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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Get ready for the pale marsh mosquito, Ochlorotatus dorsalis


Pale marsh mosquitoes will soon make their presence known along rivers, swamps, and coastal areas across the United States.  Note the tumbler in the upper left corner.


Last week while walking along a trail near the Little Patuxent River in Columbia, Maryland, I stopped by a small sparkling vernal pool to investigate the goings-on. Diving and bobbing in the water column above drowned leaves were scores of wrigglers of the pale marsh mosquito. Wrigglers are a common name for mosquito larvae. Tumbler is the appropriately descriptive name given to the aquatic pupae of mosquitoes. These spawn arose from tiny eggs laid last year on the muddy soil in the flood plain where Mother Nature informed female pale marsh mosquitoes of the near certainty that a water-filled pool would appear in the spring of 2016. Mother Nature obliged with early spring rains and minor floods to create shallow pools of water. This triggered hatching of overwintered eggs of the pale marsh mosquito.



In this vernal pool, mosquito larvae cruise the water column and dine on tiny creatures on submerged leaves.

Most mosquito larvae are filter feeders, dining on tiny organisms living in water columns or grazing on biofilms coating the surface of submerged leaves and vegetation. Development of immature stages takes from seven to ten days under optimal conditions. Of course with cooler temperatures such as the ones experienced this spring, development can take much longer. After emerging from their pupal cases, male mosquitoes dash off to consume carbohydrates and protein found in fruit, nectar, and pollen. Males are not blood feeders, but the ladies surely are. Pale marsh mosquitoes are claimed to be one of the fiercest of all biters and their activity is greatest in late afternoon and evening, unlike day time biters like Asian tiger mosquitoes and yellow fever mosquitoes we met in previous episodes.



Mosquito larvae called wrigglers filter food from the water with mouth brushes, while pupae called tumblers hang beneath the water’s surface.

Males track and find their mates by “listening” to the buzzing wingbeats of females. A specialized sensory organ called the Johnston’s organ, located at the base of the male’s feathery antennae, senses his mate’s good vibrations. After mating, females seek blood meals from feral mammals like squirrels, deer, and birds, domestic animals like cattle and swine, and hapless human adventurers wandering into the mosquitoes’ domain at the close of the day. Blood of the victims is turned into protein of the eggs. In addition to the early spring brood of pale marsh mosquitoes, several generations may occur each season. Peregrinations of pale marsh mosquitoes may take them as far as twenty miles as they search for egg laying sites or mates.



After successfully shedding its pupal skin and watching a tumbler zoom by, this little lady is ready for some blood.

In addition to their fierce bites, these tiny vampires transmit serious viruses including West Nile, Western equine encephalitis, St. Louis encephalitis, California encephalitis,  Japanese encephalitis and the bacterium causing tularemia. To catch a glimpse of these rascals before the biting starts, take a peek in a vernal pool in the next week or two while harmless wrigglers and tumblers prepare to become bloodsuckers in the weeks and months ahead.    


The great reference “Ochlerotatus dorsalis” by Eric Dryer, Heidi Liere, John Marino, and Barry OConnor was used as a reference for this episode.

To learn more about this mosquito, please click on the link below: