Wharf Borer – Identification, Occurrence and Control

The Wharf Borer beetle (Nacerdes melanura) is a wood-boring pest in the UK, especially near damp, decaying wood in coastal and riverside areas. Known for damaging waterfront timber structures, it can also infest homes and buildings with high moisture levels. This article covers identifying the Wharf Borer, its common habitats, and effective control methods.

What Does the Wharf Borer Look Like?

The Wharf Borer beetle is a slim, reddish-brown insect about 10–12 mm in length. Here’s how to identify both adult beetles and larvae:

Identifying Adult Wharf Borers

Adult Wharf Borers are often mistaken for cockroaches, but they have distinctive features that make them easily identifiable:

  • Size and Shape: Adults measure approximately 10–12 mm, with a slender and slightly flattened body.
  • Colour: Their bodies are reddish-brown or yellowish-brown, with a black band across the tips of their wing covers (elytra).
  • Antennae: The long, segmented antennae are about half the length of their body.
  • Hair Covering: The body is coated in fine yellowish hairs.

Identifying Wharf Borer Larvae

Larvae, often responsible for the most damage, are longer than adults, measuring up to 18 mm. They have a cream-white body and brown mandibles, allowing them to bore through wood.

  • Colour: Off-white, with darker brown mandibles that darken at the tips.
  • Body Structure: Larvae are sparsely covered in fine brown hairs and have a unique “button-like” swelling on the upper side between the thorax and abdomen.

Where is the Wharf Borer Commonly Found in the UK?

Wharf Borers are particularly associated with coastal and riverside areas throughout the UK, where decaying wood provides the perfect environment for their life cycle. They’re frequently found in:

  • Coastal Regions and Riversides: Docks, wharves, and other waterside structures, especially where timber is regularly submerged or damp.
  • Buildings on Historic Sites: Structures built over buried timber, often remnants of World War II sites, are susceptible.
  • Homes with Damp Wood: Wharf Borers can be found in basements or crawlspaces with high moisture and rotting wood, typically near river banks or flood-prone areas.

Regional Occurrence

In the UK, Wharf Borers are widespread in England and Wales, with sightings reported in London, areas around the River Thames, and regions near other major waterways. They have also been noted in some Scottish areas but are generally absent in Ireland.

How Does the Wharf Borer Develop and Spread?

The Wharf Borer has a complete metamorphosis with four stages: egg, larva, pupa, and adult. Their development is highly dependent on moisture and temperature conditions.

Life Cycle

  1. Egg Stage: Females lay eggs on damp, decaying timber. Eggs hatch in 5–11 days, depending on temperature.
  2. Larval Stage: Newly hatched larvae burrow into wood, feeding on its cellulose and hemicellulose. The larval period lasts between two months to two years, with longer times in colder or less favourable conditions.
  3. Pupal Stage: When mature, the larva enters the pupal stage within the wood for 6–17 days before emerging as an adult.
  4. Adult Stage: Adults live for about 1–2 weeks, emerging between May and September, particularly in June, and do not feed during this period.

Temperature and Humidity Sensitivity

Eggs require temperatures between 20–30°C to hatch, and humidity levels above 20% for development. Wharf Borers do not thrive in tropical or dry conditions, which limits their occurrence to temperate areas like the UK.

What Damage Do Wharf Borers Cause?

Wharf Borers primarily target wood already weakened by rot or decay. While they don’t damage healthy wood, they can contribute to significant structural deterioration in older or water-damaged buildings.

  • Timber Damage: The larvae’s burrowing creates tunnels up to 30 cm long, weakening wooden structures like jetties, wharves, and wooden beams.
  • Plumbing and Foundation Risks: In urban settings, they can compromise the wood around foundations and water pipes, leading to costly repairs.
  • Impact on Historic Sites: Wharf Borers are often found in older structures near the Thames and other waterways, damaging heritage timber structures.

How to Prevent Wharf Borer Infestations?

Prevention of Wharf Borer infestations in the UK involves managing moisture levels and maintaining timber structures.

Reducing Moisture

  • Ensure Proper Drainage: Reduce waterlogging by directing water away from foundations and timber structures.
  • Ventilation: Improving airflow in basements and crawlspaces can help prevent wood from becoming damp and attractive to Wharf Borers.
  • Remove Decayed Wood: Discard any rotting wood around or under buildings, as it provides ideal breeding grounds.

Using Treated Wood

When constructing new buildings or repairing older structures, treated wood is less attractive to Wharf Borers and is more resistant to moisture damage.

How Can You Control Wharf Borer Infestations?

If Wharf Borers are already present, controlling them requires a multi-faceted approach that addresses both adult beetles and larvae.

Removing Adults

Wharf Borer adults can be manually removed by vacuuming or collecting near light sources where they tend to gather.

Insecticide Treatments

For severe infestations, consider using insecticides designed for beetle control, but these should be applied by professionals if used indoors or near human activity. Insecticides work best if the infested wood is accessible; residual insecticides may also be applied to the wall and floor junctions in basements.

Moisture Control

Since Wharf Borers thrive in damp conditions, the most effective long-term solution is reducing moisture in and around infested wood. This is especially helpful when the larvae are feeding within the timber and cannot be easily reached.

  • Lower Humidity: Use dehumidifiers in crawlspaces and basements.
  • Repair Leaks: Ensure that leaks around windows, doors, and plumbing fixtures are sealed to prevent wood from becoming damp.
  • Remove Infected Wood: Replace decayed wood with treated wood or materials resistant to decay.

Trapping Methods

In cases where the beetles are entering buildings, trapping emerging adults as they exit wood or by light sources can help control their population. This is particularly useful for buildings overlying historic sites with buried wood.

Are There Non-Chemical Methods for Wharf Borer Control?

Yes, homeowners can employ several non-chemical control strategies:

  • Manual Collection: Capturing adults manually when they emerge is a simple, effective method for small infestations.
  • Reduce Larval Habitat: Removing sources of decayed wood in and around the property prevents larvae from finding suitable areas to mature.
  • Encourage Natural Predators: While this is not commonly practised in residential areas, promoting predators such as certain birds may help reduce Wharf Borer numbers in outdoor settings.

When are Wharf Borers Most Active?

Wharf Borers are typically active between April and August, with peak emergence in late spring to early summer. During this period, they are most likely to be found around waterfront structures or near light sources in homes.

By understanding their habits and life cycle, property owners in the UK can take proactive measures to protect their wooden structures from this invasive pest.