METABOLIC DISRUPTORS (SEMIO CHEMICALS) AND THEIR UTILIZATION AS A PEST MANAGEMENT STRATEGY Nalini C. Gnanapragasam (Actg He.ad, Nematology division, lea Re.6e.an.ch Institute, o-f. S/u. Lanka, TahwakeJle) Since the t ime cultivation of crops became a system of organized agriculture, insect species t ha t were pests on these cultivated crops have been controlled by various devious means. This bat t le against insect pests has been a continuous one, with a few instances of complete success. Most often the winner was the pest insect . The old tradit ional methods of control included, biological control which involved the use of certain other species of insects as ei ther parasites or predators on the pest insects . Hence the former came to be categorized with the group of beneficial insects. Such an approach had led to instances where pest insects came to be managed very successfully. The most successful one ever achieved on a plantation crop was the control of our own tea tor tr ix , with the introduction of the parasitic wasp, Macrocentrus homonae, from Indonesia. Besides the harnessing of other beneficial insect species, biological control programmes have, since of l a t e , successfully utilized parasitic nematodes in a big way, as welt as various pathogens including fungi, • bacteria and viruses tha t cause disease outbreaks in insects. On account of the crucial role tha t such disease causing organisms play in the maintenance of an ecological balance amongst insect species, insect pathology has evolved into a specialized branch of study of i t s own. The other old traditional form of insect pest management has been the adoption and modification of certain selected cultural practices, tha t render either the plant itself, or the environment in which i t grows, less favourable for the pest insects and consequently reduce the damage they cause. The use of selected resistant varieties of crop species is a well-known old technique tha t is still 16 http://Re.6e.an.ch employed as one of the most reliable means of pest m anage m ent. Since the mid-194-Os various synthetic biocidal chemicals have been widely used in insect pest control. The drastic turn of events took place with the advent of DDT, which emerged as the 'miracle insecticide 1 during World War II. Following the discovery of DDT, several other synthetic insecticides were tested and used very widely and these were even regarded as 'chemical miracles' t ha t ensured bountiful harvests. Such a pesticidal approach ' (though apparently a very successful one tha t overshadowed al l other ecologically acceptable traditional means of pest management), l a t e r led to several instances where the farmers and other cultivators had been forced to depend entirely on the use of such chemicals, so much so tha t they were on a 'pesticidal tread-mill ' . The above pesticidal approach dominated the agricultural scene for nearly twenty five years from mid- 1940s to early 1970s. Besides leading to several instances of pest resurgence, secondary pest outbreaks and the development of resistant strains, various other forms of harmful side-effects were observed on the environment, including those on man himself. Thus, the wide-scale and careless use of this resourceful management too led to an almost t o t a l dislocation of the delicate ecological balance. Chemical pesticides still have an important role to play in pest management, provided their use is carefully integrated along with other ecologically acceptable means of management, ra ther than resorting to their use as a unilateral approach. Such a situation led to the pesticide dilera ma tha t is presently confronting us. The usefulness of selective pesticides cannot be • denied and i t must be accepted tha t we will have to depend on these to meet the growing demands towards increased productivity of the various agricultural co m m oditi.es. Such selective chemicals should have the leas t impact on the ' environment. Yet, these insecticides are biocides t ha t are likely to have some form of harmful effects on the non- ta rge t organisms in the long run. I t is as a consequence of such an understanding tha t scientists are presently turning their at tention towards those 17 http://oditi.es chemicals t ha t will not have any effect whatsoever on the environment but are highly specific in their activity a t the behavioural or a t the general growth physiology level of t a rge t insect species. These are not hiocides but are chemicals t h a t cause certain physiological disorders either leading to physical deformity during development processes, sexual sterility or cause confusions in chemical communication, particularly during the process of locating mates, thereby markedly reducing the offsprings. Such group of chemicals have now come to be referred to as 'semio chemicals ' . The t ime has arrived for man to venture into outer space on journeys t h a t will take a very long t ime and as a consequence has to synthesise his own food, using his own waste products harnessing solar energy. We have to think far ahead to cope with the pressures of ever increasing demands and the consequent fight for survival. The entomologists are no exception. Novel approaches to pest management will therefore have to be worked out from now onwards. The search far Semio Chemicals: A detailed understanding of the feeding behaviour and the nutrition of the insect is the first step t h a t has to be taken before one ventures into carrying out research for selecting compounds tha t could block certain essential metabolic pathways. The field