Arsenical Insecticides

The use of arsenic compounds as insecticides is by far their most important application, more than 70 per cent, of the world's production being devoted to this purpose. The widespread dissemination of such compounds, in the form of dusts or spray liquids, coming into contact with edible products and with the soil, constitutes a problem which requires strict methods of control. The composition of the insecticidal mixture depends upon the nature of the pest to be eradicated and the sensitivity of the plant. It is essential that the most effective compound should be used and that amounts in excess of the desired toxicity should be avoided owing to possible injury to the foliage, arsenic being poisonous also to plant life, and because of the necessity of removing residual arsenic from fruit and vegetables before marketing. Arsenates are much less toxic, both to insects and plants, than arsenites, and are therefore generally employed, being less likely to cause damage to the crops. The two most generally effective arsenates are lead hydrogen arsenate, PbHAsO₄, and calcium arsenate, Ca₃(AsO₄)₂, but others may be employed. For the plum curculio the order of toxicity of the metallic arsenates is as follows: PbH > Ba > Ca > Zn > Mn > scorodite (native iron arsenate). The dust or spray mixture generally contains, in addition to the arsenate, sulphur and slaked lime, and may contain such substances as calcium carbonate, iron oxide, aluminium sulphate or silicate, casein or starch. The addition of a fish oil or a mineral oil as an emulsifier increases the adhering power. The Schweinfurt greens are frequently employed, and typical spray liquids may be prepared by passing through a colloid mill suspensions of 1 to 5 g. of copper arsenite or aceto-arsenite per litre of water with 1 per cent, of starch or 10 per cent, of kaolin.

The pests are destroyed by absorbing the powder either through the mouth or through the body by contact, and the sprays and dusts should contain minimum quantities of water-soluble arsenic, as this is mainly responsible for foliage injury; the fineness of division also has an effect, and injury is increased if the arsenate is too highly dispersed. In the United States a legal limit (0.75 per cent. As₂O₃) is imposed as to the amount of water-soluble arsenic that may be present in lead arsenate sold for agricultural purposes. It is necessary, therefore, that the insecticidal mixture should contain some substance which will inhibit the formation of soluble arsenic, but which will not itself cause injury to foliage or decrease the insecticidal value. The best results are obtained with ferric oxide or an excess of freshly slaked high-grade lime, and such mixtures cause no injury to apple and only slight injury to peach foliage. Spray mixtures containing ferric oxide adhere remarkably well to the leaves, and analysis of the latter shows that more arsenic remains on them than when lime is used.

The property of adherence of dusts to the plants determines their effectiveness. It varies considerably with composition, lead hydrogen arsenate adhering to a greater extent than magnesium arsenate and this more than calcium arsenate. The presence of slaked lime reduces the adherence. The air condition at the time of dusting also has an effect, the adhesion decreasing with increased wind velocity.

The toxic action of arsenic on foliage is ascribed to the decomposition of glucosides to form a complex with arsenic. The existence in plants of organic acids which are known to decompose arsenates led Potts to examine the water (transpiration and dew) from leaves of 50 species of plants. The aqueous solutions from only three species, known to be resistant to arsenates, were alkaline; all other species gave acid solutions. Calcium arsenate and lead hydrogen arsenate are much more soluble on leaf surfaces than in distilled water, the former being the more soluble. When sprays containing the lead arsenate also contain a large quantity of calcium hydroxide, the foliage upon which they are used may give a slightly alkaline reaction, but rain removes much of the lime, carrying some of the lead arsenate with it. The leaf surface then becomes acid and dissolves calcium and lead at the expense of the arsenate, and the free arsenic acid is readily absorbed by the tissues. Much of the protection from injury which is afforded by lime is due to the removal of arsenic with the lime by rains. At the same time, the reduction of water-soluble arsenic may be due to a definite chemical reaction, as the best results are obtained when the ratio As:Ca corresponds closely with that for the possible compound 2CaCO₃.Ca₃(AsO₄)₂. Formation of complex Ca-Pb salts is improbable. Magnesium compounds give larger amounts of soluble arsenic and are therefore less suitable as correctives.

