Pest of the Month: Tropical Nut Borer

The threat of Tropical Nut Borer in Hawai‘i creates a significant threshold on macadamia nut production and proceeds.

By Damon Adamson.

Tropical Nut Borer and damaged nut
Tropical Nut Borer and damaged nut

The Tropical Nut Borer (TNB), Hypothenemus obscurus (Coleoptera: Scolytidae), was first identified in Hawai‘i—North Kona specifically—in 1988, with consequential studies conducted in the early 1990s. Initial studies determined TNB only affected Western Hawai‘i Island and with no other reports from Maui, O‘ahu, or Kaua‘i.

But within just a few years, TNB was found on all islands throughout the state, creating a significant threshold on macadamia nut production and proceeds. University of Hawai‘i studies conducted in the 1990s suggest a larger impact on orchards within dryer climates versus consistently wetter areas.


TNB’s ability to fly accompanied with their small size (adult is approx. 1mm) allows mild winds to assist in disbursement across larger areas. However, the rapid spread of TNB was probably aided by the purchasing of unshelled nuts from infested areas.

In addition, the benefits of using husks as fertilizer resulted in some growers buying husks from processors or husking plants and inadvertently spreading infested husks in their orchards.

From the distribution and damage experienced in the drier infested areas, it is clear that TNB will continue to be a serious pest of macadamia. The damage from TNB is considered substantial, while its life cycle adapts well to established macadamia production practices.

Life cycle

The life cycle of TNB begins as deposited oblong eggs within the macadamia nut fruit husk or kernel measuring 0.03 inches long by 0.01 inches wide. They have an opaque coloration and have within a week of being laid. Two larval stages follow hatching, where they grow and eat through the surrounding area which they were laid, husk or kernel.

Three to four weeks later, the beetles enter their pupal stage where they rest and grow to 0.04 to 0.07 inches, depending on sex-females tend to be larger. This stage lasts approximately one week.

After its transformation, the adult beetle, brownish in color, is capable of laying eggs. Studies have found that eggs average about one to eight, male to female respectfully, which can easily decimate an orchard in a short period.


No single method of TNB control has been determined work in all situations; therefore, an integrated pest management (IPM) program requires the use of several methods and their usefulness depends on farm size and the economics of the operation. For these methods to be most effective, they need to be undertaken while considering several key factors:

  1. TNB reproduces primarily within nuts that have been on the ground for more than two to three weeks.
  2. TNB can fly or drift more than 200 yards away from an infested area into a new area within three to four weeks.
  3. An infected nut husk can shelter in excess of 100 beetles.

Remember, TNB reproduction and damage continues while nuts are within the harvest bags. Damaged nuts should not be left in the orchard and should be removed and destroyed by burning, composting, burying, or transporting them to the dump.

Finally, do not buy raw husk for composting around your home or orchard.


Insect and Mite Pests of Macadamia Nuts in Hawai‘i—A Quick Reference Guide, College of Tropical Agriculture and Human Resources, University of Hawai‘i at Mānoa.

Macadamia Integrated Pest Management: IPM of Insects and Mites Attacking Macadamia Nuts in Hawaii, Vincent P. Jones, College of Tropical Agriculture and Human Resources, UH Mānoa.

Damon Adamson (senior, horticulture) received a bachelor of arts in history from UH West O‘ahu. He is retired U.S. Army, born in San Diego, CA.