That part dealt with everything up to about 1900. This part covers developments made in the last century, which saw some of the most advanced techniques employed to repel biting insects as well as some very bizarre ones.
The use of black-light bug zappers where a light is used to draw insects in to an electrified metal grate are big business, with some people projecting 1.75 million unit sales a year in the US alone. These devices, commonly found in your local Chippy, do kill mosquitoes, but they also kill insects that are beneficial (ones that prey on mosquitoes) as well as prey for other animals within the local ecosystem such as birds. Of course the other issue is that employing one of these invariably increases the number of insects in the area by attracting more, without necessarily reducing the risk of getting bitten.
Ultrasonic devices were also very fashionable at one point, with several institutes investing huge sums of money in an attempt to develop a chemical free repellent that consumers would undoubtedly buy in to. The theory behind them states that because there are particular frequencies detected by mating female mosquitoes, emitting ultrasound at a specific, disruptive frequency, ultrasound would disorient the mosquitoes. Unfortunately, no proof was ever found that ultrasound had any affect whatsoever on mosquitoes.
One of the stranger methods was employed in Texas. During the 1920’s, San Antonia built several ‘bat towers’. These were designed to increase the population of bats, an animal that preys on mosquitoes. The basis for this came from an experiment made by M.D Tuttle, who released bats into a room full of mosquitoes, observing the bats eating on average 10 mosquito a minute. He extrapolated rather obtusely that a colony of 500 bats would therefore consume a quarter of a million mosquitoes every hour. Of course when put in to practice, the bats tended to feed on moths, wasps and almost anything other than mosquitoes. In actual fact mosquitoes only comprise 1% of the average wild bats diet!
A similar method was popularised in the 1960’s with the use of Purple Martins instead of bats. The ornithologist J.L Wade stated that purple martins ate up to 2,000 mosquitoes a day! He managed to attract more Purple Martins to the town and apparently the mosquito population decreased. The scientific backing surrounding this statement never materialized and independent studies of the diets of purple martins showed that in fact, they prefer larger insects like dragonflies which prey on mosquitoes.
Of course the most significant development in the last century was DEET. N, N-diethyl-2-methylbenzamide (DEET) was developed by the U.S government. There is some dispute over exactly when and who developed it, but it was originally synthesised at some point between 1945 and 1955.
The U.S government invested heavily in the development of pesticides coming out with dimethyl phthalate in 1929, Indalone in 1937 and Rutgers 612 (2-ethyl-1, 3-hexane diol) soon after. All of these predecessors to DEET failed to give repellency across a full range of insects, each being effective against only a handful instead.
It is thought that the Department of Agriculture and US Military joined forces to find an effective pesticide that was safe for topical application too. They developed n-n-diethylnetatoluamide (DEET) by 1945 (after testing many other options in clinical trials from 1942 onwards), which was then employed by the US Military before being made commercially available in 1957. It was originally a pesticide used on farmland but the military recognised its topical potential, eventually supplying its troops with vast quantities in both the Korean and Vietnam wars.
Nowadays, it is available in varying percentages. Contrary to what you may think, its efficacy doesn’t necessarily increase with concentration. An increase in concentration (if applied correctly) will only increase the duration of the repellency. 10% DEET will last around 2 hours, while 100% can last around 10 hours.
Of course now we know that the repellent is toxic and poisenous to humans as well as insects. Esteemed scientist Vincent Corbel, an expert on the matter, discovered that DEET inhibits the enzyme acetylcholinesterase, allowing for the build up of toxins in the synapse gaps of between neurons especially those found in the Central Nervous System (i.e. spinal cord and brain). DEET is therefore both a nerve agent and a neurotoxin.
What’s more, the efficacy of DEET has dropped dramatically. 100% DEET is at best 94% effective, a figure that is slowly decreasing. Some species of mosquito like the Anopheles albimanus of Central America are totally immune to DEET as a result of its overuse. The adaptability of the mosquito means that over the past 50 years it has built up resistance that will continue to spread until DEET is as effective as water.
Where else in society are we dependent on technology that dates back to the 1940’s? Nowadays, there are many different options, some as effective, some more so than DEET. Many are natural so don’t harm the environment or the user and there is even one that is both natural and more effective! For the best protection available, you need to look move from DEET. Update your travel bag and choose something that is proven to work and isn’t toxic!