECIES OF MOSQUITOS BY TOPICAL APPLICATION LDso (ng/mosquito) Compound An. sfephensi Ae. aegypti OMS-1283 OMS-1301 OMS-123 OMS-658 OMS-1211 OMS-1210 OMS-659 OMS-1285 OMS-1302 OMS-1287 OMS-5 OMS-10 OMS-1290 5.0 150 25 19 11 19 21 14 22 11 32 8.4 14 5.0 150 19 15 8.0 42 64 15 18 25 18 13 12 but, before describing the effect of structure on this and other properties, it is worth stressing that the relative potencies of even quite closely related com- pounds depend upon the test species of insect. Among the substances mentioned, the susceptibility of An. stephensi relative to Ae. aegypti for the same substance varies from a ratio of 0.3 to 1.8. For example, the diethyl esters OMS-659 and OMS-1210 have ratios of 0.3 and 0.5, respectively, while the ratios for the corresponding dimethyl esters are 1.3 and 1.4, respectively. Presumably, these changes reflect differences in penetration, distribution, meta- bolism and target susceptibility between the two species of insects. OMS-123 (fenchlorphos), OMS-658 (bromophos) and OMS-1211 (iodofenphos) form a series in which the halogen occupying the 4-position of the phenyl ring in an O,O-dimethyl 0-2,5-dichlorophenyl phos- phorothioate is successively changed from chlorine to bromine to iodine. The intrinsic toxicities decrease in the order I -Br - Cl for both species of mos- quitos, yet the contact toxicities of deposits on ply- wood show the reverse order. This seems to be a situation where the relative importance of the 382 EVALUATION OF INSECTICIDAL COMPOUNDS AGAINST ADULT MOSQUITOS. 4 TABLE 3 CONTACT TOXICITY TO FEMALE AN. STEPHENSI OF WETTABLE-POWDER DEPOSITSa Mean percentage kill of mosquitos after exposure Residual Substrate Compound to insecticides for following numbers of minutes: toxicity _~ ___ 2 5 15 30 60 in weeks b Plywood OMS-123 5 58 100 4-8 OMS-658 5 78 100 8-12 OMS-1211 0 18 77 >24 OMS-1210 0 37 93 >24 OMS-1285 3 28 92 >24 OMS-1283 2 43 98 >24 OMS-5 0 44 95 OMS-10 0 0 -c OMS-1287 0 0 -c OMS-1290 0 3 -c Plaster OMS-123 100 OMS-658 20 63 100 OMS-1211 21 58 OMS-1210 20 83 a At a rate of application of 1 g of active ingredients/m2. b Number of weeks when the percentage mortality exceeded 70 % after an exposure period of 1 hour. c Initial toxicity not high enough to warrant residual toxicity testing. various physical properties which infi and penetration has altered according of application. Lipid solubility is importance when the insecticides are deposits and the solubilities of these thr in n-hexane do fall in the same ord toxicity. Another pair of compounds, TABLE 4 DECOMPOSITION OF VARIOUS COI ON TROPICAL SOIL a AT 25 Compound OMS-123 OMS-658 OMS-1211 OMS-1210 OMS-1283 Half-life of coi on tropical (Ugand; 20 % relative humidity 3.5 3.5 4 5.5 uence pick-up strate the same structural change are OMS-5 and to the method OMS-1290. Again the iodo-derivative has a higher of particular intrinsic toxicity than the chloro- compound while present as dry the reverse is true of contact toxicity. The effects of ee compounds the same structural changes on residual action are ler as contact also shown by the results given in the tables. Where- which demon- the main cause of loss of insecticide from a deposit is evaporation, as on plywood, the residual actions follow the measured volatilities. The.4-iodo- com- MPOUNDS pound, OMS-121 1, has a very low volatility and has. i'c a much longer residual action than OMS-658 and mnpound OMS-123. On the other hand, loss of deposit and a) soil (days) toxicity which depends upon a hydrolytic decom- 80 % relative position, such as could occur on some soils or limed humidity surfaces, will show little effect from the changing halogen since the various halogens at position 4 do 6.5 not materially influence either the specific base- 6.0 catalysed hydrolysis of Table 1 or the soil-catalysedhydrolysis of Table 4. 8 8 0.5 a From Uaanda. A second kind of structural change is illustrated by compounds OMS-1211, OMS-1285 and OMS- 1210. In these, first one of the methyl ester groups& of OMS-1211 is replaced by ethyl (OMS-1285), and then the other (OMS-1210). This progressive re- 383 A. B. HADAWAY AND OTHERS placement of methyl by ethyl results in a reduction of intrinsic toxicity to the mosquitos. Contact toxicity with dry deposits follows a different descend- ing order however, OMS-1285 - OMS-1210 OMS-1211. Again, this order of contact toxicity is matched by the solubilities in n-hexane. OMS-1285 should be less persistent on plywood according to volatility measurements but all three compounds are of such a level of volatility that bioassays did not indicate any differences within the test period of 24 weeks. As usual, the diethyl ester is more stable than the dimethyl in hydrolytic decomposition measurements in homogeneous aqueous-ethanolic systems but shows little difference in decomposition on soils. OMS-658 and OMS-659 confirm the effects of the same structural changes with the diethyl compound, being less toxic than the dimethyl to mosquitos. This is the opposite of their respective toxicities to mammals. A third kind of comparison can be made between phosphates and phosphorothioates, or the corre- sponding amidates and amidothioates. Examples of such pairs are OMS-1283 and OMS-1211, and OMS-10 and OMS-5. In both cases the P=O compound has a greater intrinsic toxicity to mos- quitos than the P= S compound. This is reasonable because the sulfur compound requires to be meta- bolically converted to the oxygen analogue within the insect and it is unlikely that such a process is completely efficient. The contact toxicity of OMS- 1283 is greater than that of OMS-1211 although the difference is only small. As the