Chemosterilants are chemical substances that sterilise insects sexually to prevent reproduction. These substances may include chemical mutagens. The sterilising action of a chemical may be due to the fact that (1) it damages the chromosomes and leads to the formation of dominant lethal mutations in the semen of males or mature eggs of females; (2) it causes perishing of the cells in the premeiotic stages or aspermia; (3) it inactivates the sperm, and (4) under the influence of the toxicant, females lose the ability to lay eggs. In addition, when chemosterilants penetrate into the tissues of an insect, they may damage the somatic cells, which leads to the perishing of the insect. This phenomenon is extremely undesirable because the success of chemical sterilisation depends on the ability of the sterile insects, especially males, to compete with the fertile specimens in the natural population.

A very important property of sterile males ensuring the success of practical sterilisation of insects of the natural population is their ability to multifold mating with females. It must not be inferior to that of fertile males. Upon mating with a sterile male, a fertile female deposits non-viable eggs, thus sharply reducing the population in the next generation. The greatest effect is achieved with a ratio of sterile males to fertile males and females in the population of 3: 1:1, 5: 1: 1, or 10: 1: 1. Such a ratio is achieved by releasing males sterilised in a laboratory or by treating the natural population with chemosterilants.

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Chemosterilants do not reduce the population of a pest in a given area; hence, the insects may cause substantial harm to the harvest. In this connection, the effectiveness of using chemosterilants grows sharply when the size of the natural population is diminished by preliminary treatment with an insecticide or by using sexual attractants. Most chemosterilants relate to highly toxic compounds with a mutagenic, teratogenic, and cancerogenic action.

Modern chemosterilants belong to two groups differing in the mechanism of their action: antimetabolites and alkylating agents.

Antimetabolites are substances whose structure is very close to that of the natural metabolites of an organism. When they enter an organism, they displace these metabolites in exchange reactions. The antimetabolites of folic acid, glutamine, pyrimidine, and purine participating in the biosynthesis of nucleoproteins have the highest sterilising activity. Upon entering an insect’s organism, these substances violate the synthesis of nucleic acids (DNA and RNA) in the nuclei of the sexual cells. The effectiveness of such chemosterilants depends on the activity of the synthesising processes in the nuclei of the cells. For instance, upon emergence from the pupae male flies contain already mature mobile sperm, and the formation of nucleic acids is already completed in their spermatozoids. At the same time, the rapid synthesis of nucleic acids occurs in the eggs of matured females, and the antimetabolites can exhibit their action. This explains why the chemosterilants of this group sterilise only females well.

Antimetabolites mainly have stomach action and sterilise insects when ingested together with their food. These substances cause female insects to lose their ability to produce and deposit eggs. The minimum sterilising concentration of antimetabolites in bait is 0.0025 per cent.

Compounds of the antimetabolite group are highly toxic to humans and animals and have an embryotoxic and mutagenic action. This substantially restricts their introduction into plant protection practice.

Among the antimetabolites of folic acid, the ones studied the best are methotrexate [N-(p-(2,4-diamino-6-pteridyl)methyl methylamino-benzoyl)glutamic acid] and aminopterin.

In a concentration of 0.05-1 per cent, these substances caused the sterilisation of up to 99.5 per cent of house flies.

Alkylating substances are compounds by means of which a hydrogen atom in a molecule of a substance is substituted by an alkyl group. They readily enter into chemical alkylation reactions with various compounds of a cell, including proteins and nucleic acids. If a considerable part of a cell’s molecules are involved in the reaction, the cell may perish. The dose of an alkylating agent must be selected so that most vital molecules will remain intact in an insect’s sexual cells, thus causing sterility in the insect while keeping it alive. In their sterilising doses, alkylating substances act on the chromosomes, harming a DNA molecule at one or more points.

Chemosterilants of this kind have both a stomach and a contact action on insects and mainly sterilise males, causing chromosomal aberrations (breaking of the chromosomes) of the male sexual cells. This leads either to the perishing of the zygote formed as a result of fertilisation or to no larvae appearing from the eggs deposited by the female.

The most promising chemosterilants are derivatives of ethylene imine, whose ring is a carrier of the sterilising activity. They include tepa and its methyl analogue—metepa, and sulphur analogue—thiotepa.

These compounds sterilise various insects and mites without reducing the sexual activity of the males. At the same time, the viability of the insects diminishes somewhat. Tretamine and thiotepa also sterilise females.

The derivatives of ethylene imine are highly toxic to humans and warm-blooded animals and have a mutagenic, teratogenic, and cancerogenic action. In some cases, the sterility of mammals was noted under the influence of the alkylating substances.

Studies performed by the All-Union Research Institute of Plant Protection have established that a 0.1 per cent solution of thiotepa lowered the number of progeny of the turnip moth by sterilising 85 per cent of the males. Apholate in a concentration of its solution of 0.3-1.5 per cent and tretamine of 0.05-0.1 per cent had the same strong effect. These substances may be used for the sterilisation of males in a laboratory and then releasing them into the environment.

The sterilisation and release into the environment of sterile insects is a promising trend in plant protection. The use of chemical sterilants will reduce the scale of chemical treatments of agricultural crops, eliminate the hazard of contaminating products and the environment with poisonous pesticide residues, and exclude the negative action of toxicants on the biocoenosis. The manufacture of these substances on a large scale is being delayed owing to their unfavourable toxicological properties. The seeking of ways for the safe use of chemosterilants is going on in two directions: methods of using toxic chemosterilants are being developed that are safe for the agro-ecological system (for example, their use in traps with attractants), and chemosterilants are being developed that act selectively only on invertebrates and are harmless to humans and warm-blooded animals.

The author is an associate professor (retd.) & former head of the department of botany at Ananda Mohan College.