Mosquitoes in full force as summer wanes

Spraying now spans March to November
September 12, 2017

A wet spring and summer has fortified the mosquito population in the First State, where the peak season for mosquito-borne diseases is just getting started.

“Right now, it's kind of quiet – yet we have lots of mosquitoes out there,” said William Meredith, administrator of the Department of Natural Resources and Environmental Control’s Division of Fish & Wildlife Mosquito Control Section.

Mosquitoes pick up diseases such as West Nile virus and Eastern equine encephalitis, or EEE, from wild birds, which are immune to the viruses but serve as hosts. Mosquitoes spread those viruses from late August to early October, Meredith said.

West Nile and EEE spread to horses and humans when mosquitoes first bite an infected bird, carrying the virus in their saliva, and then transmit it with their next bite.

To reduce the spread of diseases, the state Mosquito Control Section works on land and air to spread insecticides that kill both larvae and adult mosquitoes. The section follows a grid throughout the state, and responds to residents' complaints to hit any mosquito hot spots.

“What's usually hectic is even more so,” Meredith said, estimating mosquito control can receive upwards of 2,000 public complaints about mosquitoes in a bad season. “We've got a lot more mosquito problems to deal with than in past years.”

Delaware's population boom has meant more demand for the mosquito control, Meredith said. In the 1970s and 1980s, insecticides were generally applied between Memorial Day and Labor Day. Now, mosquito control spans from March to November, working with a $672,000 operating budget supported by the state's general fund. This year's tight budget and resulting cuts did not cut into mosquito control's budget.

“We're very mindful of the importance of the coastal resort strip to the local economy,” Meredith said, adding that applications in Sussex County have increased to keep pace with population growth. “There's more pressure and more complaints. But that's also happening up in southern New Castle County.”

Through the end of August, aerial applicators and ground foggers scheduled nearly 70 upstate treatments, more than a dozen mid-state treatments and more than 60 treatments downstate.

“It’s highly variable year to year,” Meredith said. “There are different methods depending on the life stage and species, and it’s a combination of aerial work and ground work. We treat everywhere. Mosquitoes don’t know boundaries.”

Mosquitoes can pose a much larger threat than just a buzzing, biting nuisance. While rare, EEE can cause serious health problems and even death. West Nile virus, which is more common, is often undetected, but can have severe consequences for a small percentage of people who have a more sensitive reaction to the virus.

Those viruses also mean bad news for horses. This summer, a horse in Kent County was bit by a West Nile-infected mosquito; its owner found the animal suffering from weakness in its legs. A few days later, the horse was unable to stand and had to be euthanized, state officials reported.

The horse's infection marks the first time West Nile has been detected in either a human or horse since 2015. After West Nile peaked in Delaware in 2003 – sickening 17 people, claiming 2 lives and killing 63 horses – cases have significantly dropped.

Meredith said in horses, at least, cases may have dropped because an effective vaccine was developed in the mid-2000s. The only reason a horse would die from West Nile or EEE today would be because it was not properly vaccinated, he said.

That option doesn't exist for humans, however. Prevention such as applying insect repellent, avoiding mosquito-prone areas and discarding standing water where backyard species breed are the best defenses for people.

In 2015, there were five confirmed cases of people infected by West Nile virus, as reported by the Division of Public Health. But about 80 percent of people infected never develop any symptoms. About 20 percent of those who contract West Nile virus develop fever, body and muscle aches, headaches, nausea, vomiting and a rash.

“[The Division of Public Health] does not like to speculate about the variations in virus-related illnesses because it is hard to back up with the science,” said state epidemiologist Mamadou Diallo. “It is important not to view this as a trend and continue to take precautions, as case numbers could jump again at any time.”

Meredith shared those sentiments.

“There's a possibility that something could erupt here above normal,” he said.

Delaware’s marshes offer prime breeding grounds 

Mosquitoes are prevalent in Delaware, which boasts nearly 100 miles of coastline with hundreds of thousands of acres of wetlands and marshes, their prime breeding ground. As development encroaches on these wetter areas, people are living closer to mosquito territory.

“Certainly in Sussex, subdivisions are sprouting up like corn out there, adjacent to wet woodlands, and that's causing a problem,” he said. “That's pretty pervasive throughout the state.”

There are dozens of native mosquito species in Delaware, 19 of which are known to aggressively feast on the blood of mammals, including humans. The common house mosquito, which mainly breeds in backyards, is known to transmit West Nile virus, Meredith said. Saltmarsh mosquitoes, which can travel up to 10-15 miles from where they were born, are the primary carriers of EEE, which has not been detected in a human since 1979.

To detect possible problems with those two viruses, the Mosquito Control Section relies on chickens on the front lines. There are 20 sentinel chicken stations statewide, where flocks may be exposed to virus-laden mosquitoes.

Those chickens are immune to the disease, Meredith said, and blood samples give state officials a glimpse at where future problems may exist. So far this year, West Nile virus has been detected in several chickens in Kent and New Castle counties, while EEE was found in southwestern New Castle County.

“We're not doing this to cause panic for the public,” Meredith explained, adding that more specific locations will not be shared. Even though mosquito numbers are high, disease detection is relatively low so far, he said. But that doesn't mean people shouldn't still take precautions. “In comparison to past years, it's a little bit slower this year,” he said. “But that can change rather quickly.”

To monitor where insecticide applications are planned, for more information or to contact mosquito control, call 302-739-9917 or go to