tapan kumar maitra explains the resistance of harmful organisms to pesticides
THE resistance of an organism to a pesticide stems from a biological property it possesses to withstand the latter&’s poisoning action. A resistant organism functions, develops and reproduces normally in a medium containing a poison. The phenomenon of resistance and the reverse phenomenon of sensitivity are closely related to the toxicity of the relevant poison, especially to its selective toxicity, because all the factors causing toxicity also act on the resistance or sensitivity of an organism.
There is distinguished natural resistance based on the biological and biochemical features of an organism, and acquired, or specific, resistance appearing only because of interaction with the poison. Natural resistance is subdivided into specific, sexual, phase (stage), age, seasonal and temporary. This kind of resistance appeared and exists independently of the use of chemical means for plant protection.
Specific resistance is due to the features of the biology of definite species of harmful organisms (insects, mites, rodents, etc). To overcome it, special pesticides are synthesised and used that have a selective toxicity (insecticides against insects, fungicides against the causal organisms of fungal diseases). The chemicals for plant protection include pesticides both with a narrow selectivity acting only on one species of harmful organisms or on several species of the same genus (pirimicarb against aphids, barban against wild oats) and with a broad spectrum of action (phosalone against insects and mites, DNOC against wintering forms of insects, fungi and bacteria, and also against weeds). Hence, specific resistance can be successfully controlled by selecting the appropriate pesticide.
The resistance of organisms fluctuates greatly within a single species, which must be taken into account when using pesticides. In a number of cases, female specimens of insects and animals have a higher resistance to poisonous substances. Such sexual resistance is overcome by choosing the relevant doses. Changes in the resistance of harmful organisms are also noted in ontogeny, depending on the phase of development.
The most sensitive to poisons are larvae and adult insects, fungal conidia at the time of growth and plants in the germinating stage. High resistance is a feature of insects in the egg and chrysalis stages and during the diapause, of wintering spores of fungi and bacteria and of the seeds of plants.
The resistance of harmful organisms to poisons within a single phase of development changes, depending on the age, time of day and year (season). The larvae of insects are more sensitive to insecticides at an early age, while toward the time of moulting their resistance increases. The resistance of plants and rodents also grows with their age.
Insects wintering in the phase of imago or larva are characterised by seasonal resistance. At the end of summer or autumn, these species are more resistant to pesticides because they accumulate a considerable amount of fat and do not eat much. In the spring, they are more sensitive to poisons because their organism is weakened by the prolonged wintering. The main way of controlling seasonal, temporary and age resistance is the proper choice of pesticides and strict observance of the optimal periods for treating agricultural objects.
Specific (acquired) resistance signifies the ability of a harmful organism to survive and reproduce in the presence of a chemical substance that previously suppressed its development. The first reports on the appearance of races of pests that resist the action of chemical formulations relate to 1915-1916, when a race of red wild orange scale-resisting hydrocyanic acid was discovered in California. Later, the appearance in other insects of a specific resistance to inorganic compounds — lead arsenate and sulphur — and to pesticides of a vegetative origin — pyrethrum — was noted. Up to the 1940s, no importance was attached to this phenomenon because pests became accustomed to poisons quite slowly and they were controlled successfully.
With the appearance of new synthetic pesticides, their acquired resistance began to develop rapidly and at present this is noted in over 200 species of insects. Specific resistance appears in the fifth to 10th generation of the harmful organism and develops to such an extent that in some regions it becomes impossible to use certain pesticides. It has been found that upon the perennial application of the same fungicide, for instance benomyl, the resistance of fungi spores may increase from three to 12 times.
It has been established that the phenomenon of specific resistance is based on the selection from genetically heterogeneous populations of specimens having increased resistance.
The selecting factor is the pesticide. The effectiveness of this selection depends on the initial material (insects, mites, etc), the number of treatments, the pesticide dose and the genetic heterogeneity. Specific resistance appears more rapidly when a harmful organism produces more generations during a season, its heterogeneity is higher, and the dose of the pesticide is smaller. The selected race of the pest, however, in the majority of cases is less adapted to the conditions of existence, and after discontinuation of the chemical treatments the population in a certain time returns to its initial state. But when treatments with the same substance are renewed, the specific resistance appears more rapidly.
Specific resistance may be individual, group and cross. Individual resistance (only to one pesticide) is encountered quite rarely and is due to the activity of narrowly specialised enzymes decomposing the toxic substance. For example, the resistance of insects to malathion is explained by the fact that this pesticide is rapidly decomposed in the organisms of resistant insects by the enzyme malathionoxidase. Group resistance involves two or more pesticides that are related in their structure and mechanism of action and that belong to one group, appearing after the use of a substance of this group. For example, after treating insects with formulations of HCH, a race of pests appears that resists all organochlorine insecticides. The group resistance of insects or mites is due to the following reasons:
– Slower penetration of a poison into an organism and faster excretion of it. Resistant specimens excrete from two to three times more toxicant than sensitive ones.
– Rapid detoxication of a poisonous substance because of a higher activity of the enzymes or the appearance of specific enzymes. In races of insects resisting organophosphorus compounds, the activity of aliesterases and phosphatases is higher than in sensitive ones. As a result, an insecticide rapidly decomposes. Some species of insects have a set of specific enzymes that actively decompose insecticides (for instance, malathionoxidase in insects resisting malathion).
– A different penetrability of the shells of nerve cords. In the organism of resistant insects, the insecticide penetrates poorly into the nerve cells (established for polychlorocyclodienes).
– An increased lipid content in the body of resistant specimens. The result is that lipid-soluble poisons are retained in the fat layer in a considerable amount and are removed from the sphere of action.
Cross resistance involves two or more pesticides of different groups as regards both chemical structure and the mechanism of action that appears after the use of one pesticide. Such resistance is encountered rarely and has been studied very little. This phenomenon is apparently explained by the fact that a previously used insecticide increases the activity of the non-specific enzymes of the endoplasmic reticulum of the fat-body. Consequently, the new insecticide is rapidly decomposed to non-toxic products.
To manage resistant races of harmful organisms and prevent the appearance of specific resistance to pesticides, it is essential to strictly observe the rates of use of the formulations and the periods of their application. The basic factor in controlling acquired resistance is the alternation of pesticides with a different mechanism of action, both during a season and from year to year.
For example, phosalone is recommended for the first treatments of orchards against apple worms, and carbaryl for the following ones. In controlling mites, the use of dinobuton is alternated with dicofol treatment.
Specific resistance can be overcome by adding synergists to pesticides – ie, substances that amplify their action. The integrated method of protecting plants is effective in combatting specific resistance. This makes it possible to prevent the appearance of resistance of harmful organisms to pesticides, diminish the danger of affecting entomophags and lower the contamination of the environment with the toxic residues of pesticides.
The writer is associate professor and head, Department of Botany, Ananda Mohan College, Kolkata