Contents - Previous - Next

This is the old United Nations University website. Visit the new site at http://unu.edu

Hunger, technology and society

Non-toxic food protectants: comparative nutrition and physiology of pests in relation to the protection of processed foods
The role and status of women in post-harvest food conservation

Non-toxic food protectants: comparative nutrition and physiology of pests in relation to the protection of processed foods

S.K. Majumder
Division of Infestation Control and Pesticides, Central Food Technological Research Institute, Mysore, India

FOOD PREFERENCES OF STORED-PRODUCT INSECTS

Taxonomically and physiologically, stored-product insects occupy a distinct place as they attack and grow on products of low moisture content. High osmotic concentration, low moisture content, and factors that are inimical to most insect life are the ecological conditions under which stored product insects multiply, in contrast to the requirements of agricultural pests and insect vectors. Moisture conservation is a specialized trait in the physiology and morphology of stored-product insects (1). Even within this group of insects it is noteworthy that not all attack all commodities.

Curiously, the distribution of different genera and species of stored-product insects is related to ecological factors and the physico-chemical composition of food products. The phenomenon, to a great extent, is governed by the insects' habits, habitat, and nutritional needs.

Nutrition in its broadest sense is a bond between physiological and ecological phenomena associated with the process of natural selection and competition for food. This has led to the development of a high degree of specialization by insects over a wide nutritional spectrum. The ability of various insects to infest capsicum and coffee beans, to grow on tobacco and strychnine, and to survive on cellulosic materials are extreme examples of such adaptation.

The particle size of a commodity influences the type of infestation it will be subjected to. The phenomenon of specificity-as exhibited for example by Sitophilus orange on whole cereal and Tribolium castaneum on milled cereal -is due to the insects' physical-habitat and chemical requirements (2). Similarly, Caflosobruchus chinensis can thrive on milled whole pulse with husk factor, while it does not grow on processed pulse products (3).

Spiced and processed products are infested by StegobIum paneceum. This species requires phenolic compounds and alkaloids. Another type of specificity is exhibited by certain insects as they preferentially attack either germ, bran, or endosperm (4). The association of mycetomes and internal microflora (see table 1 ) also dictates the capacity
of an insect to breed on a commodity. Olfactory and gustatory factors also govern food preferences. The sterile environment of an axenic culture does not support healthy growth of these insects (table 2).

The spices in general contain volatile and non-volatile oils, proteins, fibre, starch, minerals, tannins, etc. In most cases the characteristic flavour is due to a mixture of several contituents such as alcohols, esters, phenols, etc. The volatile oils of species contain terpenes, sesquin terpenes, alcohols, aldehydes, esters, thiocyanates, sulphides, phenols, and their derivatives (5). Their physiological actions require further investigations. Spices may repel or attract some insects and may be toxic to others. The biological sequence of insect, mould, mite, and bacteria and their commensalism during big-deterioration of grain are quite common.

INFESTATION IN PROCESSED FOODS

Cereals and legumes are processed by milling, grinding, roasting, steaming, and also by formulation. Many blended and chemically fortified products are being introduced on the market. With industrialization and urbanization, the demand for processed and ready-to-serve products is increasing even in developing countries. The traditional products are also increasingly being processed and packed in dry and dehydrated forms for efficient marketing and distribution.

The world-wide emphasis on food quality and balanced nutrition, with particular reference to proteins, vitamins, and minerals in bulk foods, has led to the development of enriched products and unconventional food formulations based on cereals, oilseeds, milk powder, egg powder, fish flour, and dried meat. The rapid progress in food science and technology during the last decade has also resulted in the development of dry, dehydrated, desiccated, and freeze-dried packaged foods. This trend has opened up new ecological patterns, and consequently new problems relating to the food preferences, biology, bionomics, and development of insect populations (6). The simple example is that case-hardening of cereals and pulses and even of macaroni and pasta goods by steaming or parboiling makes them highly resistant to insect infestation. The enzymes of the digestive tract of insects seem to have specific roles in assimilating constituents of cereals and pulses. Casehardening not only induces mechanical resistance to mandibular chewing but also prevents utilization of the gelatinized starch by gut enzymes in stored-product insects.

