Mosquito control program surveilling standing, stagnant water for larvae

A dozen tiny, thread-thin and worm-like critters wriggled around a stone birdbath at Louser Memorial Park Northeast in Lebanon on a recent morning.

“Mosquito larvae,” declared Lydia Mohn as she suctioned them up with a pipette and transferred them into a sample bottle.

Mohn heads the Mosquito-borne Disease Control Program for Lebanon and Lancaster counties. Since April, Mohn and four technicians have been visiting sites with standing and stagnant water to check for mosquito larvae.

Drainage ponds, ditches, parking lots, old tires, and cemetery vases are all likely places where rainwater might puddle and mosquitoes lay eggs.

If larvae are found, they go into sample bottles labelled with the collector’s name, date, habitat, site, and approximate number of mosquitoes. The samples go to DEP for classification and testing.

Pennsylvania has 62 species of mosquitoes. While the control program team collects all specimens of mosquitoes, of particular interest is the brown Culex pipiens mosquito, a vector for West Nile Virus (WNV).

DEP has been testing for WNV since 2000 when the virus was first detected in mosquitoes in Schuykill County, said John Repetz with DEP in an email. Culex populations have increased since then, leading to more WNV detected throughout Pennsylvania, he added.

Many people who contract WNV through bites of infected mosquitoes have no symptoms. But one in five people will develop mild flu-like symptoms, and some infected individuals—particularly, the elderly and those with compromised immune symptoms—may experience a severe infection, according to the Centers for Disease Control and Prevention.

“Basically, every county in Pennsylvania has had cases of West Nile Virus,” said Mohn, who added that so far, Lebanon County is virus free. But Bucks County has recorded mosquitoes positive for WNV, according to DEP.

Others are only visited a couple of times a month although significant larval populations can trigger more frequent checks.

“This season has already been very busy,” Mohn said. “We’ve had a lot of rain which means a lot of standing water and a lot of larvae.”

Besides hand-dipping for larvae, the program team also sets traps specific for the Culex pipiens mosquito. Traps are placed in a dishpan of gravid water made with a fermented mix of hay and protein powder that creates a pungent, unpleasant smell that attracts the Culex—although other species may also get lured in.

Mosquitoes have four different life stages, three of which occur in water and include eggs, larvae and pupae. From egg to adult takes on average from seven to 14 days.

Only adult mosquitoes live outside of water, and only the female bites, according to EPA. A female must have a blood meal in order to lay eggs.

“WNV activity is dependent on weather and temperature patterns,” said Repetz in an email. “Changing climates and weather events have provided more available habitat for mosquitoes to lay eggs. The mosquito season has also extended during this time—starting sooner, ending late.”

DEP offers ‘do’s and don’t’ to protect against mosquitoes and WNV on its website.