of insect nutrition and insect behavioural physiology is one tha t is receiving the greates t attention today in many an entomological research progra m m e. Most insects cause damage to plants during their efforts a t obtaining nutrients needed for their growth and development. Therefore, an elucidation of a detailed knowledge of cri t ical dietary requirements of an insect is the first step one has to take to evolve a technique to induce disruptive physiological changes. Such an approach could be undertaken only by initially developing a completely defined artificial diet to r ea r the insect through successive generations in la* oratory cultures. Such a study would provide the essential information about those cri t ical dietary ingredients needed 18 for normal growth and development. The composition of such perfected diets could then be altered both quantitatively and qualitatively in order to create certain nutritional faults in the diet t ha t would lead to imbalances of essential nutrients. Another way is to block the availability and assimilation of the cri t ical dietary ingredients t h a t will lead to sterility and/or physical deformity and consequently lead to the development of a large population of abnormal and deformed insects t h a t will compete with normal ones in the field. The diet t ha t could be truly classified as a completely defined one (holidic diet) should contain only known ingredients of very high purity, and should enable the insect to develop normally through successive generations. To claim this as a holidic diet, there should be further proof of the complete absence of any microbial symhionts t ha t could possibly contribute certain essential dietary ingredients. A basic knowledge of the nutrition of insects has been made use of by other investigators in recen t t imes to develop and induce specific nutritional imbalances in the host plant (Prat t e t al, 1972). For example, some non- genetically linked 'resistance' has been induced by changing the physiology of host plant by the regular application of specific artificial fertil izers and other soil adjuvants for the control of the aphid, Myzus persicae (Wooldridge 6 Harrison, 1968; Harrewijn, 1972), Brevicoryne brassicae (Van Embden, 1966) and the spruce budworm, Choristoneura fumiferana (Shaw & Little, 1972). Recent studies have demonstrated t h a t a very good potential exists for creating such imbalances especially in respect of amino acids, in the case of our own t ea tor t r ix , Homona coffcarta. An exogenous supply of the amino acid, threonine was found to be very cri t ical for the development of the t ea tortr ix, since in i t s absence this insect failed to reach maturity. An additional exogenous supply of the amino acids tyrosine and cystine, provided in amounts slightly more than what is really needed, proved to be very detr imental and arrested development. Tea is a crop t h a t places a very high demand on ferti l izers, especially nitrogenous ferti l izers which could be 19 supplied in different forms, including sulphate of ammonia and urea. These nitrogenous fertil izers are usually provided along with others including muriate of potash, phosphates and magnesium salts. Thus a good potential exists for manipulative changes, which could influence the synthesis of some of the cri t ical amino acids in the t ea plant. There is already some evidence for developing such 'resistance' in young tea plants to parasitic nematodes through alteration of potash levels (Gnanapragasam, 1982). The fa t ty acids, caproic, caprylic, capric, myristic, oleic and arachidonic acids, when supplied as additional exogenous ingredients were found to be toxic to the t ea tor tr ix . The possihility of using fa t ty acids to control insects has been reported by House (1967) and Maw and House (1971). The polyunsaturated fa t ty acids, linc-laic and linolenic acids were found to be very cri t ical dietary supplements to the tea tor tr ix . Results of diet-deletion studies in which linoleic and linolenic acids were deleted but supplemented with the saturated fa t ty acids, stearic and lauric acids showed tha t the l a t t e r could adequately substitute critically needed polyunsaturated fa t ty acids. In the absence of these fa t ty acids the final moult of the pupa to adult was affected and moths failed to emerge. When provided in sub-optimal amounts, only part ial emergence of adults was observed and the few tha t did emerge were with malformed crumpled naked wings. Thus by making the above essential fat ty acids unavailable to the insect or by providing small amounts of the toxic acids in the diets, i t is possible to suppress the maturation of the tortr ix pupa to the adult. Metalic ions like copper are known to oxidise unsaturated fa t ty acids (Fruton & Simmonds, 1961). This effect was clearly demonstrated by inducing typical fa t ty acid deficiency symptom in tortr ix moths which emerged from artificial diets t reated with as low as 12.5 ppm of copper. The emerging moths were deformed. Thus the use of copper seems to have a potential to control this insect by interfering with the processes of metamorphosis to the adult (Sivapalan & Gnanapragasam, 1980). 