Autumn is a time when residents often encounter cockroaches entering their homes, but the once-popular “cockroach chalk” is no longer available in Estonian stores, and there are reasons for this, writes the Estonian media ERR News.
The Estonian satirical radio program Rahva oma kaitse recently brought to the attention of the wider public the once-widely used and praised “cockroach chalk” – a substance similar to blackboard chalk that was used to combat cockroaches. It was used to draw lines that killed the insects and prevented them from spreading further. One of its ingredients was a substance that was used to produce military explosives.
Mati Martin, chief biosystematics specialist at the Tartu State Museum of Natural History, said that the scientific name for the substance commonly known as “miinikollane” or “mine yellow” is picric acid (trinitrophenol), and it is one of the most acidic phenols.
Picric acid is a highly explosive substance, and in its mixture form was often used in the arms industry at the turn of the 19th and 20th centuries, and it also found application in civilian life. Martin noted that it was also used to dye the fabric of traditional skirts on the island of Muhu, where it was obtained from washed-up sea mines.
It is not clear whether picric acid, or “mine yellow”, was also popular for insect control at that time.
However, as early as November 1933, the magazine Ühistegelised Uudised recommended using the substance to combat cockroaches. During World War II, it was even sold in pharmacies under the name “cockroach chalk.”
Using “cockroach chalk” was simple – one just had to draw lines in places where cockroaches congregate, such as on kitchen walls and floors. The reason why the substance attracted the cockroaches is unknown, but scientists speculate that they were attracted by the binding agents or the smell of the poison itself.
Later versions of “cockroach chalk” may have also included permethrin, which is still used in insecticides. Annely Jaksen, a representative of a pest control company, recalled that the chalk was also used in Soviet times, and later heard that it contained permethrin. Uno Mäeorg, a professor of bioorganic chemistry at the University of Tartu, confirmed that insects react to certain substances that either attract or repel them. He added that since it was called “cockroach chalk,” it must have had an effect on them.
“Mine yellow” was effective in reducing cockroach numbers, but its lines were difficult to get rid of afterwards.
An article published in the newspaper Nõukogude Hiiumaa in 1987 told of cases where the lines had to be scraped off the walls along with the top layer of paint.
Picric acid and similar substances are poisonous and unstable, and when they react with metals, they become explosive. Although the chalk used during the Soviet era may have been made from more stable compounds, its use was likely phased out due to stricter safety requirements. Martins pointed out that picric acid is very poisonous and dangerous, and, in his opinion, it is no longer used in the weapons industry.
Mäeorg, on the other hand, said that the substance may not have been particularly dangerous in small quantities. He rather attributes the disappearance of chalk from store shelves to its bad reputation and social stigma. “For it to explode, it needs detonation. Just like a firing pin strikes a cartridge, picric acid requires a hard blow from an object to set it off. Today’s authorities probably wouldn’t allow it,” the scientist said.
People were not poisoned by picric acid mainly because the “cockroach chalk” was a short-term solution and was used in relatively small amounts. Martins pointed out that in many cases the effect of the poison depends on body weight – the ratio between the amount of poison and body weight. He added, however, that buildings were also “more permeable” in the past, and there was better air exchange. Also,
the effects of the poison can sometimes be detected only several years later,
or the effect manifests itself in subsequent generations.
Jaksen, who has been working in the field of pest control for 25 years, said that two main methods are currently used: gel poison and poison spraying. Gel bait is placed in places where cockroaches move. After eating the gel, they carry the poison to the outbreak, further destroying their fellow species. In turn, the poison spraying acts directly on their possible movement routes, from cracks in floorboards to corners under sinks. There it remains for several weeks. Jaksen explained that, despite popular belief, spraying does not drive cockroaches to neighboring homes, and the long-term effect also destroys eggs or future generations of cockroaches.
Despite modern techniques, the competition between pest control workers and cockroaches is still fierce. They have adapted at a remarkable pace to the synthetic insecticide DDT and others that act on the insects’ nervous system. As generations passed, gene mutations spread, making the insects more resistant to nerve poisons. Behavioral changes were also observed. For example, some cockroach populations no longer attracted glucose, so mixing the poison with sugar became useless.
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