Why Mosquitoes Develop Resistance to Pesticides


Despite their tiny size, mosquitoes have been called the world’s most dangerous creature because they spread deadly diseases. More than half the people on Earth live in an area where disease-carrying mosquitoes are present and, every year, mosquitoes cause millions of deaths from diseases like malaria, dengue and yellow fever.1

Insecticides are considered to be one of the primary defenses against insects like mosquitoes, but they come with their own set of risks, not the least of which is the potential for resistance.

The growing problem of insecticide resistance has not gone unnoticed by the World Health Organization, which reported that resistance to four commonly used classes of insecticides — pyrethroids, organochlorines, carbamates and organophosphates — is already widespread in regions of Africa, the Americas, South-East Asia, the Eastern Mediterranean and the Western Pacific.2

Now, research published in the journal Oecologia has shown that mosquitoes are also developing resistance to commonly used agricultural insecticides, but their predators are not, creating the perfect environment for mosquitoes to flourish.3 Paradoxically, in this way insecticide usage could be leading to increased numbers of mosquitoes in some regions.

Mosquito predators may die while mosquitoes thrive

Researchers from Utah State University were conducting research on Costa Rican orange plantations when they realized they were getting an awful lot of mosquito bites — more so than they received in other areas. This led to the first part of the featured study, in which the researchers detected mosquito larvae flourishing in bromeliads, plants that hold small pools of water between the folds of their leaves.

When they compared the mosquitoes on orange plantations treated for decades with dimethoate, an organophosphate insecticide, with mosquitoes found in forests not treated with insecticides, they found double the mosquitoes in the orange plantations, but a lack of damselfly larvae, which are insects known to feast on mosquitoes.4

The researchers also exposed mosquitoes to dimethoate in the lab, which revealed not only that mosquitoes from orange plantations were 10 times more tolerant of the insecticide than the pristine forest mosquitoes, but also that damselflies from plantations succumbed to the chemicals, suggesting they have not developed resistance the away the mosquitoes have.5

“This evolved resistance to pesticides may, therefore, allow W. abebela [mosquitoes] to colonize habitats free of the dominant predator in the system, explaining the higher W. abebela abundances in pesticide-exposed areas than in pesticide-free locations,” the researchers explained.6

Beyond Pesticides, a nonprofit organization leading a mission to transition to a world free of toxic pesticides, further explained that differences in biology of mosquitoes and their damselfly predators are compounding the insecticide resistance problem:7

“Life cycles of the mosquitoes studied are 12 to 24 times shorter than damselflies, providing mosquitoes with a faster opportunity to develop resistant individuals. Given this short life cycle, even a small number of resistant mosquitoes can rapidly repopulate an area with their improved genetics.

Damselflies just cannot catch up in time. This further increases the dominance of mosquitoes in an ecosystem. Without predators to tamp down their populations, mosquitoes are able to colonize new habitats. In this case, mosquitoes in chemical-dependent groves are able to lay eggs in larger bromeliads, whereas in pristine areas the presence of damselflies and other predators make this highly unlikely.”


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Mosquitoes genetically adapt while predators are poisoned

One of the mechanisms that allows mosquitoes to so efficiently develop resistance to pesticides is through the overproduction of specific enzymes, as follows:8

  • Carboxylesterases, which are efficient against organophosphate and carbamate insecticides
  • Glutathione-S-transferases, or GSTs, which are efficient against organophosphates, organochlorine and pyrethroid insecticides
  • Cytochrome P450-dependent monoxygenases, which are efficient against most insecticide types, often in conjunction with other enzymes

At least one study, in which researchers analyzed more than 760 mosquito genes possibly involved in insecticide resistance, revealed that an increase in activity of detoxification enzymes in resistant insects was triggered by a rise in the copies of genes coding for the enzymes.9,10

Mosquitoes in at least 68 countries resistant to insecticides

According to a WHO global report on insecticide resistance in malaria vectors like mosquitoes, 68 countries have reported mosquitoes resistant to at least one class of insecticide, while 57 of them report resistance to two or more classes.11

That being said, as of February 2019, only 40 countries have completed insecticide resistance monitoring and management plans as recommended by WHO’s report, leading the Organization to suggest that their understanding of the extent of the problem is incomplete.

The global fight against malaria is centered on insecticide application and distribution of insecticide-treated bed nets, but insecticide resistance renders these “weapons” highly questionable.

As for insecticide-treated bed nets, which are treated with pyrethroids, a five-year evaluation conducted across five countries found that mosquitoes were resistant to pyrethroids in all of the areas tested, but those using the nets still had significantly lower rates of malaria infection than those who did not.12

According to WHO, “They attributed the continued efficacy of LLINs [long-lasting insecticidal nets] to the barrier provided by the nets themselves and to the fact that, even in areas where mosquitoes have developed resistance to pyrethroids, treated nets may still kill the mosquitoes.”13

One WHO scientist suggested that resistant mosquitoes landing on the treated nets may not die right away, but if they continue to be exposed to the insecticides while trying to find a way through the net, it’s possible the higher exposure could eventually kill the mosquito.14 However, other studies have found that both treated and untreated nets have a “clear protective effect against malaria.”15

Insecticide usage leads to devastating ‘trophic cascades’

Applying insecticides and other pesticides is creating resistant bugs along with a host of other problems, as such chemicals do not exist in isolation. When they’re applied in the environment, they lead to what Beyond Pesticides described as “trophic cascades.”