TABLE 1. internal Fungal Flora (Toxigenic and Non toxigenic) Isolated from Some Stored-Product Insects

CLIMATIC AND GEOGRAPHICAL FACTORS

The international trade of commodities has accelerated the movement and migration of stored-product insects, but the climatic and edaphic factors, represented by the global zones, have restricted their growth and distribution. Distinct geographical distribution of the major species has been well documented. The most typical example of stored product insects is Trogoderma granarium, which is restricted in its distribution to the hot-dry belt. Its multiplication is rapid even in Sudanian and Saharan zones, where the moisture content of the grain may be even less than 3 per cent. Most stored-product insects are absent in normal samples of grains in these areas. Thus, the adaptive changes in T. granarium are specific for its geographical distribution On the other hand, the low temperature adapation of Tribolium confusum and Cadra sp. has been responsible for the distribution of these pests in the temperate and cooler zones of the world.

Chemical control and fumigation have been practiced since the twelfth century B.C. The development of organic pesticides turned the attention of workers to their extensive use because of their spectacular results (7). During the last decade opinion built up against their indiscriminate use and extensive application. This has necessitated research on the biology, ecology, nutrition, and physiology of stored product insects with a view to identifying the vulnerable points in their biology and life history and evolving measures for their control. Some of the studies carried out in these areas in recent years are reviewed in this report.

TABLE 2. Effect of Surface Sterilization on Population Numbers of insects Raised on Rice*

- Total numbers after 60 days of incubation plus 20 insects released on 20 grams of rice

TABLE 3. Percentage inhibition of Populations of Tribolium castaneum by Some inorganic and Organic Salts Added to Wheat Flour at Two Levels

NUTRITIONAL ABERRATION

Chemical candidates that are unlikely to be toxic in the human dietary, at least at low concentrations, were examined for their effects on stored-product insects (8). Sodium chloride, sodium bicarbonate, magnesium sulphate, iron phosphate, sodium bromide, potassium bromide, and citric acid were selected to determine their effects on the growth of T. castaneum (table 3).

Almost all of these chemicals reduced insect population growth even at low concentrations of 10 ppm, or 0.01 per cent. However, magnesium chloride exerted beneficial effects and resulted in enhanced population growth of test insects. In contrast to a non treated control group, the population of T. castaneum increased in proportion to the concentration of magnesium chloride in the diet. Only two salts, aluminium chloride and tricalcium phosphate (TCP) at 3 per cent concentration in wheat flour, resulted in complete inhibition of growth and development of the insects (8). Carbonates and bicarbonates tended to delay development and consequently reduced the population. The action of magnesium sulphate was similar; it extended the life cycle to about 120 days. Aluminium chloride, in spite of its high toxicity to T. castaneum, is not likely to be permitted in foodstuffs at the required concentration of 3 per cent and above. TCP not only arrested growth totally but also showed substantial inhibitory action at lower concentrations. TCP alone seems to have a great possibility for application in storage of food products, as it is also acceptable as an additive in food (8; 9).

Ionic concentration in insect blood differs fundamentally from that of most animals. Tobias 110) pointed out in 1946 that the existence of insects with low sodium and high potassium content in their blood is of physiological interes The proper functioning of vertebrate muscles and nerves is
usually thought to depend upon a plasma sodium potassium ratio of greater than 1, while the intracellular ratio is less than 1. The results reported by Majumder and Bano on inhibition of insect growth and delayed breeding brought about by the addition of an excess of some cations to their diet might have been due to an ionic imbalance in the insect system and consequent physiological disturbances (8).

TABLE 4. interaction of Some Food Additives on the inhibition of insect Populations Reared on Media Containing 1 per Cent Tricalcium Phosphate (10 mg/g)

Roeder (111) contends that calcium concentration in insect 3 blood has no particular significance. Calcium is lower in insects than in other invertebrates. Our present study has :.shown that calcium salts are more toxic than others to insects; but, strikingly, not all calcium salts brought about a high degree of inhibition in insects. This presumably indicates that it is not only the calcium ion that is toxic, but the companion anion also plays a major role in inducing growth inhibition. The toxic effects are accompanied by a series of abnormal changes in physiology and biochemistry. The symptomatic or diagnostic changes are reflected in supernumerary moulting, weight loss, delay in metamor phosis, mottling of the skin, depletion of fat, and histolysis of the tissues (8; 9; 12).

