Humming hoards

The whine of a mosquito looking for a blood meal in order to develop her next batch of eggs is a constant accompaniment to summertime activities in much of the world, and all over the United States – scientists have identified over 3,000 different species of mosquitoes, 150 of which have been found in North America.

Different mosquito species have different habitats and different behaviors – some develop in swamps, others in ephemeral ponds; some are capable of transmitting malaria, some aren’t; and some don’t bite humans at all, preferring to get the blood they need for egg production from frogs, snakes, or birds.

Adult mosquitoes don’t hatch directly from eggs – they emerge in water as larvae, which mature into pupae before developing into the winged adults that ascend in humming hoards during warm summer months. Their ability to survive to adulthood is influenced by the conditions they experience early in life, and these conditions, as reported in a paper published earlier this year by a team of researchers at Rutgers University, include the species of the other mosquito larvae around them.

The team of scientists filled cups of water with varying proportions of two types of mosquito larvae, and found that, at higher temperatures, one type (the Asian bush mosquito, typically found in clear, clean water) did not survive to adulthood unless there was a small number of the other type (the southern house mosquito, well known to thrive in polluted water, and even raw sewage) present in the same cup. Left on their own, or with too many southern house mosquito larvae present, the Asian bush mosquitoes died.

The scientists observed that the cups containing only the Asian bush mosquito larvae grew yellow and cloudy with the presence of a population of flagellates – single-celled organisms with whip-like tails – which they suspect was responsible for killing the larvae. They postulate that, in the cups where both mosquito species were present, the southern house mosquito larvae saved the Asian bush mosquito larvae from the flagellate, perhaps by eating the microorganisms – but they also point out that they were a benevolent presence only in small numbers. If the southern house mosquitoes made up more than 50 percent of the mosquito population, they appeared to out-compete the Asian bush mosquitoes, whose survival plummeted. At least in cups in a lab, the presence of just the right number of one species of mosquito allowed another species of mosquito to survive.

These findings suggest that mosquitoes that are able to survive in polluted habitats may, in some cases, change those habitats to the extent that other species can move in, too; the researchers write, “if mosquitoes are capable of increasing the geographic range . . . of other mosquitoes, this could result in the establishment of new diseases or increased transmission efficiency of existing ones, with devastating impacts on native wildlife and humans alike.”

Mosquito larvae are influenced by the conditions in their immediate surroundings, including the species of their neighbors – and those dynamics have important implications for their eventual fate as adults, buzzing in our ears.

Mosquito larvae feed and develop underwater - coming to the surface to breathe - before molting into pupae and eventually emerging as adults. 

(Original image by Mary Hollinger/NOAA Photo Library via Wikimedia Commons)



Every invasive species is a native species somewhere else. Red swamp crayfish, Procambarus clarkii, aka Louisiana crayfish, also known as crawdads, and sometimes called mudbugs, are native to the south-central United States and northeastern Mexico. Everywhere else they’ve spread to – and that’s a lot of places, including Europe, Asia, and many states outside of their native range in the United States – they’re invasive.

Not all non-native species are invasive – non-native species are organisms that don’t naturally occur in a specific place, while those that are non-native as well as destructive in some way are considered invasive. Red swamp crayfish usually fall into the later category – when introduced to a new location, they typically dominate the local habitat, to the detriment of local crayfish populations. 

Red swamp crayfish were first officially detected in Washington State in 2000, in Pine Lake, a tiny lake (just eight tenths of a mile by four tenths of a mile at its widest point) 20 miles east of Seattle. Five years after they were first recorded in Pine Lake, the invasive red swamp crayfish population was much larger than the native crayfish population. (Native crayfish population was not reported in 2008.)

Note: 2008 values were calculated from 24-hour sampling periods, with the assumption that capture rate was equal throughout the 24-hour period - because crayfish are more active at night, the calculated values may be underestimates. 

Sources: First detection in Pine Lake from Mueller 2001; 2005 values from Mueller 2007; 2008 values calculated from Larson & Olden 2008.

(Figure by Emily Benson)

The problems that invasive red swamp crayfish can cause when they spread to a new location are well known, as are the most common mechanisms of introduction. Most red swamp crayfish dispersal is due to human activity – mudbugs are considered a culinary delicacy, and live crayfish have been stocked, farmed, and traded widely throughout the world. There are, however, other ways that crayfish can spread – a study published in Aquatic Ecology earlier this year suggests that ducks and other waterfowl may be a previously unappreciated vector for transporting crayfish between lakes and ponds.

The researchers who conducted the study were interested in whether or not juvenile crayfish could cling to the feathers of a duck as it flew between bodies of water, and, if they could, how long they could hold on for. They found that crayfish are capable of hitching a ride on waterfowl, particularly in shallower water depths. With trained homing pigeons standing in as proxies for ducks, the scientists found that crayfish could survive flights as long as 37 miles in mesh bags secured to the birds.

As the researchers write, “these findings indicate that waterbird-mediated passive dispersal should be taken into account to explain P. clarkii’s rapid spread and should be considered when managing its invasions.” Humans may be responsible for the majority of the spread of this invasive species, but we’re not the only culprits.

Red swamp crayfish are freshwater crustaceans, but they can survive out of water for up to sixteen and a half hours, depending on the temperature and humidity;  Anastácio and colleagues estimate that a crayfish could walk over 700 yards in that time. 

(Image by Entomolo via Wikimedia Commons)