On fear of insects and insecticides

On a recent flight back home from a tropical island, the captain announced that a stewardess was about to spray the cabin with insecticide. "French authorities require that the cabin be sprayed with insecticide on departure. The product is harmless to humans and was approved by the appropriate authorities." A few seconds after the announcement, a stewardess gleefully walked along the length of the airplane, holding two cans over her shoulder and dispensing a mist of particles.

No one seemed to be disturbed by this scene. By total coincidence, I had just began reading chapter 7 "Needless Havoc" in Rachel Carson's Silent Spring. In this chapter, Carson describes the drastic and insanely aggressive steps taken by midwestern states (USA) to stem the westward spread of the Japanese beetle.

After a few moments of hesitation, I worked up the courage to talk to the stewardess about the product that was just sprayed around the cabin.

Me: "What is the product that you just sprayed?"
Stewardess: "it's insecticide, you know, to kill insects".
Me: "Right, but what type of insecticide?"
Stewardess: "Why? Are you allergic?"
Me: "No, I mean what kind of chemical compound?"
Stewardess: "Oh, I don't know. Let me ask the head steward".
Stewardess: "He said that I should show you the can. Is that OK?"
Me: "Yes, that is fine. I am seated in 35B".


A few moments later she returned and handed me the can. Its back label listed at least twenty chemical compounds by reference number which were meaningless and impossible to memorize. However, the back label warmed about mixing the product with water and to avoid oral absorption. It further stated that the product would "kill all insects within at most 6 minutes". How can a product be potent enough to kill insects so quickly and yet be harmless to humans? Is arthropod physiology so different than our own? With heightened interest, I proceed to study the front label. It had an illustration of an airplane under which the words "AMS 1450A - Product of France" were written. The stewardess returned and picked up the aerosol can. She left holding a plate of food in one hand (presumably to be served to a customer momentarily) and the can in the other. A little googling in the comfort of my home several hours later yielded the exact image of the front label. It is reproduced on the left side.

The term "AMS 1450A" designates an SAE specification in the Aerospace Material Specifications category. There are several AMS 1450A compliant products. These seem to typically contain 2% permethrin and 2% d-phenothrin.

Both substances belong to the family of synthetic chemicals called pyrethroids, i.e. neuro-toxins which prevent sodium transport in nerve cells. These toxins keep nerve channels in their open state, so that the nerves cannot de-excite, causing the organism to be paralyzed.

Us humans and other mammals are less vulnerable than invertebrates to pyrethroids because of our higher body temperatures, less porous skin (as compared to exoskeletons) and most importantly the detoxicating capabilities of our livers. Both permethrin and d-phenothrin are lethal to insects and to aquatic lifeforms. The lethal concentration for the rainbow trout is 1.4 parts per billion. If that was not alarming enough, pyrethroids bio-concentrate in fish and the bio-concentration factor increases dramatically (by at least 2800) when combined with other chemicals used in insecticides, e.g. piperonyl butoxide.

Here is an excerpt from the Silent Spring explaining the meaning of bio-concentration aka bioaccumulation.

Can we suppose that poisons we introduce into water will not also enter into these cycles of nature?

The answer is to be found in the amazing history of Clear Lake, California. Clear Lake lies in mountainous country some 90 miles north of San Francisco and has long been popular with anglers. The name is inappropriate, for actually it is a rather turbid lake because of the soft black ooze that covers its shallow bottom. Unfortunately for the fishermen and the resort dwellers on its shores, its waters have provided an ideal habitat for a small gnat, Chaoborus astictopus. Although closely related to mosquitoes, the gnat is not a bloodsucker and probably does not feed at all as an adult. However, human beings who shared its habitat found it annoying because of its sheer numbers. Efforts were made to control it but they were largely fruitless until, in the late 1940s, the chlorinated hydrocarbon insecticides offered new weapons. The chemical chosen for a fresh attack was DDD, a close relative of DDT but apparently offering fewer threats to fish life. The new control measures undertaken in 1949 were carefully planned and few people would have supposed any harm could result. The lake was surveyed, its volume determined, and the insecticide applied in such great dilution that for every part of chemical there would be 70 million parts of water. Control of the gnats was at first good, but by 1954 the treatment had to be repeated, this time at the rate of 1 part of insecticide in 50 million parts of water. The destruction of the gnats was thought to be virtually complete.

The following winter months brought the first intimation that other life was affected: the western grebes on the lake began to die, and soon more than a hundred of them were reported dead. At Clear Lake the western grebe is a breeding bird and also a winter visitant, attracted by the abundant fish of the lake. It is a bird of spectacular appearance and beguiling habits, building its floating nests in shallow lakes of western United States and Canada. It is called the ‘swan grebe’ with reason, for it glides with scarcely a ripple across the lake surface, the body riding low, white neck and shining black head held high. The newly hatched chick is clothed in soft gray down; in only a few hours it takes to the water and rides on the back of the father or mother, nestled under the parental wing coverts.

Following a third assault on the ever-resilient gnat population, in 1957, more grebes died. As had been true in 1954, no evidence of infectious disease could be discovered on examination of the dead birds. But when someone thought to analyze the fatty tissues of the grebes, they were found to be loaded with DDD in the extraordinary concentration of 1600 parts per million. The maximum concentration applied to the water was part per million. How could the chemical have built up to such prodigious levels in the grebes? These birds, of course, are fish eaters. When the fish of Clear Lake also were analyzed the picture began to take form—the poison being picked up by the smallest organisms, concentrated and passed on to the larger predators. Plankton organisms were found to contain about 5 parts per million of the insecticide (about 25 times the maximum concentration ever reached in the water itself); plant-eating fishes had built up accumulations ranging from 40 to 300 parts per million; carnivorous species had stored the most of all. One, a brown bullhead, had the astounding concentration of 2500 parts per million. It was a house-that-Jack-built sequence, in which the large carnivores had eaten the smaller carnivores, that had eaten the herbivores, that had eaten the plankton, that had absorbed the poison from the water.

