'World class' mosquito habitat: Inside Delaware's war with biting insects

'World class' mosquito habitat: Inside Delaware's war with biting insects
This aerial image shows mosquito ditches and water pooling on marshes in Delaware’s downstate Inland Bays. Photo courtesy of Delaware Center for the Inland Bays

It’s been nearly 100 years that Delawareans have been waging a springtime battle against a foe that’s often heard before it’s seen. A foe capable of inflicting wounds without victims realizing until it’s too late. One that is only a few millimeters in size, but capable of inflicting serious illness and even death.

Mosquitoes are capable of inflicting debilitating disease and ruining otherwise perfectly good summer nights, and have likely made whoever discovered citronella candles a bit richer. Anyone who has lived in Delaware through at least one wet spring season — especially near a waterway or marsh — knows mosquitoes are just part of life in this low-lying state.

“We’re unnaturally keeping mosquito populations much lower than they rightfully should be in Delaware, which has world-class mosquito-producing habitat,” said William Meredith, head of Delaware’s Mosquito Control Section, part of the Department of Natural Resources and Environmental Control. “The reason the mosquitoes are pretty low and not that problematic is because there are control programs.”

As the weather begins to shift and April showers bring May mosquitoes, a big part of the reason people aren’t assaulted by skeeters the moment they step out the front door is the work done by the Mosquito Control Section and decades of biological and chemical applications. But everything is weather dependent, and the wetness of May and June can play a huge role in the outcome of this summer’s mosquito season.

“Every year has its curveball,” said Meredith, noting that mosquito season has returned to normal compared to a severe outbreak in 2018. That year, weather conditions were perfect to support a brood of mosquitoes that had the agency’s complaint line nearly ringing off the hook.

From pooling water in marshes to Meredith’s true nemesis, corrugated downspouts, prime mosquito breeding habitat abounds in Delaware. Preparation for mosquito season begins long before the unofficial start of summer.

Meredith’s section is tasked with first targeting key places with “biological measures” such as open marsh water management, in which experts alter the physical properties of prime breeding areas.  Next, mosquito control experts will turn to the use of larvicides — an effective, targeted way to kill young larvae and stop the mosquito problem before it actually starts. They’ve already started treating wet woodland areas with that biological-based intervention, Meredith said in mid-spring 2022.

The last resort — and the one people are likely most familiar with — is adulticiding, which is the use of pesticides capable of killing adult mosquitoes. This is done aerially with airplanes and on the ground with spray trucks throughout Delaware’s marshes and communities, typically during the early or late hours when there is less risk of impacting other vulnerable species. While this seems the most obviously straightforward approach to mosquito control, it’s also the most complex.

“The (U.S. Environmental Protection Agency) has determined that all the modern mosquito control insecticides applied by the Mosquito Control Section can be used to kill mosquitoes without posing unreasonable risks to human health, wildlife or the environment (but this is not to say that there are no risks at all),” Delaware’s mosquito control spray policy states.

The Asian tiger mosquito. This species was introduced to the area by humans in the 1980s, and thrives when there is plentiful breeding habitat thanks to water standing in manmade containers. Pexabay photo

Mosquitoes themselves pose significant health threats as carriers of diseases such as the West Nile virus, Zika virus and equine encephalitis (EEE), but new studies are finding the chemicals used to fight these foes could be posing health threats as well. One of the main pesticides used in mosquito control across the country, known as naled, is now undergoing review by the EPA, which recently found that young children exposed to aerial spraying of the chemical may suffer adverse health impacts. The chemical has been banned in the European Union since 2012, according to multiple media outlets.

“All of these things have some harms associated with them,” said Lori Burd, environmental health director at the nonprofit Center for Biological Diversity. “It’s always a balancing test. Unfortunately, we just see it being really common for mosquito districts to go straight to aerial spraying a really intense chemical.”

In Delaware, though, aerial spraying of the more dangerous chemicals like naled is considered a last resort. Among the list of fewer than a dozen chemicals used by agencies like Delaware's to kill adult mosquitoes, many also pose risks to the environment, particularly for vulnerable pollinators such as bees and aquatic species.

“No pesticide is 100 percent safe in terms of its possible impacts on non-target species,” Meredith said. “That’s a fact of life. We acknowledge it. As such, we’re in the game of minimizing the risk and trying to reduce potential impacts.”

How can Delaware balance the risk of mosquito-borne disease with the risk of chemical contamination, especially as climate change threatens the region with a wetter, peskier future? It may come down in part to asking why mosquito control is needed: for actual public health or just to make summer nights on the deck more bearable.

A world without mosquito control

Long before Delaware’s Mosquito Control Section existed — actually, not so long after the United States of America officially existed — living with mosquitoes was just a way of life. A dangerous one.

In 1793, about 10 percent of nearby Philadelphia’s population was wiped out (and twice as many sickened) by a yellow fever pandemic caused by the Aedes aegypti mosquito, a species that thrives in urban settings and is still monitored today.

