Some factors affecting the distribution and rate of action of insecticides.

Bull. Org. mond. Sante Bull. Wld Hlth Org. 11971, 44, 221-224 Some Factors Affecting the Distribution and Rate of Action of Insecticides A. B. HADAWAY 1 The amount of insecticide that reaches a critical site ofaction within an insect is affected by a number of physical and chemical processes both within and outside the insect. This paper reviews some of these processes, with particular attention to those external to the insect, whose action commences at the moment an insecticide is released into the environment. On the basis of research that has been conducted on such processes, conclusions are drawn as to the properties that are desirable, from the point ofview ofinsecticidal efficiency, in new compounds. To fulfil its function as an insecticide, a chemical released into the environment must first reach the target insect, then pass through the chemically complex natural barrier of the integument, and finally arrive in sufficient concentration-either in unaltered form or as an active derivative-at a critical site of action. During this journey various physical and chemical processes progressively reduce the amount that passes from one stage to the next, until only a minute fraction of the dose reaches the site of action and is responsible for the biological response. Ideally we should know what factors determine the progress of a compound through the different stages. Unfortunately these processes become more difficult to study, and hence our knowledge of them decreases, as the distance from the point of release of the compound increases. Thus, after many years of intensive research the site of action of a compound may not be known with certainty, although in most cases it is generally considered to be in the central nervous system. Again, surprisingly little is known about the route taken by an insecticide from the integument to the site of action. The general assumption has been that it passes through the integument into the haemolymph, in which it circulates to the central nervous system and other body tissues. LeRoux & Morrison (1954) concluded from the topical application of '4C-labelled DDT to houseflies that the blood appears to transport but not to accumulate the activity, since the radioactivity in 1 Director, Tropical Pesticides Research Unit, Porton Down, Salisbury, England. 2630 the haemolymph amounted to only 1 % of the total recovery. There is, in fact, little quantitative evidence of the passage of insecticide from the cuticle into the haemolymph. Matsumura (1963) found that malathion topically applied to adult Periplaneta americana is selectively absorbed in the initial stage by the cuticle, and studies by Lewis (1965) indicated that DDT very rapidly saturates the epicuticular wax layer of adult blowflies (Phormia terraenovae) but does not diffuse very freely into the haemolymph. Burt & Lord (1968), however, showed that the haemolymph of Periplaneta americana can carry relatively large quantities of the moderately watersoluble compound diazoxon. This concept of the movement of insecticides from the integument in the haemolymph has been challenged by Gerolt (1965, 1969, 1970), who has produced evidence suggesting that dieldrin and the more water-soluble oxime carbamates migrate laterally in the integument and reach the central nervous system by way of its rich supply of tracheae. Whatever the route taken, however, there is no doubt that within the insect a chemical is exposed to the action of a number of enzymes and biochemical processes, resulting in its conversion to an active derivative or in its degradation and excretion. Only very limited quantitative data are available on the rates at which insecticides penetrate the integument of the insect, and on the factors that affect these rates. This is particularly true for practical formulations of insecticides. The Tropical Pesticides Research Units have for many years been concerned with the efficient, economic, and safe use - 221- 222 A. B. HADAWAY of insecticides in developing countries in tropical areas and have accordingly conducted research on the factors affecting events that take place largely outside the insect-i.e., those that occur from the moment an insecticide is released until it is taken up by the cuticle. Our work thus has a practical bias, but it has indicated some of the properties that are desirable in new insecticides. Starting at the one extreme, the point of release into the environment, considerable information has been accumulated on the factors determining the distribution and deposition of insecticide sprays. Earlier research on the production and behaviour of spray drops of different sizes, carried out in connexion with the control of tsetse and locusts by aerial spraying, paved the way for the equipment and techniques now used in the ultra-low-volume application of insecticides for area treatment against adult mosquitos and for crop protection (Yeo, 1960; Sayer, 1959; reviewed by Hadaway & Johnstone, 1969). Vector control, however, depends less on the direct impaction of insecticides on insects than on their uptake by the insects from deposits in buildings, on clothing, or on vegetation. Although the direct topical application of compounds in solution to the integument can provide valuable information on some aspects of insecticidal activity, it omits the vital stage of movement from a deposit to the cuticle. Olson & O'Brien (1963) found that the degree to which six solutes penetrated the cuticle of the cockroach (entry through the epicuticular lipid being gained via a small volume of organic solvent) was proportional to the extent to which they entered an aqueous phase. Our studies with adult mosquitos indicate that limiting factors in the uptake of insecticides from deposits can include their solubility in lipids as well as their partition characteristics. The proportion of the dosage applied to a material that remains at the surface available for contact with insects is related to the physical properties of the insecticide and its formulation and of the material to which it is applied. Insecticides in the form of emulsions or solutions often penetrate the substrate to a greater extent than those in the form of suspensions of solid particles, which can be filtered off at the surface, and for general purposes water-dispersible powder formulations are preferred to emulsions and solutions. If the active ingredient of a waterdispersible powder is a liquid, however, then on dispersion in water a considerable proportion may be displaced from the inert carrier and form an emulsion that penetrates porous substrates. Insecticides can diffuse from deposits of waterdispersible powders to the integument of insects where the two are in contact, and the duration of contact required to produce a given biological response is a measure of the rate of entry of insecticide. The same response may be obtained after a shorter contact time if the insect can pick up particles of insectic (...truncated)