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MEMORANDUM |
From: Ray Legge, Resource Manager Huntley Meadows Park
To: Joseph Alexander, Supervisor, Lee District
Subject: The Frogs of Manchester Lakes
Date: August 7, 1986
The abundant frogs that appear on the sidewalks, doors, walls, and other exterior surfaces at Manchester Lakes are gray treefrogs. These common frogs are small amphibians, between 1 1/4 and 2 1/2 inches in length. Gray treefrogs are mottled gray in color, but an individual's color may change from off-white to dark gray to a light green. As the name implies the gray treefrog is an accomplished climber because of its suction-cup-like toes. Gray treefrogs may be distinguished from other frogs appearing in the area by their suction-cup toes, grayish color, and white patches below both eyes.
Gray treefrogs gather around slow-running bodies of water in this area during nights of May through August to breed and are rarely seen at other times. Males attract female treefrogs with a short musical twill. Mating occurs in the water and eggs are laid in groups of 10 to 40. The eggs hatch into tadpoles in 4 to 5 days. The tadpole stage lasts 45 to 64 days during which time the frogs eat plant material. They do not eat mosquitoes in the tadpole stage. During metamorphosis the frogs switch from gills to lungs, from a plant diet to a meat diet, and from swimming to walking or jumping. After the tadpoles transform into adults the frogs spend most of their time on land. As adults, treefrogs eat insects, either from the ground or from trees or buildings.
Other frogs such as Bullfrogs, Green Frogs, Southern Leopard Frogs and American and Fowler's Toads are present at Manchester Lakes but are found only in low numbers and do not climb on buildings.
Gray treefrogs are probably so numerous at Manchester Lakes for several reasons. Manchester Lakes probably resides on habitat formerly occupied by treefrogs. Frog numbers will probably decrease with successive summers as frogs move or are unable to reproduce. The numbers of tadpoles and adult frogs will decrease as predators such as fish and insects appear at Manchester Lakes.
Gray treefrogs are not intelligent creatures, nor are they attracted to lights, but they are smart enough to frequent areas where insects are to be found, such as any outside lights left on. Treefrogs are largely nocturnal hunters and usually rest in hiding places during the day. Light posts are one possible way to lure treefrogs from porches.
There are no frog-killing agents or repellents yet developed, as frog plagues are largely unknown. Other sprays such as insecticides are largely ineffective and impractical. Because treefrogs are vertebrates like humans, substances which harm them are likely to be dangerous to dogs, cats and humans.
Although gray treefrogs are eaten by many animals, it is not practical to "import" such predators. It is also unlikely that the large number of gray treefrogs at Manchester Lakes will attract undesirable predators such as raccoons and snakes.
Partial chlorination of Manchester Lakes may kill some eggs and tadpoles during their aquatic life but chlorine will damage other aquatic life more than gray treefrogs.
The seasonality of treefrog abundance at Manchester Lakes is largely unexplained. As gray treefrogs begin breeding no earlier than May, reaching greatest numbers in June or July, adults would not be congregating in great numbers until June and young would not begin their terrestrial life until at least mid-June. As Manchester Lakes residents reported greatest numbers of frogs in April, another earlier-breeding species may be responsible for the high April numbers. It may have been spring peepers, a frog that resembles the gray treefrog but may be distinguished by the x-mark on its back.
The presence of the gray treefrogs around the houses of Manchester Lakes indicates that insects are plentiful and these frogs undoubtedly catch thousands of undesirable insects. Although the frogs are found in large numbers on the outside walls they are not likely to seek shelter inside houses. Frogs require fairly cool, very wet areas for resting and hibernation.
Although frogs are unusually numerous at Manchester Lakes and thus bothersome and unattractive to some, easy solutions are not known. Lakes attract frogs, and lights attract insects which attract frogs. Chemicals which may kill or repel frogs are ineffective and likely to cause more problems than they solve.
These notes were compiled by Fairfax County Park Authority staff with assistance from Dr. Roy McDiarmid, a frog expert at the Smithsonian Institution.
cc: Gary Roisum
Gil Aldridge
Bill Beckner