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Adult - Shore flies in the genus Scatella are small (2 mm), black flies with reddish eyes and gray
wings with clear spots. Shore flies resemble eye gnats, fruit flies, or vinegar flies in general shape.
Shore flies are sometimes confused with darkwinged fungus gnats which are about the same size
and color. (Darkwinged fungus gnats are shaped more like miniature mosquitoes and have
relatively long legs and 50 antennae.)
Egg - Shore fly eggs are about 0.42 long and 0.2 mm wide. They are fairly rounded at both ends
although there are small lobes on one end. The surface of the egg is covered with minute, faint
polygons. The eggs remain white throughout the development of the embryo.
Larva - The tiny first stage maggots have two spiracles only at the rear. The next two larval stages
have two spiracles on the rear and a spiracle on either side near the head. The mature maggot is
about 2.6 mm long and 0.9 mm wide and 0.5 mm high. The hind spiracles are black and are
located at the end of small but conspicuous tubes. The anterior spiracles each resemble the fingers
of a slightly inflated rubber glove but these spiracles are microscopic and inconspicuous.
Puparium - The puparia of Scatella shore flies are brown to dark brown, curved, and tapered on
both ends. The puparia are about 2.6 mm long and 0.9 mm wide. At the hind end, the spiracles
resemble stiff antennae. On the front, the anterior spiracles stick out sideways like microscopic
horns. The puparia are curved toward the top side of the developing fly.
Figure 102: Shore fly
Scatella stagnalis (Fallen) and perhaps other spp., Ephydridae, DIPTERA
Zoom Fig. 102: Full view Shore fly A-adult, B-egg, C-larva, D-pupa
Distribution - Shore flies in the genus Scatella are found throughout the United States, Canada
and northern Mexico.
Scatella stagnalis also occurs widely in Europe and Asia. Most species naturally occur on muddy
or marshy lake shores and intertidal zones of beaches.
Food Plants - Adult and immature shore flies feed on microscopic algae, dinoflagellates, bacteria,
cyanobacteria, and other unicellular forms.
Damage - Neither adult nor immature shore flies feed upon ornamental plants. The damage caused
by shore flies consists primarily in the excrement ("fly specks") left on the foliage of bedding plants
and other ornamentals. Because shore flies are often confused with darkwinged fungus gnats,
control efforts are often wasted (darkwinged fungus gnats may be harmful to plants but are
relatively easy to control; shore flies are harmless but are very difficult to control with insecticides).
Sometimes shore flies become so abundant in greenhouses that the sheer numbers of flies becomes
a deterrent to customers browsing or even employees working.
Life History - Scatella shore flies are commonly found in greenhouses where they breed in algae
growing on the potting mix, pots, benches and floors. Females scatter eggs right on the surface of
the potting mix. The eggs hatch in 2 to 3 days. The larvae are found within the crust of algae and
very top layer of potting mix. The maggots feed on bacteria and yeasts as well as diatoms and
flagellates growing on the surface of the potting mix. The larvae mature in 3 to 6 days and pupate
inside the skin of the last larval stage (this kind of pupa is called a puparium). The last larval skin
affords the relatively tender and completely helpless pupa protection from environmental hazards
(including insecticides). Some of the puparia are found on top of the potting mix or are very close
to the surface. A new generation of adult flies emerges 4 to 5 days later. The adults crawl about on
the surface of the potting mix, on the plants or they fly about the pots and plants. The flies move
and fly rapidly but generally stay close to their breeding sites. The adults feed primarily on diatoms
and flagellates on the surface of the potting mix or mats.
Perhaps because of the high biological activity of the surface of the potting mix, the protection
afforded the pupa by the last larval skin, and the water repellent property of the adults, insecticides
do not seem to readily suppress shore flies in greenhouses. Cultural methods of shore fly
management include avoiding excessive use of water during irrigation, using the minimal optimum
levels of fertilizers for adequate plant growth (and lower levels of excess fertilizers that encourage
algal blooms), and a drier greenhouse environment. Algae on mats, benches, the walls or other
structural members, and the soil beneath benches should be eliminated by using an approved
algicide. Because shore fly maggots can also develop on rotting vegetable matter, general
greenhouse sanitation should also help suppress shore fly numbers. For specific chemical
recommendations see your Cooperative Extension Service publications on ornamental plant pest
University of Florida/IFAS Reference to Pest Control Guides