An important role in eliminating the harmful action of chemical means of plant protection on the environment is assigned to the rational application of pesticides in integrated plant-protecting systems. The latter is based on the greatest possible utilisation of environmental factors causing harmful organisms to perish or limiting their vital activity. The main task of such systems is keeping the number of harmful insects at a level where they do not cause perceptible harm by using a collection of measures instead of a single one.
A system of antismut measures was developed and implemented in our country. Its result was a reduction in the losses of the grain crop due to smut from 10-30 per cent to 0.01-0.02 per cent. The use of an integrated system made it possible to suppress the mass reproduction of locusts. Systems of measure to control grapevine mildew, cotton wilt, tobacco blue mould, beet weevils, and spider mites on cotton markedly lowered the losses of the crop.
The fundamental principle of rational chemical pest control consists of taking complete account of the ecological situation on agricultural lands, having an exact knowledge of the criteria of the strength of the harmful species, and also of the number of beneficial organisms suppressing the development of the pests.
There are four main ways of improving the safety of chemical methods in protecting plants.
1. Improving the assortment of pesticides with the aim of diminishing their toxicity to humans and beneficial animals, lowering their persistence, and increasing their selectivity.
2. Using optimal ways of pesticide application such as pre-planting treatment of seeds (which eliminates early spring and late autumn treatments in orchards), band or strip treatments, and the use of granulated formulations.
3. Optimising the use of pesticides with a view to the economic expediency and necessity of employing pesticides for suppressing populations (with account taken of the economic threshold of harm for each species of pest in a zonal aspect).
4. The strictest regulation of the use of pesticides in agriculture and other branches is based on a comprehensive study of their sanitary and hygienic characteristics and the conditions for ensuring safety when working with them.
At present, compounds that are highly toxic and persistent in nature are being replaced with ones that have a lower toxicity and persistence.
To preserve beneficial insects, highly selective pesticides must be used for a chemical treatment that is poisonous only to definite targets and is not a hazard to the natural enemies of the pests. Among them are tetrachlorvinphos, phosalone, and DAEP. Fungicides are virtually harmless to beneficial insects. An important way of improving the selectivity of pesticides having a broad spectrum of action is the rationalisation of the ways of applying them with a view to the economical threshold of harmfulness for each pest species. This makes it possible to reduce the area or number of chemical treatments with no harm to the culture being protected. It is sufficient to carry out selective treatments using pesticides having a definite selectivity with respect to bollworms and mites (dicofol, dinobuton, phosalone).
Chemical measures for controlling beet leaf miners on beet plantations are justified if 8-10 eggs are detected on an average on one plant in the stage of four leaves, 12-14 eggs in the stage of six leaves, and 22-25 eggs in the stage of eight leaves.
When using insecticides, it is safest to employ systemic substances, especially in the treatment of seeds, or the incorporation of granulated formulations into the soil when or after planting. An important advantage of using systemic insecticides when planting is the prolonged protection of the plants in the most vulnerable stage of their development when the seeds and seedlings may be destroyed very easily or greatly damaged by pests.
Systemic and some contact (heptachlor) granulated insecticides effectively protect the seeds and seedlings of various cultures from pests dwelling in the soil (click beetles, larvae of dor bugs, etc.), from insects inside the plants (the larvae or maggots of cabbage, onion, carrot rust-flies, Hessian, winter, and fruit flies) and on their surface from sucking pests (aphids, mites). The incorporation into the soil of fertilisers with pesticides when planting not only combines two operations but also facilitates an increase in the crop. For example, the twofold sowing and postemergence application of granulated superphosphate with dimethoate to control cutworms increased the yield of cotton by 500 kilograms per hectare. Low-volume spraying and ultra-low-volume spraying are the most productive and make it possible to complete treatments in a very short time before the pests manage to cause significant harm.
In a number of parameters, evening and especially night spraying is very promising first of all for orchards and blossoming alfalfa. Nights are usually windless, which noticeably diminishes the danger of drift, and improves the settling and uniform covering of the plants with the pesticide. At night, some pests are more active (cutworms, mosquitoes, etc.), which ensures greater contact with the pesticide.
It is most promising to control mites and insects that have developed resistance to certain pesticides by alternating pesticides from different classes with a different mechanism of action.
To prevent contamination of the soil with pesticide residues, the incorporation of persistent pesticides into the soil must be limited as much as possible, while where necessary, rapidly decomposing substances should be placed into the drill rows, holes, and hollows (locally), which reduces the consumption of the pesticide.
Persistent substances must be used in the conditions of multiple crop rotation, where the rotation of the crops is attended by alternation of various pesticides, thus preventing their accumulation in the soil. Organophosphorus compounds may be used to treat plants on the same field every year. Heptachlor, technical HCH, and carbaryl should be used every other year. Toxaphene should be used once in four or five years, i.e. to treat not over two fields in a 9-10-field crop rotation. During a single season, it is good practice to use different pesticides, even if they are similar in their spectrum of action so that they will be able to decompose as much as possible during the vegetative season.
The employment of toxicant rotation with the observation of two or three-year intervals between repetitions of the same pesticides or the alternation of different ones during one season is an essential condition for the organisation of the chemical protection of plants in perennial plantings, and also in specialised farms with a high degree of rotation of the same crop.
The author is an associate professor (retd.) & former head of the department of botany at Ananda Mohan College.