Maryland Grows

It is a common misconception that all mosquitoes feed on blood. Unlike ticks, which require a bloodmeal to progress from one life stage to the next, mosquitoes largely rely on plants throughout their life cycle. Only adult female mosquitoes bloodfeed in order to acquire protein to lay eggs; otherwise, adult mosquitoes feed on plant sugars to gain energy for flying, mating, and metabolic demands. Once those eggs are laid in standing water, mosquitoes hatch as aquatic larvae that eat microbes supported by decaying organic matter, called detritus. This detritus often comprises plant parts like leaves, seeds, and fruits that fall from nearby vegetation into larval habitat. The next life stage before adulthood, the pupa, is non-feeding, so the amount and quality of detritus mosquitoes receive as larvae are crucial. Just like we tell children to drink their milk to grow up to be big and strong, mosquito larvae with plentiful and nutritious resources can reach adulthood faster, grow larger, live longer, and lay more eggs as adults. Yikes! 

Leaves may vary drastically in their chemical composition, affecting which microbes they support, how quickly they decay, and what beneficial nutrients or toxic secondary metabolites they release as they decompose. Researchers are still working to identify important traits in a detritus resource base, but a trend has emerged: non-native plants seem to support non-native mosquito populations better than native plants do.

Native species, whether plant, animal, or microbe, are those that occur naturally in the region where they evolved. Over the course of time, these species have adapted to the local environment and developed relationships with other native species. Conversely, non-native species are those which evolved in a different region from that in which they can now be found. A small percent of non-native species establish and spread rapidly throughout their new home, with the potential to harm human well-being, environmental health, and/or economic prosperity. Some researchers speculate that non-native species share certain traits that make them successful invaders. The Asian tiger mosquito, Aedes albopictus, is one of the most successful invaders of the past century, arriving in Texas from Asia in 1985 and reaching us here in Maryland by 1987. The tiger mosquito is capable of spreading viruses such as dengue, West Nile, and zika; it is also a nuisance vector, driving people indoors with its aggressive daytime biting behavior. 

One study in Florida found that the tiger mosquito was more abundant in field sites where the invasive Brazilian peppertree grew, and in follow-up lab studies, tiger mosquito larvae supplied with those leaves had higher survival rates and larger adult body sizes than larvae raised on leaf litter from the native southern live oak tree. Another Florida study found that adult female mosquitoes, who use chemical cues to select aquatic habitat that will be most beneficial to their offspring, preferentially laid eggs in water-holding containers with non-native zoysia grass over native slash pine or southern live oak tree leaves. In an Ohio study, tiger mosquito larvae developed faster and grew larger on detritus from invasive Amur honeysuckle leaves than on native northern red oak leaves. 

Here in Maryland, we are investigating the effects of common native and non-native tree leaves in socioeconomically diverse neighborhoods of Baltimore and the suburbs around DC. Through plant surveys, we found that lower-income Baltimore blocks often have tree canopies dominated by the invasive tree of heaven and princess tree. Meanwhile, higher-income blocks in Capitol Heights frequently contain native red maple and American elm. The non-native white mulberry and the native black walnut trees are fairly evenly distributed across the region, regardless of socioeconomic status. In follow-up lab experiments, the tiger mosquito was nearly three times as likely to survive on a detritus base that contained at least one non-native tree species, and it reached adulthood 4-8 days faster than when it was raised on exclusively native plant detritus. While we have more data to collect, preliminary results suggest that this invasive mosquito may benefit from the presence of non-native leaf detritus. 

Do you have any of these trees in your yard? Invasive tree of heaven and native black walnut both have alternate, feather-compound leaves, but can be easily distinguished from each other in the fall when they fruit—the female tree of heaven has small, flat, papery fruits, similar to what you see on maples, while the walnut has, well, large, round nuts. Before the fruits appear, you can check the edges of the leaflets—the walnut will be toothed all around the edges, like a saw, while the tree of heaven often has only one pair of teeth at the base of the leaflet. You can also crush a leaflet and take a whiff—the black walnut has a pleasant, spicy scent, while the tree of heaven smells like rotten peanut butter.

The invasive princess tree will likely stand out in your yard by the size of its leaves. Or do you have the native common catalpa? Both trees have very large, paired, hairy, heart-shaped leaves, but, as above, can be easily differentiated by the shape of the fruits this fall. The princess tree has round, wooden capsules, whose husks can be spotted through the winter, while the common catalpa has long, slender pods.

Mulberries are currently fruiting, which will help you spot them, but the non-native white mulberry can be tricky to distinguish from its native cousin, the red mulberry, which both have fan-veined leaves that may be lobed or heart-shaped (often a mix of both on the same tree!). You can try a taste test (the red mulberry is a deeper color with a blackberry-like flavor, while the white mulberry is lighter and blander), or feel the leaves—the red mulberry is hairy, while the white mulberry is hairless.

You can likely already picture the fan-lobed maple leaf, like on the Canadian flag, when trying to seek it out. Determining exactly which native maple you have in your yard is a bit too technical to describe here, but there’s a quick and easy test to check if you actually have the non-native Norway maple: if you break off a leafstalk and see a milky sap, it’s non-native!

Elms have feather-veined leaves that are asymmetrical at the base. Native elms, like the American elm, have double-toothed leaves, meaning that each of the saw-like teeth along the edge has its own serrations, too. The non-native Siberian elm, meanwhile, will only be singly toothed. 

Take a walk around your yard to see how many native vs. non-native trees make up the canopy—these are the leaves that may fall into larval mosquito habitat. Will the mosquitoes in your yard grow up to be big and strong? For tips on removing larval habitat altogether, check out this Maryland Grows post from June. 

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.