Explainer: What’s up with the mosquitoes?

An MRCU staffer fogs a yard in October 2019. - Photo: Taneos Ramsay

It is no illusion. Grand Cayman is buzzing with mosquitoes.

The current boom in blood-sucking insects has resulted from a combination of factors, including tides and rainfall, reduced aerial spraying, and the species of mosquitoes overrunning the island, explains James McNelly, director of the Mosquito Research and Control Unit.

He explained some of the factors behind the mosquito surge in responses sent by email to the Cayman Compass.

What’s that biting my ankle (and my neck, and my forehead)?

That would be the Aedes taeniorhynchus or black salt marsh mosquito. Each female can lay batches of 100 to 200 eggs each, McNelly explains.

While most female mosquitoes require a blood meal to produce eggs, this is not the case for black salt marsh mosquitoes, giving the species a breeding advantage.

“No blood is required to lay the first 100 or more eggs. This in part is responsible for the huge populations experienced on the island. Laying that many eggs without ‘having to leave home’ for blood is a huge advantage for this mosquito,” McNelly says.

A male black salt marsh mosquito. – Photo: University of Florida

Black salt marsh mosquitoes prefer mangrove swamps, or habitats that hold standing water, to lay eggs. While eggs are laid on dry ground, they can only hatch after coming in contact with tides or rainwater.

“Both tides and rainfall in June have created a perfect storm in terms of contributing to the current, annoying situation,” McNelly says.

“It doesn’t help that these mosquitoes are strong flyers and will travel many miles from the water in which they were produced. Females can easily fly several miles in search of blood. It’s also important to note that these mosquitoes are not involved in disease transmission. However, they are definitely annoying.”

McNelly estimates that Grand Cayman has between 10,000 and 15,000 acres of potential breeding habitat, including the freshwater aquifer and Central Mangrove Wetland, for black salt marsh mosquitoes. Control operations are limited in these areas.

What has been done to control mosquitoes in recent weeks?

The MRCU aerial programme serviced all three islands in June to limit production in mangrove swamps, McNelly says.

Aerial spraying covered 110,165 acres in June. During 19 nights that month, two to five trucks were used for additional spraying.

Treatments included application of residual larvicide (lasting around one month) across 1,500 acres in the Sister Islands and a combination treatment of two residual materials across 2,500 acres in Grand Cayman.

An additional 1,138 acres were treated with a ‘single brood’ material.

McNelly says these numbers are abnormally low due to reduced flight and airport operations.

“The existing larviciding blocks or maps used by the aerial team need to be re-visited in light of the current situation with biting mosquitoes,” McNelly writes. “These blocks, which direct our aerial larviciding efforts, were reduced by 22% in 2018. The most notable reductions took place in Lower Valley/Bodden Town/Frank Sound/North Side areas.”

Did the coronavirus lockdown affect mosquito-control operations in any way?

Yes, McNelly says, explaining that limited air traffic control operations resulted in fewer mosquito-control flights by MRCU.

“This severely impacted our larviciding operations… They were closed to us on weekends until a few weeks ago,” McNelly writes.

“There is still a fair amount of coordinating that is required, but this has gone smoothly, and the airport has been accommodating when we have requested the ability for aerial operations to proceed.”

High winds and Saharan dust have also impeded aerial operations in recent weeks.

How is the mosquito population measured?

MRCU monitors 29 New Jersey light traps that have been in place for decades to track nuisance mosquito species in Cayman.

“This network is in need of some modification and perhaps modernization. Light serves as the only attractant that draws the mosquitoes to the trap,” McNelly writes.

“More than several of these light traps are compromised by the fact that they are situated directly under or are very near street lights. These and other light sources that result from the expansion of the island’s population and infrastructure influence the trap network’s ability to effectively monitor mosquito populations.”

Is there concern at this time about vector-borne diseases, such as dengue or chikungunya?

MRCU says no, in part because these diseases are not endemic to the Cayman Islands and international travel remains limited.

“These diseases travel to the island through people infected elsewhere. Monitoring takes place through physicians and hospitals which collaborate with [Health Services Authority] and provide information on potential and/or confirmed cases. HSA in turn provides the information to MRCU and we deploy mosquito surveillance and control measures,” McNelly writes.

“If anything, the lack of travelers to the island helps mitigate potential importation of disease.”

MRCU carries out surveillance and control measures directed at disease-carrying mosquito species, like Aedes aegypti and Aedes albopictus. McNelly notes, however, that two traps used to monitor these populations were impacted by COVID-19 measures.

“We deploy a wide area larviciding strategy from the planes to control the larval stages of the vectors (a strategy used successfully in several US programs),” McNelly writes.

Disease prevention officers also carry out inspections of residences and hand treat containers.

What does MRCU’s current mosquito-control plan look like?

Mosquito control in Cayman includes surveillance, targeted larvicide treatments and spraying, and removal of standing water, McNelly explains. Such measures are part of MRCU’s ‘integrated pest management’ programme.

“Surveillance is always the foundation. Surveillance allows for informed decisions related to control,” he writes.

“If larvae are found in the water – mangroves, pasture, puddles, wastewater, storm drains, tires, etc. – larviciding takes place. This can be done with one or more formulations that last variously from 24 hours to a month or more. The duration of control impacts the price.”

If larvicide treatments are insufficient, then spraying takes place to target flying mosquitoes.

“This is the last ‘tool in the toolbox’. A truck spray impacts mosquitoes for roughly 300 feet from the road in the direction of any breeze/wind. A plane obviously does a more complete job of spraying an area,” McNelly writes. “We use different classes of chemicals and rotate their use to avoid resistance.”

Residual spraying is used to target areas such as shady walls, rows of vegetation and trash piles where mosquitoes enjoy hiding during the day.

Truck-mounted equipment is employed in areas that are inaccessible by aerial spraying due to towers, and when diseases such as dengue have been detected.

“Mosquito Research and Control has been surveying and treating the production sites and emergent, flying and biting mosquitoes throughout the COVID pandemic. We have done so whilst, and most importantly, ensuring that our personnel and staff adhere to all the proper precautions related to risk of COVID transmission,” McNelly writes.

“We have tried not to allow COVID to impact our service, but it cannot be denied that it has influenced and impacted operations to some extent.”

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