If a soap is added to a lead hydrogen arsenate-hydrated ferric oxide mixture considerable damage and leaf drop is caused, especially if the soap contains a strong base, as in the case of commercial potash fish-oil soap or potassium oleate. Laboratory tests show that more soluble arsenic is formed than with a soap of a weak base such as triethanolamine oleate. The addition of cryolite to an arsenate inhibits the formation of soluble arsenic; fluosilicates cause decomposition. These fluorine compounds may be employed as insecticides instead of arsenates and, in some cases, e.g. against the Oriental fruit moth and the plum curculio, are more effective and considerably less dangerous, since their residues are easily reduced below the limit for human consumption. Manganese arsenate mixtures may be obtained with minimum quantities of soluble arsenic. The commercial salt, " Manganar," does not react with lime- sulphur mixtures to produce soluble arsenic, and a product containing less than 1 per cent, of the latter results 3 by reaction of manganese dioxide and arsenious acid in aqueous medium and in the presence of lime at 100° C. and under a pressure of 80 lbs./sq. in.

The control of apple pests is best accomplished with lead hydrogen arsenate-lime-sulphur mixtures, the method of spraying being more effective than dusting for such pests as the curculio, codling moth or apple scab. The addition of a mineral oil, or a fish oil such as herring oil, to the spray mixture is advantageous, giving better coverage. The fruit after treatment retains varying quantities of lead and arsenic on the surface and the amount of arsenic retained may exceed the British limit of tolerance (0.01 grain As per lb. fruit) and may also cause damage to the fruit; also the toxicity of the lead is cumulative. It is essential, therefore, that these residues be removed immediately after harvesting. The amount of residue varies very considerably with the composition of the spray, the time of spraying, the amount of rainfall between the last application and harvest, and with the variety of apple. For the accurate estimation of such residues it is necessary to analyse a sample of about 50 apples picked at random. The spraying should be done early in the season and, if later applications are necessary, substitutes should be used, otherwise heavy residues remain. Fruit on the lower branches generally retains the largest quantities of arsenic; most of this is found on the skin, about half on the sides and half on the stem ends, very little penetrating into the pulp of the fruit. Pears, which may be treated similarly, retain only slight residues, except in very dry seasons.

Hand wiping or washing with water fails to remove all the arsenic; 2 per cent, aqueous sodium hydroxide or 1 per cent, by volume of commercial hydrochloric acid in water is more effective, but calyx scald due to soluble arsenic is liable to follow, in which case the arsenic can be isolated from the injured tissues. The best treatment is to wash the apples immediately after picking in 1 per cent, hydrochloric acid for 1 minute at 13° to 21° C. and rinse in water immediately. The addition of sodium chloride or sodium sulphate to the hydrochloric acid increases the efficiency of the wash. By such means the residual arsenic is reduced to negligible amounts, and the keeping qualities of the fruit are not affected. Even heavily sprayed tomatoes may be cleaned successfully in this way. The treatment is not satisfactory, however, if oil sprays have been used or if the fruit has become waxy in storage. Accumulations of oil or wax on the skin prevent contact with the acid wash and should be first removed by dipping in a suitable solvent. Petroleum emulsion at 35° to 40° C. is a satisfactory oil remover, while methyl alcohol has been found to be an efficient wax solvent.

With fruit other than apples and pears, where washing is not practicable, calcium arsenate is often a more suitable spray than lead arsenate. The latter is the best control for grape berry moth, but a practical method of residue removal is not available. The problem is more difficult than with apples owing to greater fruit surface per given weight, close packing of berries, which favours clotting and retention of residues, and the highly perishable nature of the fruit. A mixture of lead arsenate and lime is safest to use for peach foliage and fruits which are extremely sensitive to arsenicals. The injury is reduced to a minimum where nitrogen fertilisers are used.

The application of arsenates to such trees as the orange, grape-fruit and citron is found to result in a lowering of the H⁺-ion concentration of the fruit juice, and excessive amounts affect the keeping qualities of the fruit. Judicious spraying, however, besides controlling harmful insects, may thus be beneficial in reducing excessive acidity of some varieties of fruit. Cherries may be treated similarly to apples. A satisfactory wash, which reduces the residues to about 0.005 grain per lb., is a 0.3 per cent, hydrochloric acid solution with 3 minutes immersion. The cherries should stand a few hours after picking, or cracking may occur during the washing operation. For the plum curculio a dust containing 5 per cent, of normal lead arsenate is useful as a spray, and higher concentrations give no better control. Calcium arsenate and acid lead arsenate are of equal effectiveness and are superior to basic lead arsenate as determined by the time required to kill the potato and tomato looping caterpillar.

The larvae of coleoptera in sugar beet fields may be exterminated by repeated spraying with a lead arsenate spray of about 0.5 per cent, concentration during spring. The larvae in all stages of development die within 24 hours. For sugar cane pests an adherent dust containing 1 part of white arsenic or lead arsenate with 5 to 6 parts of finely powdered phosphate rock and 5 per cent, of a neutral mineral oil has been found effective in Hawaii, while an effective spray mixture is also made by agitation of lead hydrogen arsenate with water and fish oil in the proportions 8:14:2. In South Africa a spray mixture of calcium arsenate, lime, molasses and citronella oil has been found effective in killing locusts on sugar cane plantations.

