Nacerdes melanura (Linnaeus, 1758)

Wharf borer

An orange-and-black beetle whose larvae honeycomb the wet pilings of old wharves, and whose relatives can blister skin.

Order: Coleoptera
Family: Oedemeridae
Group: Fabric & structural beetles
Ontario status: Not established in Ontario, coastal wharf/piling species; reference for damp-wood borer comparisons

Why it matters

A structural pest of waterfront infrastructure elsewhere in the world; in Ontario, a reference species for the damp-wood borer category, and a cautionary note, since its family can cause skin blisters.

Origin

Coastal Europe and worldwide ports, it travels in ships' timbers and dock wood.

Behavior

Adults are often seen on flowers near the coast in summer. Larvae tunnel in the sodden wood for a year or more, honeycombing it.

Living conditions

Waterlogged, decaying wood: wharf pilings, dock timbers, wet fence posts, damp cellar wood. The wood must be wet and rotting.

Signs of presence

Slender orange beetles (10-12 mm) with black-tipped wing covers near old docks
Honeycombed, punky pilings that crumble
Oval emergence holes in wet timbers

Weaknesses

Requires saturated decaying wood, dry the timber and the colony dies
Restricted to waterfront and very damp sites
Conspicuous adults are easy to spot

Myths and facts

Myth Wharf borers sting.
Reality They don't sting or bite. But like other oedemerid (false blister) beetles, handling them can irritate skin, look, don't touch.

Research notes

Morphology

Adult 10-12 mm, slender, orange-yellow head and pronotum, elytra orange with black tips. Larva white, up to ~20 mm, with dark mandibles. Emergence holes oval, ~5 mm.

Diagnostics

Orange/black coloration + slender body + wet rotting timber habitat = wharf borer. Unmistakable among wood borers.

Easily confused with

Old house borer, Hylotrupes bajulus Old house borer is black, attacks dry seasoned softwood; wharf borer is orange, attacks only saturated decaying wood.

Life cycle

Adult: Emerge in summer; visit flowers; live weeks
Egg: Laid in cracks of wet decaying wood
Egg to adult: 1-2 years
Generations per year: Less than one
Nymph or larva: Larvae tunnel 1-2 years in saturated timber

Development

Complete metamorphosis in waterlogged wood.

Reproduction

Moderate fecundity; females oviposit in wet rot.

Distribution

Global: Ports and coasts worldwide
Native range: Coastal Europe; spread via shipping
Ontario: Not established; reference species

Ecology

Decomposer of waterlogged wood; part of the waterfront decay community. Oedemeridae contain cantharidin-like defensive compounds (vesicants).

Resistance

Unknown, no documented resistance.

IPM implications

Wharf infrastructure logic: replace or isolate the saturated timber, the infestation is a moisture problem, and borer control without timber replacement fails. Handler caution for the vesicant chemistry.

Chemical ecology

Cantharidin-related defensive chemistry (skin-blistering in handlers); floral visitation by adults.

Genetics and DNA

No reference genome published as of 2026.

Taxonomy

Order Coleoptera, family Oedemeridae (false blister beetles). Nacerdes melanura = 'black-tailed.'

Open questions

Cantharidin-pathway biochemistry in Oedemeridae; northern range limits under warming coasts.

References

Ebeling, W. (1978). Urban Entomology. University of California, Riverside.