That is bio-concentration, the tale of the house that Jack built.

D-phenothrin is also extremely toxic to bees, with 2 micrograms being sufficient to kill a bee. Curiously enough, cats are also very susceptible to pyrethroid insecticides. After coming to contact with pyrethroids, cats show at best severe clinical signs such as convulsions, vomiting or diarrhea or at worst die. Why are these "harmless" insecticides deadly to cats? Presumably because cats meticulously groom their coats and lick their paws thus ingesting the toxins dispersed on their outer body. (Both permethrin and d-phenothrin are relatively persistent pyrethroids.) In short, in the minute of the minutest concentrations these neuro-toxins are deadly to insects and to aquatic life and toxic to mammals in higher concentrations.

Returning the subject back to the aircraft cabin, flight attendants are particularly exposed to aircraft desinsection. Given the nonchalant attitude of the stewardess I witnessed, flight attendants appear to be oblivious to the health risks involving the frequent application of these neuro-toxins.

How about long term affects of these chemicals? Studies indicate that pyrethroids can cause anemia, liver failure, kidney failure, hormonal imbalances, miscarriages, hydrocephaly and/or brain atrophy in offspring, and may be carcinogenic in the long term. We should also keep in mind that mixtures of low concentrations of pesticides can become toxic. For example, it has been known for a long time that malathion, a pesticide that is widely used in agriculture and well-tolerated by humans, becomes deadly when mixed with other undisclosed but readily available chemicals. And you were worried about second-hand smoking!

Worried about our own health, it is all too easy to forget the havoc insecticides and herbicides are wreaking on the environment. According to government surveys, "almost every time and place that you observe a stream or river in a populated area you are looking at water that contains pesticides, inhabited by fish that contain pesticides." Pest control seems like a trivial problem compared to irreversible poisoning of our soils and drinking water. Are the overseeing authorities sleeping on the job? The Center for Biological Diversity alleges just that in a incriminating report entitled "Silent Spring Revisted". On the other side of the spectrum, a medical officer from the World Health Organization claims that pyrethroids are not toxic to humans and health risk is in not disinsecting aircraft.

It appears that public awareness of the dangers posed by insecticides is null or non-existent. For example, the policy document of the Swiss green party mentions support for banning Genetically modified organism's (GMO's) but does not once mention insecticides or herbicides. The policy document of the Canadian green party mentions insecticides only once in relation to banning their use on school premises. Like that is a big help! We collectively seem to fear insects more than we fear insecticides. The latter although unseen and intangible are far far more dangerous to our environment and by ricochet to us.

The phrase "No more bees, no more pollination, no more plants, no more animals, no more man" has been attributed to Einstein. However, the same can be said of earthworms as well as many other lifeforms. Earthworms, by their burrowing and digestive actions, considerably improve soil fertility. They are quite sensitive to the application of fertilizers and pesticides, in particular arsenical ones. Coincidentally, cigarettes have become increasingly toxic over the years because the soils of tobacco plantations are now thoroughly impregnated with residues of a heavy and relatively insoluble poison, arsenate of lead.

Thought experiment

Dragonflies are the natural predators of mosquitoes and are known to be effective in controlling mosquito populations. Given the much longer life-cycle of dragonflies and their lesser numbers compared to mosquitoes, Darwin suggests that dragonflies will acquire immunity to pyrethroids later than mosquitoes. Consequently, we could reasonably assume that for a period of time, dragonflies will be more susceptible to pyrethroids than mosquitoes. In this state, the predator-prey relationship is inverted: pyrethroid-immune mosquitoes will contain enough toxin to poison dragonfly feeding on immunized mosquitoes. It follows that the application of a wide-spectrum insecticide such as the pyrethroids in a given region could actually cause mosquito populations to increase in that region due to the elimination of dragonflies, the natural predator of mosquitoes. The astute farmer noticing the increase in the mosquito population will be tempted to spray insecticide on his farmland in even higher dosages, further polluting the environment. The need for higher dosages will in turn pressure the supervising bodies such as the EPA to modify the authorized toxicity limits. Unfortunately, this scenario is not merely fictional. On November 9th, 2011, the EPA issued a risk assessment for the pyrethroid class of insecticides and has decided to reduce the safely factor from 10x to 1x for adults and children over 6 years of age. WARNING: The moment you understand what this means, your head may explode instantly.

With the exception of India, today all nations ban the production and use of DDT. However, DDT is orders of magnitude less toxic than the toxins in use today. The only difference is in chemical persistence. DDT lasts 30 years whereas today's toxins, e.g. pyrethroids, last typically only a few months. Considering that insecticides are applied repeatedly and everywhere, we can conclude that not much has changed since 1962 when Rachel Carson published the "Silent Spring". The names of the toxins have changed but not the fundamental approach to pest control: "Kill them all, for the Lord will recognize His own." Instead of targeting all-living things, we now target all-living things except mammals. How light-handed of us!

There is plenty of evidence suggesting that the pyrethroid class of insecticides pose a risk to humans. Even if pyrethroids were perfectly safe for humans and all other mammals (which they clearly are not), targeting such a large class of living creatures, the insects, is indiscriminate and ultimately irresponsible.

PS. Come to think of it, an adult mosquito flown from the southern hemisphere could not survive in the dead of winter in Paris. Thus, not only is aircraft desinsection is environmentally dangerous it also attempts to fix a non-problem, at least in winter.