solubilities are of the same order, 1.4 and 3.0, respectively, and the intrinsic toxicities have the opposite influence of a similar magnitude, these properties probably cancel each other. On the other hand, the P= S compound, OMS-5, has an appreciably better contact toxicity than the P=O compound, OMS-10. In this case, while the latter has the higher intrinsic toxicity, the former is much more soluble and this is presumably the factor of decisive importance. When similar pairs of compounds have been examined previously, the phosphate has usually had the higher melting point, lower volatility and greater persistence on non-porous surfaces. OMS-1283 is an exception to this finding, having a lower melting point and higher volatility than OMS-1211. A comparison of the OMS-10-OMS-5 pair, however, supports the more general conclusion. As expected, the phosphates are much more susceptible to specific base-catalysed hydrolysis and also hydrolyse more rapidly on soils than do the phosphorothioates. Consequently, they are unlikely to have any prolonged activity after sorption on many soils. The compounds OMS-1301 and OMS-658 show the effect of placing a chlorine atom in the 5-position of the phenyl ring in O,O-dimethyl O-4-bromo-2- chlorophenyl phosphorothioate. It causes a large increase in intrinsic toxicity for both species of mosquitos tested. The same substitution also reduces volatility by a factor of 3 although OMS-658 is still relatively too volatile for satisfactory residual activity. The phosphoramidates and amidothioates such as OMS-1302, OMS-1287 and OMS-1290 have moder- ate intrinsic toxicities but such low contact toxicities that comparisons of the relative effectiveness of spray deposits cannot be made. Poor contact activ- ity is a feature of this class of compounds even though some have very favourable solubility pro- perties and is worthy of further investigation. The phosphoramidates are especially liable to decompo- sition, both in basic solutions and on soils. All three compounds have low volatilities and should have a long residual activity against susceptible insects on non-porous surfaces. This was expected since it was already known that substitution of -NH2 or alkyl-NH radicals for one of the -OCH8 groups in a substance like OMS-123 results in a decrease in volatility of about 10 times. When this change is combined with the substitution of heavier halogen atoms at the 4-position of the ring, which also cause decreases in volatility, it is not surprising that OMS-1302, OMS-1287 and OMS-1290 are very persistent. This group of compounds, therefore, displays a wide range of intrinsic and contact toxicities to mosquitos, and of properties such as volatility and stability which influence their persistence on various surfaces. Compared with many standard insecticides used for mosquito control, however, even the best of this group have only moderate toxicities, especially when they also possess the desirable attributes of persistence and stability. The members which com- bine the best features are the O,O-dialkyl O-4-iodo- 2,5-dichlorophenyl phosphorothioates, such as OMS- 1210, OMS-1211 and OMS-1285. Phosphates are less stable and phosphoramidates and phosphor- amidothioates have very poor contact activity. 384 EVALUATION OF INSECTICIDAL COMPOUNDS AGAINST ADULT MOSQU1TOS. 4 385 RtSUM1 EVALUATION DE L'ACTIVITi INSECTICIDE DE DIVERS COMPOSES SUR DES MOUSTIQUES ADULTES: 4. ACTIVITE INSECTICIDE ET PROPRIETES PHYSIQUES DE QUELQUES ORGANOPHOSPHATES HALOGENES Un groupe de composes comprenant des O,O-dialcoyl O-phenylphosphates halogenes, des thiophosphates et des amidothiophosphates presente une gamme etendue de toxicites, en application topique et par contact, a 1'egard des moustiques adultes (Anopheles stephensi et Aedes aegypti) et tout un eventail de caracteristiques physiques, volatilite et stabilite notamment, dont dependent l'effi- cacite insecticide et la remanence. Leur 6tude permet de determiner l'effet des modifications de la structure chimique sur le pouvoir toxique et les proprietes physiques. L'OMS-123 (fenchlorfos), I'OMS-658 (bromofos) et l'OMS-1211 (iodofenfos) sont trois composes dans les- quels l'halogene situe en position 4 sur le cycle phenyle d'un O,O-dimethyl 0-2,5-dichlorophenylthiophosphate est le chlore, le brome ou l'iode. La toxicit6 intrinseque, en application topique sur le moustique, decroit selon que l'halogene est l'iode, le brome ou le chlore, mais la toxi- cite de contact des d6pots d'insecticide varie de fagon inverse. Les solubilites dans I'hexane sont modifi&es dans le m8me sens que les toxicit6s de contact. Le remplace- ment du chlore par le brome et surtout par l'iode a pour resultat de reduire la volatilite du compose et d'augmenter sa remanence. D'autres investigations ont porte sur les effets du remplacement successif de l'un et l'autre radicaux methyle de I'OMS-1211 par un radical 6thyle, donnant les composes OMS-1285 et OMS-1210. Cette modification entraine une reduction progressive de la toxicit6 intrinseque, tandis que la toxicit6 de contact decroit de I'OMS-1285 A I'OMS-1210et i l'OMS-1211. Ona proced6 a des comparaisons du m8me ordre entre phosphates et thiophosphates ou entre amidates et thioamidates corres- pondants. En general, m8me les plus actifs des composes 6tudies ne font preuve que d'une toxicit6 mod6ree pour les mous- tiques adultes, surtout s'ils possedent en outre les qualit6s requises de persistance et de stabilit6. REFERENCES Hadaway, A. B. & Barlow, F. (1963) Bull. ent. Res., 54, 329 Hadaway, A. B. & Barlow, F. (1965) Bull. Wid Hilth Org., 33, 129 Hadaway, A. B. & Barlow, F. (1966) Bull. ent. Res., 56, 569