Mosquitoes are so tiny – their average weight is 2.5 milligrams, their fastest flight is 1.5 miles per hour; and they drink only 5-millionths of a liter of blood when feeding.
At most, their wingspan is between 7 millimeters and 9mm – and that is for the largest species, Psorophora ciliata. The wings on the related Psorophora columbiae extend only between 4mm and 4.5mm from tip to tip.
The disease, disruption and dismay they cause, however, is world-class, beyond all proportion to their size. The time and money governments, research institutions and private organizations spend to exterminate them boggles the mind. The Cayman Islands budgeted nearly $6 million for mosquito control in 2015-2016 – and the Cayman Islands has only a handful of genuses.
In fact, the Mosquito Research and Control Unit, founded in 1965 by Marco Giglioli, lists only seven, although within those seven, 36 species occur locally.
The psorophora genus alone, for example, boasts 51 species globally. The MRCU lists only two in Cayman, however, ciliata and columbiae, which are the largest of our local “mozzies” – and among the most annoying.
The website for the City of Jacksonville Mosquito Control states the obvious: “Psorophora columbiae are definitely a pest mosquito,” going on to describe a situation anyone familiar with pre-1965 Cayman will recognize: “These mosquitoes have been known to kill cattle because of the high volume of adults that swarm at one time. This can result in suffocation and severe blood loss … in the animals.”
Black-and-silver and sometimes called the “dark rice field mosquito,” the creatures do not carry disease, however. They breed in standing water on pasture land and hatch in large numbers after heavy rains.
Curiously, ciliata larvae feed on the larvae of other mosquitoes, particularly some of the aedes genus, three of which (and mainly the Aedes aegypti) plague Cayman, spreading chikungunya, Zika and dengue fever.
Aedes aegypti, nicknamed the “Yellow Fever” mosquito, comes with a list of medical horrors – neurologic diseases, meningoencephalitis, cranial nerve palsies, rheumatic disorders, spondyloarthritis, undifferentiated polyarthritis, myocarditis, two kinds of ocular diseases, acute renal disease, severe bulbous lesions – and particular cautions for the pregnant.
Since July 28, the MRCU and U.K.-based genetic engineering firm Oxitec released millions of modified male mosquitoes in a 300-acre area of West Bay, seeking to control a Zika outbreak. A court challenge to the release led some to ask why “safer” ciliata could not be substituted for the altered Oxitec creatures.
The answer, according to the Entomology and Nematology Department at the University of Florida, is that not only are the numbers of ciliata larvae insufficient, but, frankly, “the pestiferous nature of [ciliata] adult females effectively undermin[es] any potential benefits of using the larvae as a biological control agent.” Simply said, they are too great a nuisance to be useful as a control method.
Aedes aegypti is complemented locally by the relatively benign aedes albopictus, which, according to Oxitec, “has the capacity to spread diseases, but is relatively inefficient at … transmission.” While the aegypti “feeds almost exclusively on humans,” lives “in and around the home” and often helps itself to “multiple bites to get a blood meal,” its albopictus counterpart “is an aggressive biter and will tend to secure its blood meal more easily from a variety of sources,” biting dogs, birds, cattle and other creatures “more in forested areas and underbrush.”
The third aedes mosquito in Cayman is aedes taeniorhynchus, the “black salt-marsh mosquito.” The MRCU calls it “the most-abundant pest mosquito in the Cayman Islands,” requiring most of the unit’s operational efforts to control.
“They readily enter dwellings for blood meals,” although “birds usually are preferred over man, cows, and horses,” the group says, warning that nigripalpus has transmitted St. Louis encephalitis to humans in Florida.
“Culex mosquitoes are generally weak fliers and do not move far from home, although they have been known to fly up to two miles,” the website says. “They live only a few weeks during the summer,” but females emerging late in the season hibernate until spring when warm weather brings them out “in search of water on which to lay their eggs.”
“They breed mostly in fresh water and are easily identified when biting as they stand ‘end-up,’ unlike other mosquitoes that keep their bodies flat,” says the unit’s website. One anopheles species, albimanus, can carry malaria.
The Florida Keys, in 2001, reported a whole new anopheles species, grabhamii, and the Galveston County, Texas, mosquito control unit reports two other indigenous groups: anopheles quadrimaculatus – found predominantly in marshes, breeding in grass fields and retention ponds and preferring cattle to humans – and anopheles crucians, the dominant species.
They are active year round, although, curiously, they prefer colder weather, although otherwise are similar to quadrimaculatus.