Three Ways of Controlling Mosquitoes, that You’ve Never Heard Before
In the throes of the Vietnamese war, Ho Chi Minh, the leader of the North Vietnamese army, ran into an unexpected problem. Thousands upon thousands of his soldiers were returning from the field, sick and dieing. The cause: malaria. He made an urgent plea for help to the leader of China, Mao Zedong.
Mao, meanwhile, had recently begun the Chinese cultural revolution, a social upheaval where thousands of scientists and intellectuals were persecuted, banished and executed for their anti-communist ideologies. Yet, he considered Ho Chi Minh’s request and ordered over 500 scientists assembled as part of a secret military project.
Project 523.
Nearly 50 years later in 2015, Tu Youyou, a research leader of project 523, was awarded the Nobel prize in Physiology or Medicine, for developing Artemisinin, a group of drugs that saved the lives of millions of people from malaria.
A scourge upon the human race
Malaria is a disease caused by Plasmodium, a parasitic single-celled microorganism (protozoan). The protozoan is found in the blood dozens of wild animal species. When an Anopheles mosquito bites an animal or human infected with Plasmodium, it picks up the parasite. It grows within the mosquito until it is injected into the next host and the cycle repeats.
By the time you finish reading this article 7 people will have died from malaria. By the end of the day that number will have risen over 2000. In the course of human history, malaria has likely contributed to nearly 50% of all human deaths. Mosquito borne viruses are undeniably one of the greatest public health problems facing humanity.
We have been making great progress. In 2015, compared to 2010 (WHO): nearly two times more children with fevers were tested for Malaria, the percentage of people with the highest risk of malaria in Sub-Saharan Africa who sleep under a mosquito net nearly doubled, and most importantly malaria mortality rates decreased by almost 30%.
It’s slow but we’re making progress.
DDT: damn disastrous toxin
Now, there have been set backs.
Dichlorodiphenyltrichloroethane (DDT) is a pesticide that worked fantastically. against mosquitoes. In the 1930’s Malaria was responsible for the deaths of a few thousand Americans each year. By the early 1950’s DDT application eliminated malaria from Europe and North America and probably saved thousands of lives. Unfortunately, DDT had some serious drawbacks.
Research in the pesticide showed that it was responsible for bird population declines in many North American and European birds of prey. When DDT breaks down, it produces a compound that causes to eggshell thinning. Furthermore, it is toxic to an array of living creatures, readily bioaccumulates in organisms and stays in the environment for up to tens of years before breaking down.
Therefore, more recent research has focused on eco-friendly mosquito control using green chemistry, behavioral modification and even sound traps.
Toxic plant-based nanoparticles
Plants can exhibit properties toxic to mosquitoes. Toxins may be poisonous to mosquito eggs, larvae, pupae, and adults as well as interfering with the reproductive behavior of mosquitoes and their growth cycles.
In one study low concentrations of mosquito repellent essential oils of frankincense, lemon, thyme,and dill were applied to beakers containing mosquito larvae. After 12 hours 100% of all mosquito larvae were killed by the natural essential oils. No other ingredients were required and no toxic byproducts were produced.
Obviously, plant-based pesticides offer a clean, eco-friendly alternative to chemical pesticides.
Nanoparticles produced from plants, can also be impressive toxins to mosquitoes while being cheap to produce, eco-friendly and simple to develop. Non-plant produced nanoparticles on the contrary often require hazardous chemicals, high temperatures and specialized energy sources.
Fungi, bacteria and plants can all produce nanoparticles. The process involves adding silver nitrate, which is taken in by the plant and then output as silver nanoparticles. The silver nanoparticles are then tested on batches of mosquitoes to test their toxicity. In the case of Aloe Vera, only a few 10’s of ppm (parts per million) of silver were enough to kill 50% of all mosquitoes in experiments.
Of course nothing in research is ever simple and the complex reactions between mosquitoes and plant-derived green nanomaterials are still being studied.
To learn more about plants and nanoparticles, we have an article reflecting on the effects of zinc nanoparticles in biodegradable sunscreens.
What makes a mosquito sexy?
Instead of trying to kill mosquitoes, why not just sterilize them?
This was the basis of a technique called the sterile insect technique (SIT). The technique consists of releasing male insects that have been sterilized by ionizing radiation e.g. X-rays, into the wild. These males then hopeful begin to compete with fertile males, and ultimately diminish natural populations.
But before any large-scale sterilization can take place, the mating patterns of mosquitoes had to be studied.
Cue Barry Manilow …
While it may seem silly on the surface to study sexuality in mosquitoes, a number of curious findings have been recorded.
First of all, it turns out that male mosquitoes attempt to force monogamy onto females. After insemination female mosquitoes are temporarily incapable of being fertilized due to a variety of physiological reasons. Furthermore, male mosquitoes don’t prefer to inseminate female mosquitoes, which have already copulated.
Some researchers have recorded that female mosquitoes inseminated by sterile males, will go on to mate with fertile males. Others have noted that mosquito courtship in laboratories not equivalent to behavior in the wild. These are just some of the many problems that remain unresolved with genetic control approaches.
Booster Shots
The sterile insect technique sound fantastic on paper, but unfortunately has some issues in practice.
In the wild, mosquitoes and their larval habitats are widely distributed and it’s practically impossible to disseminate enough sterile males over a large enough area to effectively decrease a wild population.
A method was developed, whereby laboratory grown male mosquitoes were irradiated and then coated with a pyriproxyfen powder (a powerful larvicide) and released into the wild. The males would either mate with the females and pass on the infertile gene, or the larvicide would be transfered onto the females. The females would then deposit the larvicide into various bodies of water as they laid their eggs.
This method is known as “boosted SIT“.
Sound Traps: a novel tool
As it turns out, when you call out to a mosquito, it comes a buzzin’.
After the advent of electricity, people began to notice that mosquitoes were attracted to the hum of generators. In a quest to understand why, one researcher in the late 1800’s captured and pinned a mosquito underneath of a microscope. He then struck tuning forks of various frequencies and observed as the fibers on mosquitoes antenna vibrated more and less for certain frequencies.
We now know that that mosquito was just trying to hear his love. Research has show that female mosquitoes beat their wings twice as slow as males, therefore males can actively search for females by listening to the sound of their wing beats.
This led to the development of an eco-friendly mosquito trap. The sound trap method involves a speaker, which attracts the males into a trap. Once trapped the males are irradiated and released to compete with other males. This is more effective then outright killing the males themselves.
Sound traps, while effective, only work in close proximity to mosquitoes. Amplification is difficult and more research needs to be done on the locations and reasons for mosquito swarming.
The state of affairs
Each method is complementary to the next. Sound traps combined with boosted SIT, “eco-friendly insecticides” and changing human behavior all need to be combined to combat the spread of mosquito-borne diseases.
Do you have any questions, or know any other eco-friendly methods to control mosquitoes? We’d love to hear your comments below.