Extremely interesting results have been obtained on the interaction of TCP with additives such as organic acids, amino acids, sugars, and some inorganic salts. These results have thrown light on the probable mode of action TCP on insects. Among the acids, vitamins, sugars, and inorganic salts, three distinct groups can be classified according to their interactions with TCP toxicity for insects. A potentiation or reversion index has been computed on the basis of insect populations in the treated samples (table 4). The rest of the compounds were indifferent in action. The potentiation of toxicity by four salts, three sugars, three amino acids, and three vitamins was very significant. ln many cases salts present as impurities in TCP samples were antagonistic and reduced its toxic action against insects (table 5) (13).

Organic acids such as tartaric and citric acids reversed the action of TCP. Interestingly enough, there was potentiation of toxicity by some sugars, particularly monosaccharides. The antidotal effects of trehalose, norleucine, and glutathione and the total reversion of TCP toxicity by cholesterol and ergosterol are the most important findings in this study of insect physiology. Our investigations also demonstrated that increased calcium ingestion by insects caused a significant decrease in cholesterol and fat levels in the blood. The significant weight loss and scanty distribution of ill-defined fat within the tissue of larvae subject to toxicity are indicative of the aberration in energy metabolism caused by TCP (8; 12). investigation using TCP labelled with 45Ca or 32p showed maximum accumulation of these compounds in tissues within 72 hours of ingestion. These experiments also confirmed that calcium plays a major role in shell and muscle-cuticle formation (14; 15).

TABLE 5. Calcium and Phosphorus Contents of Commercial Tricalcium Phosphate Samples

TABLE 6. Distribution of Radioactive ⁴⁵Ca and ³²p in Tissues of T. castaneum Larvae Fed a Diet Containing 2,620 Counts/Min or 3,360 Counts/M in, Respectively

In this context, it is interesting to note that the moulting frequency of Emerita asiatica was governed by sea water calcium, and, further, Sitaramaiah and Krishnan have suggested a relationship between the sea water calcium concentration and calcification of the cuticle (16). In the larval stage, the action of TCP, even at the lowest concentration in the diet, gives rise to supernumerary moulting. The exact mechanism of the absorption of calcium phosphate from the insect gut is not known. However, radiographic studies indicated the transfer of Ca from the diet to the insect tissues (14). Haemolymph and muscle-cuticle complex showed high accumulations of ⁴⁵Ca and ³²p (table 6).

Urist (17) speculated on what normal life would be in higher animals if the two vital elements bound in the bone tissue, calcium and phosphorus in the form of calcium phosphate, made a gel instead of a hard substance. Calcium ions in the bloodstream are essential to the clotting
mechanism and also to muscle contraction. Phosphorus metabolism is probably responsible for the superiority of vertebrates over all other living things. Combined with fats, carbohydrates, and proteins, phosphorus is a vital constituent of every cell. The breakdown and synthesis of glycogen would be impossible without the phosphates.

MacGregor (18) emphasized the existence of ionic exchange between the mineral component of the skeleton and the calcium (Ca++) and inorganic phosphate (P - - -) ions of the circulating body fluids of man. Bone mineral must have a narrow solubility range for calcium (Ca++PO₄- - -) at physiological pH, temperature, and ion strength. He hypothesized that this might be quite different from that of the physicochemical equilibrium of calcium and phosphorus in insect haemolymph, particularly in stored grain insects.

The above review makes it quite evident that the substances that inhibit the growth of insects need not be toxic to other forms of life. The inhibitory effect of TCP belongs in this category. The interactions of sugars on the toxic effects of TCP were reported by Majumder and Bano (19). The potentiating action of glucose and the antidotal effect of trehalose appear to have far-reaching implications in the search for a specific insecticide. The non reducing disaccharide alpha alpha trehalose is the major sugar in insect plasma, and it is believed to play a significant role in carbohydrate transport in insects, by conveying glucose units from fat body glycogen to the site of metabolism in other tissues (20). Since glucose, fructose, and sucrose occur in small or trace amounts in the blood of insects, and trehalose is present to the extent of 90 per cent of the blood sugar level, the role of TCP seems to be related to the imbalance created in the energy metabolism of insects. The data on the population and histopathological aspects of insects fed glucose- or trehalose-treated TCP have indicated such a possibility. Furthermore, at low concentrations, the supernumerary moulting in larvae and nodular outgrowth on the pupal case in insects maintained on diets containing low concentrations of TCP throw light on the comparative physiology and biochemistry of mammals and arthropods in the manifestation of malignancy (8; 9; 12; 19).