For the larvae of the Colorado potato beetle the established lethal dose of lead hydrogen arsenate is 0.30 mg. per gram of body weight; that of Paris green is less than one-third that amount. The quantity of arsenic remaining in potato fields after treatment is so small as to offer no danger of intoxication. Calcium arsenate is most efficient and economical for the control of the potato flea beetle. A study of the use of similar dusts for the control of June beetles on oak leaves showed that the death of the beetles, which occurred within 72 hours, was due to their eating the poison and not to contact with the dust.

For the Mexican boll weevil a specially prepared calcium arsenate containing up to 20 per cent, of arsenic pentoxide is effective. This is prepared by heating together white arsenic and precipitated chalk in the presence of excess air at 650° C. The cotton plant is not injured by this preparation. In Peru about 30,000 acres of cotton fields are dusted annually from aeroplanes with calcium arsenate. Acid arsenates of calcium appear to be more toxic to boll weevils and to locusts than the basic arsenates. This is probably because the latter must be partially hydrolysed to compounds giving more soluble arsenic before toxic results are produced. The extensive application of such sprays to cotton plants is frequently followed by heavy infestations of the cotton aphis. This appears to be due in the first place to the positive photo- tropism of the winged females to white substances such as the arsenate, chalk or flour. Increase of the aphis population is then aided by the destruction by the spray of the hymenopterous parasites of the aphis.

For the larvae of mosquitoes, copper aceto-arsenite is more efficient than either calcium or lead arsenate. The relative toxicity of dilute acid and alkaline solutions of sodium arsenite to mosquito pupae, which have no mouth opening, has been determined under laboratory conditions. With 0.01 to 0.03 molar sodium arsenite solutions, those at pH 5 were about 4.5 times more rapid in toxic action than those at pH 11. Adsorption appears to be an important step in the process of penetration, and the greater toxicity of the acidic solutions is attributed to greater ease of penetration of the tissues by the un-ionised molecules of the weak arsenious acid contrasted with the difficulty of penetration by the ions of the alkaline solutions. In the case of the larvae a much smaller difference was observed, probably on account of the buffering effect of the intestinal contents. The larvae are more susceptible than pupae to arsenic because the walls of the digestive tract are more permeable than the outer body covering.

Experiments with house flies pointed to a considerable buffering action in the intestine. Solutions of arsenious acid and of the stoichiometric quantities of sodium hydroxide and arsenious oxide to form normal sodium arsenite, containing 15 g. of sucrose per 100 c.c., were fed to adult flies. The pH values of the former solutions were 6.58 to 6.96 and of the latter 11.3 to 11.4, but the toxicities were equal, being 0.14 mg. As per g. body weight - a large value for an insect. None of these solutions was repellent to the flies, but if the pH was increased beyond 11.4 repellent action was observed; house fly bait therefore should not contain more alkali than is necessary to hold the arsenic in solution. The eradication of the tsetse fly by similar means is difficult. There is not much chance of a poisonous dose being taken from the skin of a dipped animal, but a toxic dose can be taken up from an arsenic- impregnated area by means of the proboscis.

A considerable demand for crude white arsenic or sodium arsenite for the destruction of grasshoppers has arisen in recent years. A good standard bait contains 5 lbs. As₂O₃ to 100 lbs. of wheat bran; this is attractive and palatable to the grasshoppers, but the amount required depends on the size and feeding capacity of the insects, so that it is economical to destroy them when young. The Mormon cricket may be controlled by dusts containing calcium or sodium arsenite with 3 to 4 parts of slaked lime, and no serious injury to the alfalfa or grain crop involved occurs.

A sodium arsenate spray gives the best control of powdery mildew of cucumber, and is also effective against American mildew of gooseberry, and apple mildew.

Arsenic pentoxide is found to be effective in weed eradication; thus New Zealand hard-fern may be destroyed by midsummer spraying in dry weather with a solution containing 1 lb. of As₂O₅ in 32 gallons of water, and effective control of acacia scrub or thorn bush has been obtained in South Africa by brushing the freshly cut stumps with a solution containing 1 to 1.5 lb. As₂O₅ per gallon of water. "Wild morning glory" and cactus may similarly be controlled by the use of soluble arsenicals.

