Black Carpenter Ant Infestation in a Garden

Eastern black carpenter ants (via Wikimedia Commons, CC BY-SA 4.0)

Black carpenter ant infestations in a garden can cause significant damage to outdoor structures and plants. They build their nests in wood, including trees and wooden garden structures, causing tunnels and galleries as they burrow. An infestation can weaken the structural integrity of a wooden structure and kill plants by girdling the trunk.


  • Presence of ant trails or swarms of ants
  • Seeing large, winged reproductive ants (swarmers) in or near a structure
  • Damaged or hollowed out wood in trees or wooden structures
  • Frass (sawdust-like material) near ant activity
  • Presence of irregular or smooth galleries in wood, which are used by the ants to move around
  • Stains, discoloration, or softening of the wood near the ant activity
  • A sawdust-like material near the ant activity, which is the result of the ants burrowing through the wood
  • Piles of dead ants near the nest site
  • Unexplained, persistent, and intense ant activity in or near the garden
  • A sweet or pungent odor near the ant activity, which may indicate the presence of ant pheromones.

What is a Black Carpenter Ant

  • Black or dark brown in color, with a glossy and smooth appearance
  • About 1/4 to 1/2 inch in length, with some individuals reaching up to 5/8 inch in length
  • Has a distinctive large head and mandibles relative to its body, which are used for chewing and excavating wood
  • May have wings, but wings are not always present, and only reproductive individuals have wings
  • Antennae are elbowed and have a gradual curve, with 12 segments
  • Body is segmented, with three distinct regions: the head, thorax, and abdomen
  • Body is shiny and smooth, with no noticeable hair or fur
  • Has powerful mandibles that are used for biting, chewing, and excavating wood
  • Legs are long and spiny, with a two-segmented waist (petiole) that separates the thorax and abdomen.

Treating a Black Carpenter Ant Infestation

It may be necessary to use multiple methods to effectively control a black carpenter ant infestation in a garden. For example, physical control measures can be used to prevent ant access to the garden, while chemical control measures can be used to kill the existing colony. Cultural and biological control methods can be used to create an environment that is less hospitable to ants and more supportive of ant predators. Additionally, ongoing monitoring and maintenance, including regular inspections and timely repairs, can help prevent new ant infestations from taking hold.

Chemical Control

  • Using insecticide sprays, dusts, or baits to kill the ants and their colony
  • Selecting a product that is labeled for use against carpenter ants and specifically targets their biology and behavior
  • Applying the product directly to ant trails, nests, or infested wood, paying attention to entry points, windowsills, and other areas where ants may be entering the garden or structure
  • Re-treating as necessary according to the product label instructions and observing the efficacy of the treatment
  • Taking precautions to protect plants, animals, and the environment, including avoiding the application of insecticides on blooming plants and near sources of water, and following all safety instructions and application rates listed on the product label

Physical Control

  • Removing infested wood and replacing it with new, treated wood that has been treated with a wood preservative or borate product
  • Sealing cracks and crevices, especially around windows, doors, and utility lines, using silicone or silicone-latex caulk, steel wool, or other appropriate materials
  • Trimming trees and shrubs to eliminate contact with buildings and reduce the number of harborage sites for ants
  • Clearing debris and trash from around the structure and garden to reduce harborage sites for ants and other pests
  • Installing door sweeps and weather stripping to reduce ant access to the structure, and repairing or replacing damaged screens and windowsills
  • Using sticky barriers or tapes along ant trails to prevent ant movement and forage

Cultural Control

  • Maintaining a clean and well-manicured garden, including removing piles of leaves, wood, and debris, and regularly raking and removing clippings and mulch
  • Keeping trees and shrubs trimmed and well-pruned, and avoiding unnecessary damage to the bark and branches, which can create entry points for ants and other pests
  • Repairing leaks and other sources of moisture, such as clogged gutters and downspouts, to reduce the availability of water that can attract ants and other pests
  • Storing firewood, lumber, and other materials away from the garden and structures, and using elevated racks or pallets to keep these materials off the ground and dry

Biological Control

  • Using parasitic nematodes, such as Steinernema carpocapsae, to attack and kill the ant larvae and pupae in the nest
  • Encouraging the presence of ant-predators, such as birds, reptiles, and other insects, in the garden
  • Planting ant-repelling plants, such as mint, tansy, or pennyroyal, in the garden or near the structure