Vertebrates are characterized by an endoskeleton with 99 per cent of body calcium bound in the hydroxyapatite crystallites of bones and teeth. Their requirement for calcium phosphate should be in sharp contrast with that of invertebrates, where the endoskeleton is absent. In this context it is worthwhile to mention that cholesterol and ergosterol and vitamins of the D group can lessen the toxicity of TCP to insects. These vitamins are responsible for mobilization and transport of calcium in the vertebrate system. The reversion of toxicity with vitamin D seems to point out the differential action of TCP in insects and mammals. It is well known that the fat-soluble vitamins, A, D, K, and E, are absent in insects, while they are essential in vertebrates. Thus, the physiological and biochemical differences between mammals and arthropods appear to be implicated in the action of TCP as a metabolic poison for insects.

The significance of calcium as a structural element and of silicon in plants and lower forms of life has been over looked in the study of evolutionary morphogenesis. The enzyme differences relating to taxonomy and the molecular basis of the evolution of enzymes have all given rise to potential differentiation in the phylogeny and ontogeny of organisms. A role for the structural elements in the evolution of vertebrates and invertebrates is indicated in this review. The recent advances in research on the biochemistry of supporting materials have been documented by Tracey (21).

FUTURE POSSIBILITIES

Hydrated silica, calcium carbonate, magnesium carbonate, calcium phosphate, strontium sulphate, and iron oxide are among the inorganic salts forming the essential structure for the life of an organism, and these have perhaps more immediate importance to biochemical investigation than those that appear adventitious or excretory in origin. Even in the area of insect pathology and the field of bacterial control of insects, recent findings relate to the mode of action of Bacillus thuringiensis in which the parasporal body plays a pivotal role. Silicon has been discovered as the structural matrix of the rhomboidal crystal in which amino acids form the endotoxin moiety. The toxicity of the crystalline parasporai body has been attributed to the effect of silica on the permeability of the paratrophic membrane in the midgut of insects. Similarly, virus infectivity might be inhibited by calcium or silicon.

There is no doubt that the phylogenetic development of insects has not received due attention, and almost nothing is known about the comparative biochemistry of arthropods and related phyla. Though the evolutionary morphogenesis and biochemical evolution were simultaneous, studies in the areas of these fundamental questions have been negligible. In this review some of the promising inorganic chemicals and ions showing selective toxicity to insects have been used as examples of the possibility of devising inorganic protestants.

This article attempts to cite a few promising areas of research for selective control of storage insects. The greatest weakness or vulnerable point in the morphology of insects is in their cuticular structure. Pesticides and solvents in the cuticular lipoid or wax may enhance absorption and transportation of organic pesticides to the site of action where the biochemical lesion is induced (21). The cuticle is an integral part of the exoskeleton. Insects do not have endoskeletons and bones. In spite of the basic physiological and metabolic differences between chordates and non chordates, they have not been studied from the point of view of control. There is no doubt that the phylogenetic tree needs biochemical and physiological interpretation from this angle.

The exoskeleton of insects is composed of chitin, a glucosamine polysaccharide. It is, therefore, possible to destroy the structural integrity with a specific enzyme like chitinase without affecting vertebrates. Recent success of Smirnoff (22) in using chitinase to increase the effectiveness of B. thuringiensis against a resistant forest insect indicates its future possibilities.

Metamorphosis in some insects and the high metabolic rate in all are also vulnerable points in insect life. Curiously, almost all insects concentrate magnesium from their food, and the phytophagous insects contain amounts of potassium in their haemolymph too high to be tolerated by mammals. The rapid growth in the larval stage and the histolytic changes in the pupal stage are peculiar aspects of insect life history. They also provide clues to specific insect control.

