Maryland Grows

Imagine the perfect summer day. What are you doing? For most people reading this blog, I’m guessing it involves being outside in your yard—maybe working in the garden, reading in a hammock, or sharing an al fresco lunch with family and friends. Sounds idyllic! But, then, the dreaded whine, the too-slow slap, and the ensuing itchy bite left behind by a mosquito. On repeat. It’s enough to drive many of us indoors, even on the perfect summer day.

More mature readers may think to themselves “I don’t remember it being this bad when I was younger!” and you’re probably right. You are almost certainly being bitten by a relatively new invader called the Asian tiger mosquito (scientific name: Aedes albopictus) that was first observed in Maryland in 1987 and has adapted to thrive in urban and suburban areas. Named for its striped legs, the tiger mosquito is capable of spreading viruses such as dengue, West Nile, and zika. It is aggressive, and, unlike most other mosquito species that feed at dawn and dusk, it seeks its blood meals during the day.

One survey found that the tiger mosquito’s biting rate is 10 times higher than what Virginia residents were able to tolerate. In another, 61% of residents surveyed in the Washington, DC area reported that being bothered by mosquitoes caused them to change their behavior to avoid taking walks, gardening, or participating in other outdoor activities. So, if you’re hiding inside on a sunny day, you’re not alone.

What to do? Avoid them? Deter them with citronella candles? Swat them one by one?

The most effective action you can take to reduce the mosquito population in your yard is to cut them off at the source—their larval habitat. After a protein-packed blood meal, female mosquitoes lay their eggs in water-holding containers. Once flooded, the eggs hatch into an aquatic larval stage; after roughly a week of feeding on microbes in the water, the larvae develop into pupae and then emerge in a couple of days as the adults you know and hate.

If you’re committed and persistent in eradicating places for mosquitoes to breed, a practice known as source reduction, you can reduce the number of mosquitoes in your yard. For successful source reduction, follow the four “E”s:

Eliminate

The first step is to eliminate every water-holding container from your yard that you can. Throw away pieces of trash. Drill holes in the bottom of the tire swing. Store kids’ toys, wheelbarrows, watering cans, etc. in a shed or garage, under a deck, or on a covered porch. The tiger mosquito can breed in as little as one teaspoon of water, so be thorough in your search!

Exchange

Next, consider exchanging containers that cannot be eliminated. Swap an open rain barrel for one with a lid. Substitute corrugated extension spouts with splash guards, open chutes, or flat extensions at a downward angle. Trade lawn chairs with solid seats that allow water to pool with slatted chairs that do not.

Empty

Be vigilant about regularly emptying any water-holding containers that cannot be eliminated or exchanged—ideally twice but at least once weekly. Replace the water in pet bowls. Dump out flower pot saucers. Shake out tarps.

Emaciate

This one sounds weird, but stay with me. There may be standing water on your property that would be undesirable or impossible to eliminate, exchange, or empty, such as a bird bath, pond, or structural element (a common one I see in urban environments is the remainder of metal fence posts that were sawed off rather than torn out after the removal of a chain-link fence). For these sources, you can emaciate mosquito larvae with Bacillus thuringiensis subspecies israelensis, or bti for short; when larvae eat this normally soil-dwelling bacterium, the toxins released by the bti paralyze their gut, causing them to starve to death. A brutal way to go for mosquito, black fly, or fungus gnat larvae but harmless to other insects, pets, and people. Bti can be purchased online or
at most garden centers—a common brand to look for is Mosquito Dunk®.

Adhering to the four “E”s of source reduction can help prevent mosquito infestations on your property so you can enjoy those perfect summer days!

But what about mosquitoes on your neighbors’ properties?

Bonus action: Encourage

Encourage your neighbors to practice source reduction as well! Devoted source reduction in your own yard may not protect you if your neighbors are harboring abundant larval habitat. Tiger mosquitoes don’t tend to fly far, especially when resources are plentiful, but depending on how close you live to your neighbors, they may cross into your yard. Local mosquito populations are best controlled when the entire community participates.

Bonus bonus action: Entice

Want to do even more? Perhaps you’re feeling a little vengeful from summers past? Entice mosquitoes into a larval habitat trap. You can purchase or DIY something called a Gravid Aedes Trap, or GAT for short, that targets species in the Aedes genus, like the tiger mosquito. Standing water inside the trap encourages gravid, or pregnant, female mosquitoes to lay their eggs, but a mesh cover above the water prevents any mosquitoes in the next generation from escaping. The clear sides around the dark funnel top confuse the females who enter, making it likely they’ll be stuck, too—especially if you add sticky paper or cover the inner walls with cooking spray or oil to coat their wings. This makes the GAT a double-kill trap, affecting two mosquito generations at once. Just be sure to check the trap regularly to ensure that water is present but below the netting. University Park, MD created a community-wide mosquito control program in their town and created a helpful PDF document for GAT deployment.

By eliminating, exchanging, and emptying water-holding containers in your yard and emaciating mosquitoes in remaining larval habitat (plus encouraging your neighbors and enticing egg-laying female mosquitoes!), you can once again enjoy your summer outside. Easy, brEEzy, bEautiful, mosquito sourE rEduction. Happy mosquito hunting!

By Sarah Rothman, PhD Candidate in the University of Maryland Department of Environmental Science and Technology. Sarah is studying the relationship between non-native mosquitoes and plants across socioeconomic gradients. Learn about the EcoHealth lab at www.leisnham.org. After graduating, she hopes to work in the public sector conducting ecological research and outreach on topics related to invasive species, restoration, and environmental justice.