The so-called cockroach chalk, fondly recalled recently on the satirical radio show "Rahva oma kaitse," was a common household insecticide whose active ingredient was picric acid. Known colloquially as miinikollane ("mine-yellow"), the compound was primarily used in the arms industry as an explosive.

"According to systematic nomenclature, the name of miinikollane, or picric acid, is 2,4,6-trinitrophenol. It is a solid, yellow, crystalline substance and one of the most acidic phenols," said Mati Martin, chief specialist in biosystematics at the University of Tartu Natural History Museum.

Although picric acid–based mixtures such as melinite were widely used in the arms industry around the turn of the 19th and 20th centuries, the same compound also found its way into civilian use. Martin noted, for example, that miinikollane was reportedly used to dye the fabric of traditional Muhu skirts. The yellow dye was obtained from naval mines dismantled on the shoreline.

Whether the miinikollane popular among the public was ever officially registered as an insecticide is unclear. However, picric acid was already being recommended for cockroach control as early as November 1933 in the journal Ühistegelised Uudised. Allegedly, after World War II it was even sold in pharmacies under the name "cockroach chalk."

Attracted and destroyed roaches?

Using the chalk was simple: lines were drawn on surfaces where cockroaches moved, such as kitchen floors or walls. Although the exact reason the substance attracted cockroaches is unknown, scientists suspect it may have been due to the filler or the smell of the poison itself. "Perhaps even the smell of melinite was attractive to them," suggested Mati Martin. The biologist added that the filler used in making the chalk sticks may have been suitable food for cockroaches.

It is also possible that later versions of the chalk contained permethrin as the active ingredient. This chemical compound is still used in insecticides today. "For example, we also used chalk during Soviet times. Later, I heard that it contained permethrin," recalled Annely Jaksen, a representative of the pest control company Insecto.

Uno Mäeorg, associate professor of bioorganic chemistry at the University of Tartu, confirmed that insects always react to certain substances, either being attracted to or repelled by them. "Since it was called cockroach chalk, it must have worked on them," he added.

Although miinikollane was effective in eliminating cockroaches, some reports suggest that removing the lines drawn on walls or floors later proved difficult. In 1987, the newspaper Nõukogude Hiiumaa recalled that when picric acid from naval mines was used, sometimes even a new coat of paint was not enough to cover it. The marks had to be scraped off along with the top layer of material from the walls.

Toxic and dangerous

Picric acid and related compounds are toxic and unstable, especially when they react with metals and form highly explosive picrates. Although the chalks used during the Soviet era may have been made from more stable mixtures, their use was likely discontinued due to stricter safety regulations. "It is a very toxic and dangerous substance. I don't think it's even used much in modern arms production," said Mati Martin.

According to Uno Mäeorg, the substance may not have been particularly dangerous in small amounts. Instead, he sees its disappearance from the market as linked to its bad reputation. "Miinikollane simply has that kind of 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 chemist explained.

Why didn't people suffer poisonings? Because cockroach control was generally short-term and the quantities used were small, the substance likely had little effect on humans, Martin said. "In many cases, the impact of a poison depends on body mass — that is, the proportion between the amount of poison and body weight. If the critical threshold is higher, the effect is weaker," he added.

At the same time, he noted, houses in the past were likely "draftier," with cracks in windows and doors allowing for greater air exchange. As a result, the risk of poisoning was already lower to begin with. "One should not forget that the effects of poisons are often detectable only years later or may even appear in subsequent generations," the scientist reflected.

Modern roach control

Although picric acid–filled naval mines no longer litter Estonia's shores as they did in the mid-20th century, cockroaches have not managed to take over the cities. According to Annely Jaksen, who has worked in pest control for the past 25 years, two main methods are used in modern cockroach control: gel baiting and spraying.

With gel baiting, poisonous bait is placed in areas where cockroaches move. They consume it and carry it back to the nest. Spraying, by contrast, involves coating all possible travel routes — from gaps in baseboards to under the sink — with a fine mist, leaving behind a toxic layer that remains active for weeks.

"Spraying doesn't scatter cockroaches into neighboring apartments, as some poisons do, but kills them. The long-lasting effect is also helpful because if there are eggs anywhere, those won't survive either," Jaksen explained.

Still, pest controllers and cockroaches remain locked in a constant arms race. Cockroaches, among other things, adapted remarkably quickly to the notorious synthetic insecticide DDT and other pyrethroids that target insects' nervous systems. Through artificial selection, gene variants spread that made the poison less effective on their nervous system and allowed cockroaches to break it down more efficiently.

With other poisons, behavioral resistance has also been observed. For instance, some cockroach populations stopped being attracted to glucose, a sugar that had previously lured them in. As a result, mixing poisons with it became useless.

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