The hormones and metabolic antagonists can play a critical role in disturbing the growth and metabolism at any of the life stages of insects as well as during metamorphosis. Attempts to employ juvenile hormone mimics posed the problem of extending the period of active destruction by larvae and raised the vexing question of the mode of action of a halogenated hydrocarbon. There appear to be some promising lines of research in employing folic acid antagonists and anti-metabolites for blocking cholesterol metabolism in insects. Many such examples of the structural, physiological, and immunological differences between mammals and insects have been cited by Majumder (23), with special reference to devising specific and selective controls for insects.

The weak points in the ecology behaviour of stored product pests need elaborate investigation. Their food habits and special affinity for certain substrates, as well as their chemotropic responses, require fresh inquiry. There are many good examples of ways to control destructive insects that should serve to encourage basic research on the biology, nutrition, and physiology of insects infesting stored products. Among them are:

- the use of activated kaolir-ite,
- taking advantage of intergranular space as a limiting factor in the spread of infestation,
- the use of meta-hydrogen halloysite for its molecular sieve effect on cuticular lipid and as effective sorptive dust to attack the vulnerable points of morphological specialization,
- the use of the tricalcium phosphate-vitamin-glucose system that brings about a unique metabolic aberration in the biochemical physiological specialization of insects as a group,
- combining chitinase or alkaiigenic salts with B. thuringiensis to enhance its pathological insult.

REFERENCES

1. W. Ebeling, in Hilgardia, 30: 531 (1960).

2. A. Bano, H.R.G. Rao, and S.K. Majumder, in Proceedings of the 2nd All-India Congress of Zoology, Banaras, 1960, 2: 393 (1962).

3. H.R.G. Rao and S.K. Majumder, in J. Econ. Entom., 57:1013 (1964)

4. R.T. Cotton, Pests of Stored Groin and Grain Products (Burgess Publishing Co., Minneapolis, Minn., USA, 1963).

5. V. Sreanivasamurthy and K. Krishnamurthy, Some Aspects of Food Technology in India (CFTRI, Mysore, India, 1959).

6. S.K. Majumder, in Proceedings, 3rd International Congress of Food Science and Technology, Washington, D.C., 3: 518 (1970).

7. A.K. Majumder, "Fumigation and Gaseous Pasteurization" (Academy of Pest Control Science, Mysore, India, 1973), p. 43.

8. S.K. Majumder and A. Bano, in Nature, 202: 1359 (1964).

9. A. Bano and S.K. Majumder, in Proceedings of a Symposium on Posts, Mvsore, India (1964), p. 177,

10. J.M. Tobias, in Cell Comp. Physiol., 31: 123 (1948).

11. K.D. Roeder, Insect Physiology (John Wiley, New York, 1953).

12. A. Bano and S.K. Majumder, in 1 Invert. Path., 7: 384 (1965).

13. M.Rm. Rao, J.S. Venugopal, and S.K. Majumder, in Life Sci., 10:1081 (1971).

14. M.R. Gupta, R. Radhakrishnamurthy, and S.K. Majumder, in J. Stored Prod. Red, 7:207 (1971).

15. M.R. Gupta, R. Radhakrishnamurthy, and S.K. Majumder, in Indian J. Entom., 34:207 (1971).

16. P. Sitaramaiah and G. Krishnan, in Indian J. Exp. Viol., 4: 34 (1966).

17. M.R. Urist, in Discovery, 8: 13 (1964).

18. J. MacGregor, in 3rd European Symposium on Calcified Tissues ( 1966), p. 138.

19. S.K. Majumder and A. Bano, in Nature, 210: 1052 (1966).

20. G.R. Wyatt and C.F. Kalf, in J. Gen. Physiol., 40:833 (1957).

21. M.V. Tracey, "The Biochemistry of Supporting Materials in Organisms," Adv. Comp. Physiol. Siochem., 3: 233 (1968).

22. W.A. Smirnoff, Milieu (1974), p. 1,

23. S.K. Majumder, in Proceedings of a Symposium on Pests, Mysore, India (1964), p. 256.

Continue

Contents - Previous - Next