20 Copper fungicides are applied in t ea fields to control blister blight leaf disease. When t ea fields are sprayed with copper fungicides (treated a t the ra te of 180 g of cuprous oxide/150 l i t res of water/ha against blister blight (De Silva, 1966) the copper content in harvested leaves, one week following t rea tment , has been found to be in the order of 25-40 ppm. A residual amount of copper up to U0 ppm (150 ppm in the dried Black Tea) is the accepted tolerance l imit for this metallic ion in t ea . However, such t rea tments are usually given in the wet months which i s non-seasonal for tor tr ix outbreaks. I t is worthwhile to t ry such t rea tments in the tortr ix season as welL The present investigation has also shown t h a t , as i n the case of a l l o the r i n s e c t s (o ther than some spec ie s i n which some of the c r i t i c a l d i e t a r y i n g r e d i e n t s a r e provided by the a s s o c i a t e d symbionts) , t he t e a t o r t r i x depends e n t i r e l y on an exogenous supply of s t e r o l s , which is very vital for normal growth and development. They are important components of membrane structures, are precursors of growth and moulting hormones and are also constituents, of the surface wax of insect cuticle. Since insects cannot synthesize sterols on their own, certain compounds t h a t are likely to have an inhibitory action against the assimilation of sterols will make the l a t t e r unavailable to the insect, thereby blocking the physiological process of development. Amongst various tested metabolic disruptors, compounds like the group of azasterols, 25-azacoprastane and the non-steroidal amine-N, N dimethyl-tetradecanamine in very minute amounts were found to very significantly suppress development of this insect . This suppression seems to be due to the possible blockage of the required amount of dietary s terol and consequently interfering in the formation of ecdysteroids, which are the moulting hormones, essential for metamorphosis from l i rva t o pupa and then to adult. Non­ steroidal amines, having the structures with chain lengths C12 - C H are also known to possess s t ructural similarity to juvenile hormones. Therefore the compound N, N dim ethyltetradecana mine could have functioned as an antimetabolite of the juvenile hormone consequently disrupting development. 21 The results of such investigations have thus opened up new avenues for further studies and exploiting the possibility of using such physiological control agents in the field, which has absolutely no harmful effects on the environment, being extremely specific to the t a rge t insect only. Selective metabolic disruptors and a n t i m e t a b o l i t e s , which are now given the specific name of 'Semio Compounds' can now be tested in the field for thei r potential use in an integrated pest management programme. Possibilities also exist for the incorporation of selected chemicals in the diet t ha t lead to sterility and thus making possible the mass rearing of sterile males in the laboratories t h a t could be released from t ime to t ime to compete with natural field populations and thus achieve some degree of control. The study of nutritional requirement have also offered several useful clues for the selection of specific feeding a t t r ac tan t s t h a t should be further exploited a t the pract ica l leve l by offering such diets in the field. Such diets can be incorporated with selected che m osterilants t h a t are likely to induce sterili ty in the natural field populations. This programme could be coupled to one of baiting males with the use of female sex pheromones which work is already in progress, and thus evolve a balanced integrated programme of management of pest insects in t e a . In a similar manner, detailed studies on the nutrition of other insects , including the shot-hols borer is likely to open up new vistas in pest management. References DE SILVA, R.L. (1966) . Recent experiments with new fungicides for the control of blister blight (Exobasi- dium vexans Massee) on t ea . Tea Q. 37, 121-127. FRUTON, J . S . & SIMMOHDS, S. (1961) . General Biochemistry (2nd Ed). John Wiley, USA. 1075 pp. GNANAPRAGASAM, N.C. (1982) . Effect of potassium fert i l iza­ tion and of soil temperature on the incidence and pathogenicity of the root-lesion nematode, (Pratylen- chus loosi Loof on t ea . (Camellia sinensis L.) Tea Q. 51, 169-174. 22 HARREWIJN, P . (1972) . Wing production by the aphid Myzus persicae related to nutritional factors in potato plants and artificial diets, pp. 575-588 In: Insect and Mite Nutrition. Ed. J.G. Rodriguez, North Holland PuhL Co. Amsterdam & London. 702 pp. HOUSE, H.L. (1967) . The nutritional s tatus and larvicidal activities of C^ to C 1 . saturated fa t ty acids in Pseu- dosarcophaga affinis. (Diptera:Sarcophagidae) on synthetic diets. Can. Ent. 99, 1310-1321. MAW, M.G. & HOUSE, H.L. (1971) . On capric acid and pota­ ssium capricate as mosquito larvicides in laboratory and field. Can. Ent. 52, 237-240. PRATT ( J r ) J . J . , HOUSE, H.L. & MANSINGH, A. (1972) . In­ sect Control Strategies based on Nutritional Princi­ ples: A Prospectus, pp 651-668. In: Insect & Mite Nutrition. Ed. J.G. Rodriguez, North Holland PuhL Co. Amsterdam & London. 702 pp. SHAW, G.G. & LITTLE, C.H.A. (1972) . Effect of high urea fertilization of balsam fir t rees on spruce budworm development, pp. 589-597. In: Insect & Mite Nutri­ tion, Ed. J.G. Rodriguez, North Holland PubL C. Amsterdam & London. 702 pp. SIVAPALAN, P. & GNANAPRAGASAM, N.C. (1980) . Influence of copper on the development and adult emergence of Homona coffearia (Lepddoptera: Tortricidae) reared in vitro. Ent. exp. & appl. 28, 59-63. VAN EMBDEN, H.F. (1966) . Studies on the development of insects and host plant. II. A comparison of the reproduction of Brevicoryne brassicae and Myzus per­ sicae (Hemiptera:Aphidae) on brussels sprout plants supplied with different ra tes of N and K. Ent. rsp.