Biology Now, 2e

Zika-Busting Mosquitoes ■ 345

engineering, or GE. In contrast to insecticides,

GM mosquitoes can directly target just one insect

species, says DeGennaro. “You’re only affecting

one species of mosquito, the species that is the

primary vector for Zika, dengue, and more.”

Early field tests of GM mosquitoes in Brazil

appeared to stop the spread of dengue. But could

they do the same for Zika?

Rapid Spread

Before it hit Florida, Zika virus struck Brazil like

a tsunami. In a span of 3 months in early 2015,

Brazil documented nearly 7,000 cases of mild

illness similar to dengue fever, though no one yet

suspected Zika. That’s the problem with a newly

emerging infectious disease: no one is looking for it.

Then, in May, a national laboratory reported

that Zika virus was circulating around the country.

Two months later, Brazilian health workers began

reporting neurological disorders associated with

Zika infection, including brain inflammation. In

October came the worst news of all: reports of an

unusual spike in newborns with microcephaly.

That dramatic spread and link to microceph-

aly put Zika on the map, but it isn’t actually a new

virus. Zika virus—a round particle with a dense

core packed with RNA (Figure 19.4)—was first

leave (emigrate) the population. Birth and immigra-

tion increase population size; death and emigration

reduce it. Environmental factors also have a strong

impact on population size. Mosquito populations,

for example, increase in warmer, wetter weather

when the conditions are ripe for reproduction.

To target mosquito populations, scientists

rely on population ecology, the study of the

size and structure of populations and how they

change over time and space. Population ecology

can help us determine where mosquitoes live,

eat, and breed. Mosquitoes congregate and lay

their eggs at water sources, and many prefer

urban environments with trash receptacles and

concentrated groups of people. At these loca-

tions we find the highest population density, or

number of individuals per unit of area.

To c a l c u l a t e p o p u l a t io n d e n s i t y, t o t a l p o p u-

lation size is divided by the corresponding area

of interest. For example, in an urban neighbor-

hood of Rio de Janeiro, Brazil, where people were

being regularly infected with dengue virus, scien-

tists captured mosquitoes in mosquito traps and

calculated the neighborhood’s population of female

mosquitoes to be 3,505 pregnant females. The

neighborhood has an area of 911 acres, so the female

mosquito population density was 3.85 mosquitoes

per acre (3,505 mosquitoes divided by 911 acres).

Keep in mind that population density is often diffi-

cult to measure, because it depends on an accurate

count of the population size. Individuals may be

hard to detect, may move between populations, or

may inhabit a complex, hard-to-define area.

Scientists can track population density to see

how well eradication efforts work against mosqui-

toes. As mentioned earlier, one of the most popular

ways of reducing mosquito populations is poison-

ing them with insecticides. But that’s not ideal,

scientists agree. “Insecticides affect the entire

insect population usually, and that leads to overall

imbalances in the environment,” says DeGennaro.

“So putting a chemical into the environment is

something you need to do very carefully.”

An alternative approach, thanks to advanced

genetic technologies, is to tweak the genes of

the mosquitoes to sabotage their reproduction.

If the mosquitoes can’t reproduce, population

size will plummet, and fewer mosquitoes means

fewer people being bitten and infected. This

gene tweaking is called genetic modification,

or GM—altering the genes of an organism for a

specific purpose. It is also referred to as genetic

Zika virus

particle

Figure 19.4

Zika is a spherical RNA virus