The name ‘wharf borer’ or ‘wharf beetle’ comes from the larval stage, which is well adapted to live in the old wood piles and logs of harbors and wharves, commonly found in marine coastal areas and inland lakes worldwide. The larvae primarily damage old, damp, rotten, and decaying wood. Wharf borer is widespread in temperate regions globally, including the Palearctic, Afrotropical, Australian, Nearctic, and Neotropical zones. In the United States, it is widespread in the eastern and western coastal areas and the Great Lakes region.
Plant Pest Diagnostic Clinic (PPDC) at the Ohio State University received a few photographs of this beetle from a homeowner in Lake County, Ohio. The residence is located very close to Lake Erie. When I visited, the area immediately surrounding the house was water-logged due to recent rain. This was a good indication of the prevailing moisture condition under the house which is one of the requirements for the survival and development of the larvae. There was no evidence of dead or living beetles anywhere outside the house, and there was no obvious logs or other pieces of decaying wood. However, beetles were observed trapped inside the home ventilation system, unable to find a way out. At each ventilation duct on the first floor, numerous beetles were trapped, and the homeowner used insecticide to kill them. The house, built on a concrete slab a couple of years ago, replaced an old house. The old house was constructed with anchored wood buried in the soil. It is possible that some of the old rotten timber from the previous house remains under the concrete and contained developing wharf borer larvae. The emerging adult beetles likely found a way into the ventilation system, as the beetles were only found inside the house, not outside.
Taxonomy and Identification
While some scientists believe this beetle is native to the western Palearctic region and introduced to the New World through timber trade, driftwood, or other wood products, Balsbaugh et al. (1979), suggests it is native to the Great Lakes region in the United States and spread to Europe in a similar manner. Therefore, the origin of this beetle is still debated. Wharf borer, which is about 7-15 mm long, belongs to the order Coleoptera and the family Oedemeridae, commonly known as false blister beetles. The common name comes from the beetle’s ability to cause skin blisters when squished. It has a long, slender body with semi-hardened elytra. The coloration of the beetle varies from dull yellow to orange, with the tips of the elytra being purplish-blue to black. They also possess long, serrated antennae that are about half the length of their body.
The family Oedemeridae is widespread throughout the world with more than 100 genera and 1500 species in three subfamilies. North America has about 100 species of Oedemeridae in 18 genera. The genus Nacerdes contains about 27 species in two subgenera (Nacerdes, and Xanthochroa) worldwide. Warf borer, N. melanura, is the only species of this genus occurring in the United States.
Life Cycle
Usually, wharf borer adults begin emerging from wood between March and July in the Northern Hemisphere. Soon after emergence they mate, and the adult female searches for decaying, old timber or old moist softwood to deposit her eggs. The eggs are tiny and very hard to detect. The eggs will hatch and larvae will emerge after 5-7 days.
Immediately after the emergence of larvae, they start to feed on the moist wood, consuming cellulose and hemicellulose in the wood, tunneling, and creating long galleries. These larvae produce cellulase enzymes enabling them to feed on wood, similar to many other wood-boring beetles. Depending on the temperature and food availability, the larval stage can last for two months to two years. It is reported that this beetle's association with decaying or rotten wood is linked to the presence of fungi (Pitman et. Al., 1993). Pitman et al (2003) also showed the presence of this beetles far from coastal areas infesting old damp cork flooring in United Kingdom, indicating the adult beetles can fly far distances.
When the larvae develop a certain size of head capsule, it’s ready to pupate. Pupation is reported to last about 10 days. The exact time of pupation depends on the temperature and the relative humidity of the decaying wood, as adults will only emerge when the environmental conditions are favorable.
As soon as the adults emerge, they navigate out of the wood towards the light, mate, and the female will start searching for old moist timber to lay eggs. The adult wharf borer has been reported to live about 1- 2 weeks. Flying adults are short-lived, free-living, and may or may not feed during their lifetime.
Damage and management of wharf borers
Feeding by the larvae of wharf borer causes damage to old decaying wood. There is no evidence that this species attacks living, healthy trees or hardwood timber. To manage populations of wharf borer, it is important to remove any larval habitat such as buried timber, logs, or even old stumps and roots before building any house or structures, especially if the location is closer to a water body.
Pitman et al (2003) reported developmental thresholds for wharf borer life stages due to temperature and relative humidity, which could inform management options. The optimal temperature reported for egg development was between 20C to 30C, with the upper and lower thresholds for development being 35C and 10C, respectively. This explains its absence in tropical climates. Relative humidity also played a major role in egg development, with the lower threshold being 20%. Temperature was a critical factor for the development of larvae and pupae as well. For example, low winter temperatures in temperate regions do not result in mortality but will adversely affect development by increasing the length of time required as larvae and pupae.
To manage this pest, it is crucial to reduce the moisture level of the infested timber. If it is possible old timber is buried under the soil and difficult to locate, an attempt should be taken to slow the development and/or prevent infestation by reducing the moisture level of the larval habitat. This can be achieved by preventing water logging conditions around the house or other structures, collecting trapped beetles in a confined place such as the home. If the beetles are emerging outside the house, then prevent them entering the house.
For the homeowner in Lake County, Ohio where the larval habitat and point of emergence remain unknown but adult beetles have infested the ventilation ducts, the population could be reduced by collecting all the beetles as they emerge before they can escape outside. In this situation, the most effective method of control is to collect the adult beetles over the next couple of months as they emerge, and continue to monitor their populations, which will hopefully decline. The homeowner may need to repeat this process for one or two years during the spring and early summer when the adults emerge to completely get rid of the borers.
There are fumigants and residual insecticides available for controlling wharf borer, but these insecticides are only effective if the larval habitat are easily accessible and known, because often the larvae are feeding deep within the decaying wood.
Management of Wharf borers using chemicals including fumigants should only be handled by professionals, since they are highly toxic.
Reference
Arnett R.H., Jr. 1951. A revision of the Nearctic Oedemeridae (Coleoptera) American Midland Naturalist 45: 257-391, https://www.jstor.org/stable/2421732
Balsbaugh Jr., E. U., Kopp, D.D., Scholl, C. 1979. The wharf borer, Nacerda melanura L., (Coleoptera: Oedemeridae) in North Dakota. The Coleopterists Bulletin, 33, 455–458. https://www.jstor.org/stable/4000082
A. J. Pitman, A. M. Jones & E. B. Gareth Jones (1993) The wharf-borer Nacerdes melanura L., a threat to stored archaeological timbers, Studies in Conservation, 38:4, 274-284, DOI: https://doi.org/10.1179/sic.1993.38.4.274
A J Pitman, E B G Jones, M A Jones & P Oevering (2003) An Overview of the BIOLOGY of the Wharf Borer Beetle ( Nacerdes melanura L., Oedemeridae) a Pest of Wood in Marine Structures, Biofouling, 19:S1, 239-248, https://doi.org/10.1080/0892701021000049584