SCIENCE

Can something as simple as a treated net change the way mosquitoes actually behave? The answer may be yes. (NOVA Next)

Learn more about the science of mosquito nets with our great little study guide.

Discussion Ideas

• New research indicates that mosquitoes may develop behavioral resistance to insecticide-treated nets (ITNs). What are ITNs? Take a look at our study guide for some help.
  • Insecticide-treated nets are just what they sound like, mosquito nets treated with insecticide.
    • Mosquito nets, also called bednets, are hung over beds, desks, or even cooking areas. Some tents are made of mosquito-net fabric, to help campers avoid the pesky insects. Light and air can easily penetrate mosquito nets, but mosquitoes cannot. The sharp proboscis of the mosquito can penetrate the net, however. (The proboscis is the long, narrow mouthpart the mosquito uses to pierce the skin and suck the blood of its victim.) Because people can still be bitten through most nets, mosquito nets are hung from frames and not draped directly onto people’s skin.
    • The insecticide used in ITNs is deadly to mosquitoes, but generally harmless to people and pets. It slowly breaks down when the material is washed or constantly exposed to sunlight. Some ITNs need to be replaced every six months, while newer versions last up to three years. ITNs that last three years are called LLINs, or long-lasting insecticide-treated nets.
  • ITNs are one of the most effective methods of preventing mosquito-borne diseases in the developing world, reducing the number of malaria cases by 20%. Most of these cases involve children not yet five years old.
    • ITNs are crucial in the developing world, where lack of money, lack of available medications, and lack of distribution networks can prevent communities from accessing anti-malarial drugs.

• What is behavioral resistance?
  • Behavioral resistance describes an evolutionary process in which a species alters its behavioral adaptations—the way the organism acts in its habitat.
    • Behavioral resistance is distinct from biological or physiological resistance, in which a species’ physical adaptations are altered. For example, “[I]f we kill all the individuals who are susceptible [to a control agent such as a pesticide] the ones who are left over—the ones who by some quirk of biology or genetics can withstand our weapons—will become the parents of future generations.”
    • Behavioral resistance is also distinct from a learned or behavioral reaction. Unlike a reaction, behavioral resistance is a genetic characteristic that not learned but shared by an entire species or specific community.

• How might behavioral resistance among mosquitoes mitigate the efficiency of ITNs? In other words, how might their behavior change?
  • Mosquitoes may start to prefer activity during the day or dusk instead of at night.
  • Mosquitoes may start to prefer activity outdoors instead of indoors.

• Are there examples of mosquitoes or other pests developing behavioral resistance to pesticides?
  • Yes. Different species of mosquitoes have changed their predatory behavior to outside and earlier in the day in the Solomon Islands, Papua New Guinea, Vanuatu, and Tanzania.
    • Remember, however: Correlation is not causation, and in the words of one evolutionary biologist, “in general, studying behavior is hard.” For example:
      • The same mosquito species that show a change in response to nets in one place show no change in another.
      • There is so much geographical and seasonal variation in mosquito numbers that it’s challenging to nail down whether the effect you’re seeing is from the bed nets or indoor insecticides and not from something else.
      • If the insecticides on the net act as repellents—if they are unpleasant for mosquitoes to be near, rather than merely lethal to those that touch it—the change in behavior won’t be genetic, but just a reaction.
  • A fascinating study in behavioral resistance is the corn rootworm, an insect “that lays its eggs in cornfields so larvae will come up the next year and feast on the roots. Farmers evaded it by rotating crops so what was a cornfield one year will be soybeans the next. By the 1990s, however … rootworms had changed—instead of hatching every year, one species was hatching every other year, to be there when the corn returned. Another species was leaping into neighboring soybean fields to wait for them to take their turn as cornfields in the next season.”

• So should we stop using ITNs?
  • NO! ITNs are our most effective tool against malaria in the developing world. According to NOVA Next, “[t]he reductions in transmission are nothing short of astonishing—25%, 50%, even 80% fewer cases in some places.”

• How can scientists and communities combat behavioral resistance among malaria-vector mosquitoes?
  • There is no single magic bullet (or insecticide). “The problem, of course, is that we have to stay at least one step ahead of evolution. Coming at it with many different tools is our best to bet to help keep resistance, whether it is physiological or behavioral, under control.” Tools may include:
    • poisoned sugar traps. Mosquitoes often supplement their blood meals (yes, that’s what they’re called) with sips of nectar or sugar water. “Sugar traps with an oral toxin—one that only affects mosquitoes, not bees, for instance—could help drive their numbers down even if the insecticides on the nets and in the houses don’t work as well as before.”
    • stricter building codes to make houses more mosquito-proof
    • insecticides that target mosquitoes at different phases of their life cycle (including the larval stage)

TEACHERS TOOLKIT

NOVA Next: How Mosquito Nets Can Shape the Evolution of Behavior

Nat Geo: Insecticide-Treated Nets—Keeping malaria at bay study guide

CDC: Mosquito-Borne Diseases

WHO: Fact Sheet: World Malaria Report 2016Fact Sheet: World Malaria Report 2016

CDC: Insecticide-Treated Bed Nets