Other species of these tiny, flying, biting machines, like the Asian tiger mosquito, are experts at making the most out of a little: they love breeding in manmade containers such as tires and old swimming pools, but even something as small as a bottle cap filled with water will do just fine.

“Some of these container-breeding species in urban and suburban areas are very difficult to control,” Meredith explained. When they’re able to breed and grow in inaccessible containers, larvaciding isn’t an option. Control experts are forced to turn to the more risky adulticides, which do the job in eliminating those challenging species on contact.

Delaware officials rely on the EPA’s safety determinations for these products to decide when, where and how much of a particular chemical is safe and appropriate, he said.

“If we thought there was any potential of any significance for adverse health impacts, we wouldn’t be doing it,” Meredith said. “We rely on the EPA science to say it’s safe.”

There are about 57 species of mosquitoes found in Delaware, only 19 of which are considered “aggressive biters of humans, as well as other mammals and birds” and of concern to the state’s mosquito control officials.

Many of the “bad actor” species commonly and naturally occur in the marshes, tidal wetlands and other habitats of coastal Delaware. Some, like the species that slaughtered Philadelphians (and also Wilmingtonians) in the late 1700s, and the highly aggressive Asian tiger mosquito, were introduced to the area by humans.

Another four species are either rare in the First State or unlikely to bite people, but still capable of causing harm. Among them are Aedes aegypti, the yellow fever mosquito, which could be found as far north as New England in Colonial times, according to Meredith.

According to data from the U.S. Centers for Disease Control and Prevention, in recent years Delaware has only reported about one or two cases per county per year of serious mosquito-borne disease in humans. In 2018, the last notable mosquito outbreak in the state, there were 10 reported human cases of West Nile statewide, the majority of which occurred in New Castle County. That year, though, there were no reported human cases of EEE or Zika.

As climate change drives a shift in seasons — the growing season has now extended by weeks, marked by an earlier final frost in spring and a later first frost in fall — as well as stronger storms and sea-level rise, experts like Meredith expect wetter coastlines will bring more fertile mosquito grounds. And the more mosquitoes there are, the more risk of mosquito-borne disease.

“The marsh is getting wetter along the seaward edge [and] at the same time, the marsh is also getting wetter toward the landward edge,” Meredith said. “That creates new breeding habitat we didn’t have to contend with before. … We’re always on the lookout now for new habitat farther inland.”

Civilian Conservation Corps workers dig a grid ditch through a Delaware salt marsh. Photo courtesy of the Department of Natural Resources and Environmental Control/Delaware Mosquito Control Section

Ditching the ditches and finding balance

In the 1930s, Civilian Conservation Corps crews created in the wake of the Great Depression to provide American jobs dug ditches in marshes all along the East Coast, including in Delaware, as a way to address mosquito populations. While it initially worked, this parallel grid ditching caused severe hydrological and environmental problems for these sensitive habitats.

Delaware ditched the ditching by the 1960s, and around that time, the state had also started spraying chemicals to fight this flying foe.

In the mid-20th century, frequently used pesticides included dichlorodiphenyltrichloroethane, better known as DDT, a broad-spectrum pesticide with long-lasting environmental effects that was ultimately banned in the early 1970s.

In 1980, Delaware also began its open marsh water management biological controls, which target salt marsh mosquitoes. This labor-intensive process includes selectively installing ponds and ditches where mosquitoes are known to prolifically produce. Meredith said this method results in 90 percent of marsh-based mosquito control. These engineered solutions can last 15-20 years before maintenance is needed, Meredith said, and significantly reduce the need for chemical controls in those areas.

The other type of biological control used in Delaware, albeit on a much smaller scale, is the stocking of freshwater habitats with mosquito-eating fish, mostly in manmade ponds where the idea is that no native fish species would be adversely impacted.

Where mosquitoes are able to breed, the agency mostly relies on a natural bacteria-based solution called Bacillus thuringiensis, or BTI. BTI targets mosquito larvae specifically and studies show that its only adverse impacts are on a small number of similar species such as fly larvae.

When it comes to dealing with adult mosquitoes, though, today’s pesticides may be better than the DDT that’s blamed for the previous decline of some bird species such as the bald eagle, which is now significantly rebounding since the chemical’s ban in 1972. But these modern-day pesticides still are not perfectly safe.

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A study by the national nonprofit the Xerces Society for Invertebrate Conservation published in 2000 found that chemicals used for mosquito control in national wildlife refuges were having a negative impact on local native species such as dragonflies.

Aside from the serious disease potential presented by mosquito bites, the primary reason for controlling mosquitoes in Delaware all these decades – as well as the driving force behind ditching in Delaware in the 1930s and a big reason for the public calling Delaware’s Mosquito Control Section – is because biting skeeters interfere with comfort and quality of life.

But just because the EPA regulates a pesticide doesn’t necessarily mean it’s safe,  the Center for Biological Diversity’s Burd reiterated.

“Our EPA allows a tremendous amount of chemicals that are not allowed in other countries,” she said. “Our system for pesticide regulation is basically innocent until proven guilty.”

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