Government mosquito control campaigns will often warn residents against standing pools of water that harbor mosquito larvae; including the water contained within the central cups of many species of bromeliads, called tank bromeliads, because they accumulate water in their leaf axils. Consequently, when the Zika virus was big news in south Florida, people were ripping out and throwing away bromeliads in their landscapes. These tank-forming bromeliads have developed a bad reputation for promoting mosquito breeding, but are they really cause for such concern?
Bromeliads 101
There are over 3000 known species of plants in the bromeliaceae family, collectively known as bromeliads or air plants. They are native to South and Central America into Mexico, the West Indies and Florida (only one species hails from Africa); and are adapted to a range of habitats from coastal desert to rainforest to limestone cliffs. The diversity of this plant family is incredible, and in my opinion, there are no other plants that can take the place of bromeliads in garden design. You can mount them, hang them in trees, or allow them to form a colony as a groundcover. Some can grow 6 feet across (Alcantarea genus), or be as tiny as ball moss (Tillandsia genus). They can add pops of color to shady areas, where much of the other foliage is dark green, and most have spectacular flowers. Some can tolerate full sun and some are even salt-tolerant.
There are 3 main types of bromeliads: terrestrial (growing in the ground and absorbing water and nutrients from their roots), epiphytic (absorbing water and nutrients from the air and only using roots for structure), or lithophytic (like epiphytic, but growing on rocks). They are masters of nitrogen-depleted, harsh environments; translating to a very easy-care plant for the south Florida landscape.
Bromeliad Adaptations
Amongst the 3 main types of bromeliads, there are tank and tankless types. These plants have developed a number of adaptations in order to attain water and nutrients in their harsh environments:
Interlocking and rosette-type leaf arrangements and spines trap debris and moisture, which in turn provides habitat for insects, frogs, lizards and other terrestrial animals.
One-way valves called absorptive foliar trichomes; they are like little scales on the surface of leaves that allow for the absorption of water and nutrients, instead of using the roots. Tankless bromeliads (like Spanish moss, an air plant) have a higher density of trichomes, and thus have a silvery appearance. Tankless bromeliads are also called atmospheric bromeliads because they can uptake dissolved forms of nitrogen from the atmosphere. Tank bromeliads have a higher density of trichomes near their center cup, in order to absorb nutrients from the water stored there.
Water-storing phytotelmata (a term used to describe the ability of non-aquatic plants to host small aquatic environments); these are exclusive to tank bromeliad types that store water in their center cup. Tank bromeliads host a tiny freshwater habitat that includes invertebrates, bacteria, fungi and other microorganisms. All of these microorganisms produce enzymes that help break down debris that falls down from the tree canopy after it rains. This breakdown of debris is necessary for the plant to uptake nutrients for growth. The phytotelmata is also home to terrestrial animals like frogs and lizards, which provide the plant with more readily accessible nutrients in the form of excrement.
Tankless bromeliads are adapted to seasonal drought and can store more nutrients than they need in the rainy season.
Bromeliads and Mosquitoes
There are over 3000 species of mosquitoes in the world (each one of them just as annoying as the next), and 80 of these are known to occur in Florida. However, only three are known to lay their eggs in bromeliad tanks. Of these three species, two are known to transmit diseases. The other species, from the genus Wyeomyia, is not known to transmit disease and interestingly enough, plays a beneficial role in the reduction of disease-spreading species.
First of all, there is extreme competition for survival between mosquito larvae. In a University of Florida study of bromeliads planted in urban areas around south Florida, 98.8% of larvae found in bromeliad tanks were from beneficial Wyeomyia mosquitoes, which outcompeted disease-spreading larvae to such an extent that the disease-spreaders never reached adulthood. If you're concerned about mosquito-borne diseases or just want to reduce the numbers of these pests (don't we all), I've outlined a few tips below:
Mosquito Prevention Tips
Make a home for predators: Mosquito larvae are eaten by fish, small crustaceans, dragonfly and damselfly nymphs, and frogs. Adult mosquitoes are eaten by frogs, dragonflies, damselflies, birds, and bats. I have never seen larvae in the bromeliads of my own garden, but I've seen plenty of tree frogs and lizards seeking shelter within their rosettes; I suspect they're well-fed.
Proper maintenance is key: Don't allow the bromeliads to form vast colonies, especially Neoregelia species, which tend to spread quickly. Mosquitoes will naturally be drawn to dark, moist and protected spaces, so ensure there is proper air flow around your bromeliads. If you're a big fan of the lush jungle look, be aware that dense vegetation will attract them. The good news is, for most mosquito species, the water held in tank bromeliads is not large enough (or permanent enough, with our frequent rain) to lay eggs. Also, mosquito larvae do not eat, but filter nutrients from decomposing matter within the water cups, just like the plant does. So, do not allow excessive debris to decompose within the water cups (the water should be clear), or you will essentially be fattening up the mosquito larvae for survival.
Don't use pesticides: I always discourage the use of pesticides in the home landscape because beneficial insects are affected. In my opinion, there is no need to make such an effort; after all, you're just there to enjoy your garden, you're not a commercial nursery or farming operation. Use a garden hose and flush out the water of the bromeliad tanks every few days instead. This will help to flush any larvae out onto the ground, where they will not survive.
Form a barrier around yourself: According to a University of Florida efficacy study, OFF! Deep Woods brand repellent offered 5 hours of protection from mosquito bites (the most effective brand in the study). Homemade essential oil-type repellents only offered less than 25 minutes of protection. The active ingredient in OFF! is DEET, a U.S. EPA approved repellent. A good old-fashioned mosquito screening material for your favorite outdoor hangout does the trick as well.
Mosquitoes are picky about where they lay their eggs: The Wyeomyia mosquitoes prefer to lay their eggs in pale green tank bromeliads. Disease-spreading mosquitoes, like Aedes aegypti (vector of the Zika virus and Dengue fever), prefer to lay their eggs in black containers of water. Once the eggs make contact with water, they take about 3 days to hatch. Afterwards, if the larvae store enough nutrients, they live for about 7 days until they turn into pupae. I recommend flushing out the tanks of bromeliads with fresh water every 3-7 days, or letting them dry out a bit during the dry season.
I've seen more mosquitoes swarming around the fake plant in the dark corner of my patio than my collection of bromeliads in the landscape. Do I think bromeliads are a significant vector for mosquito breeding? No, not if you've established your garden as a diverse ecosystem. In my opinion, containers of standing water and deeply shaded and wet areas are the worst mosquito attractants. Do I think there is still room for the use of bromeliads in the landscape? Yes!
What do you think? Do you love or hate bromeliads?
-Ivy