“Beyond direct toxicity, pesticides can significantly reduce, change the behavior of, or destroy populations of plants and animals. These effects can ripple up and down food chains, causing what is known as a trophic cascade. A trophic cascade is one easily-understood example of ecosystem-mediated pesticide effects,” the organization noted.16

They used the example of the organophosphate insecticide malathion, which is sometimes applied to aquatic environments for mosquito control. It’s typically applied in low doses over the course of several weeks for this purpose. Researchers tested the effects of applying the chemical in this way as compared to a one-time exposure, such as may occur due to agricultural runoff, to a variety of plankton and tadpoles.

Both types of exposure were damaging, though the continued low-dose exposure was the worst. According to Beyond Pesticides, the trophic cascades in aquatic environments caused by insecticide exposure show the complexity of ecosystems and the widespread effects that insecticides cause:17

“In both instances, malathion’s impact on zooplankton caused a trophic cascade. By depressing the zooplankton population, phytoplankton flourished. The increase in free-floating algae clouded water, decreased light penetration, and led to reduced periphyton growth.

Decreases in periphyton algae, the primary food source for tadpoles, retarded growth and development in leopard frogs, which prevented many from metamorphosing before the vernal pool in which they resided dried up (though wood frogs were generally unaffected).

While zooplankton in the single-application mesocosm eventually experienced a population rebound, it took nearly a month and a half before this occurred. Overall, frogs in single-application mesocosms fared slightly better than those in chronically exposed tanks, which experienced an ongoing state of disruption that never permitted zooplankton populations to bounce back.”

Genetically engineered mosquitoes released

In January 2018, lab-bred Aedes aegypti mosquitoes carrying wolbachia bacteria were released in South Miami, Florida. It was the first phase of the Miami-Dade County Mosquito Reduction Test Program, which targeted a one-half square-mile treatment area that received the altered mosquitoes and a corresponding control area within the city.

Over a six-month period, the genetically engineered mosquitoes were released into an approximately 170-acre area. Interest in releasing lab-made mosquitoes has peaked in recent years in response to the Zika virus scare, which has since petered out in the U.S.

The project was conducted by the Miami-Dade County Mosquito Control & Habitat Management Division in collaboration with MosquitoMate, Inc., which created the technology.

MosquitoMate’s lab-bred male mosquitoes are infected with wolbachia bacteria, which is naturally occurring in up to 60 percent of insect species, but not in Aedes aegypti mosquitoes. When the male wolbachia mosquitoes mate with female mosquitoes in the wild (which do not carry the bacteria), the resulting eggs do not hatch, which means the number of Aedes aegypti mosquitoes in the area should ultimately decrease.18

The project was said to be a success, showing a 75 percent reduction in mosquitoes in the area, due to egg hatch failure.19,20 However, once genetically engineered mosquitoes are released (as they already have been), there’s no stopping them from mingling with wild mosquitoes.

While this may help to reduce the spread of certain viruses (although this remains to be seen), it may also have other unintended, as yet unknown consequences. There’s also the potential ramifications to the ecosystem of eliminating the insects, which can occur whenever any species is removed or drastically reduced — even species we deem to be pests.

While mosquitoes are primarily viewed as a nuisance and vector for deadly diseases like malaria, there may be “undesirable side effects” of eradicating them entirely, according to Florida University entomologist Phil Lounibos, Ph.D. BBC News reported:21

” … [Lounibos] says mosquitoes, which mostly feed on plant nectar, are important pollinators. They are also a food source for birds and bats while their young — as larvae — are consumed by fish and frogs. This could have an effect further up and down the food chain …

He warns that mosquitoes could be replaced by an insect ‘equally, or more, undesirable from a public health viewpoint.’ Its replacement could even conceivably spread diseases further and faster than mosquitoes today.”

Tips for avoiding mosquitoes

It’s definitely something to consider, although you can also take sensible precautions to avoid getting bitten. Wear long sleeves and pants if you know you’ll be outdoors in a mosquito-prone area and use natural insect repellants (not synthetic chemical versions), like cinnamon leaf oil, citronella essential oil or catnip oil, as necessary.

If mosquitoes are bothering you in your backyard, a house fan can keep them away while you’re outdoors, as can the strategic planting of marigolds, which mosquitoes tend to stay away from.

Draining standing water, including pet bowls, gutters, garbage and recycling bins, spare tires, bird baths and children’s toys, is also important to encourage mosquitoes to live elsewhere. This is where mosquitoes breed, so if you eliminate standing water you’ll eliminate many mosquitoes. Finally, try installing a bat house, as mosquitoes are one of their favorite meals.

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