Vegetation in fish ponds may be controlled by treatment with white arsenic or commercial sodium arsenite if applied in concentrations of 1 to 2 parts As₂O₃ per million. Several applications each year may be needed. Under these conditions the natural fish foods are uninjured and small fish are not adversely affected. Hard waters require a somewhat higher concentration of arsenic. The latter disappears rapidly from the waters, probably owing to precipitation.

It will readily be understood that the incorporation of arsenicals in the soil is a dangerous practice and may cause, as the concentration of arsenic increases, considerable injury to crops and to animals which feed on them. The total amount of arsenic present in the soil is not necessarily related to the toxicity, but the concentration of soluble arsenic is a more reliable index. Arsenious oxide, sodium and potassium arsenites, and even so-called " insoluble " arsenates, decrease the transpiration of oats, tomatoes and potted plants, and 2 parts per million of calcium arsenate may provide sufficient soluble arsenic to retard definitely root and top growth of arsenic-sensitive plants. An investigation of the growth and yields of cotton crops in S. Carolina, where calcium arsenate was administered for boll weevil control over a number of years, showed that, on all light soils, when amounts as low as 50 lbs. per acre were applied, the yields of cotton were seriously decreased. The addition of lime helped to overcome the deleterious effect of the arsenate. The growth of cow peas was similarly injured. The addition of ferric sulphate to the soil (240 lbs. per acre) in some cases greatly reduced the injury, and it was observed that less injury was shown by soils rich in iron. Laboratory experiments show that a mixture of 20 per cent, red clay soil of high iron content with top soil or grey sandy soil is generally sufficient to remove any injurious concentration of arsenic. The addition of arsenic generally tends to decrease the pH value of the soil. The plants and fruit grown on arsenic-rich soil may absorb a considerable amount of arsenic, but this depends upon the form and manner in which the arsenic has been applied. Thus, addition of 0.001 to 0.004 per cent, of As₂O₃ in the form of lead hydrogen arsenate or copper aceto-arsenite over a period of 5 years did not result in an increase of arsenic content of the grain and straw of mustard plants grown on the treated soil, nor was there any effect on the yield of the grain. Single applications of 0.02 to 0.04 per cent, of As₂O₃ as the copper compound had a very deleterious effect on the germination of mustard, but single applications of 0.01 to 0.02 per cent, in the form of acid lead arsenate had no significant effect on the germination, growth, yield of seed or arsenic content of the plants. In the soil, lead hydrogen arsenate appears to be converted slowly into a basic form, but it can be applied at the rate of 1500 to 2000 lbs. per acre without affecting the growth of plants. It is the least toxic of the arsenates to plants, and the danger of animal poisoning from its use as a spray arises more from its lead content than from arsenic. In China, powdered white arsenic is frequently mixed with the ashes of burnt twigs or grass and added to soil in order to kill worms. The mixture generally contains less than 10 per cent, of As₂O₃ and is used on soil where vegetables such as cabbage are grown.

The wholesale use of toxic sprays and dusts may be accompanied, on occasions, by considerable danger to animals. It is recorded that after a stand of oak timber extending over 3500 acres had been dusted with calcium arsenate from an aeroplane, many wild animals and a great number of bees were found dead; also a number of domestic cattle which had fed on grass from the forest or from the landing field showed the characteristic symptoms of arsenical poisoning. Such dusting is particularly dangerous in bee-keeping districts, especially in wet weather. It has been found that about 0.002 mg. As is a lethal dose for a bee, and the smoke from metal works may contain sufficient to cause widespread destruction. The use of arsenical sprays in closed spaces, such as greenhouses, is undesirable owing to the possibility of the formation of volatile compounds by the agency of fungi which may be present.

The fungicidal properties of arsenic compounds may be applied to the preservation of wood. When wood is treated with arsenical solutions, the wood tannins tend to fix some of the arsenic, and if a soluble chromate or dichromate is added to the solution, chromium also is fixed in the wood fibre after drying. If a solution in which the proportions of arsenate and dichromate are As₂O₅:K₂Cr₂O₇ = 1: > 1.25 is used, no arsenic can be leached from the treated dried wood after shaking 6 hours in 20,000 parts of water. If the ratio is 1:1, about 1 per cent, of the arsenic is soluble and this gives the most suitable degree of toxicity; with higher proportions of arsenate the amount of soluble arsenic rapidly increases. The concentration of the solution used for impregnating the wood should be about 2 per cent, each of arsenic pentoxide and alkali dichromate; if fire-resistant properties are required, soluble phosphates may be added without affecting the applicability of the solution.