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Greenhead & Deer Flies in Delaware
Biting flies of salt marsh edges and summer woods. Only the females bite.
| Peak season | Jul |
|---|---|
| Size | 1/4 – 1 1/8 inches |
| Risk | Health — painful bites (females only) |
How to identify them
Greenheads are a salt marsh horse fly. Horse flies are about 3/8 to 1 1/8 inches long and usually gray or blackish. Deer flies are smaller, about 1/4 to 1/2 inch long, black or brownish, usually with dark areas on the wings.
Greenhead & Deer Flies in Delaware
Greenhead larvae develop in coastal salt marsh, in the upper zone reached by daily high tides. Rutgers entomologists report that greenhead numbers peak in July, with flies active from late June into September. Deer fly eggs are laid in damp places, and their larvae develop in wet areas. Only females bite; they need blood to produce eggs. DNREC’s Mosquito Control Section doesn’t treat biting flies; it refers non-mosquito pest questions such as biting flies to University of Delaware Cooperative Extension.
When they’re active
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What you can do now
- Wear long sleeves and long pants, and use an insect repellent. Repellents and protective clothing may help, though no option stops every bite.
- Near the marsh, a greenhead box trap can help. Set it out when the first greenheads appear, around mid- to late June, and keep it running through August.
- Plan marsh-edge outings around the July peak when you can.
Is spraying worth it?
Spraying isn’t the answer for these flies. University of Maryland Extension says pesticide control is not feasible because of the flies’ habits. For greenheads near the marsh, Rutgers recommends box traps. For other biting-fly questions, DNREC suggests contacting University of Delaware Cooperative Extension.
Greenhead & Deer Flies: common questions
When are greenheads worst in Delaware?
Rutgers entomologists report a July peak, with greenheads active from late June into September. Only the females bite.
Do greenhead box traps work?
Rutgers reports that in Delaware, three to five traps near isolated homes resulted in an almost complete reduction in greenhead attacks. Whether trapping shrinks the overall population is still an open question.
Can I spray my yard for greenheads or deer flies?
University of Maryland Extension says pesticide control is not feasible because of the flies’ habits. Protective clothing, repellent and, near the marsh, box traps are the practical options.
Where in Delaware
Greenheads breed in coastal salt marsh, so they are worst along marsh edges. Rutgers reports that in Delaware, three to five box traps near isolated homes nearly eliminated greenhead attacks.
Where greenhead & deer flies rank on each county’s top-pest list:
- New Castle County Not in the county top six
- Kent County Not in the county top six
- Sussex County Not in the county top six
Towns where greenhead & deer flies are on the local top-pest list: