Does orange oil work for drywood termite management?

Drywood termites are wood-eating pests that can damage buildings or other wooden structures through their constant feeding activity. They are less dependent on moisture than subterranean termites, which allows them to live above ground in studs, rafters, and window and door frames without any contact with soil. Managing them usually requires chemical treatment, including fumigation, localized treatments, or both.

For the purpose of this article, I will focus on the efficacy of orange oil, containing the chemical d-limonene, in comparison with several other commercial termiticide products. Orange oil gained popularity in the 2000s as a least-toxic and eco-friendly product for managing drywood termites and other wood-destroying insects in wood. While the use of the product gained a lot of momentum because it was considered a least-toxic product, some groups started to question its efficacy and wonder how d-limonene compares with synthetic products made for termite management. Comparing products head to head is not as easy as it sounds. To do so, one needs to look at many factors, such as short-term and long-term efficacy, speed of mortality, insecticide formulation, application methods, concentrations, etc.

Luckily, there is a very useful research report on this very topic by Dr. Mike Rust and Jody Venturina from the University of California, Riverside. In this report, called “Evaluation of Chemical Localized Treatment for Drywood Termite Control,” submitted to the California Structural Pest Control Board, the researchers looked into several different products and active ingredients commercially available for localized drywood termite management. When looking at their mortality data for drywood termites exposed continuously to orange oil (Table 1), it becomes obvious that a 1-day-old orange oil deposit was very lethal to termites, as it killed 100% of termites by day 14, with 33% mortality observed by day 7. However, 30- and 60-day-old deposits of orange oil failed to cause more than 50% mortality, even after 26 days. The 30-day-old deposit caused 50% mortality, while the 60-day-old deposit caused only 11% mortality by day 26. For comparison, the untreated termites, which were not exposed to any insecticide, had higher mortality by day 26 than termites exposed to the 30- and 60-day-old orange oil deposits. This suggests that orange oil lost much of its residual potency within a month. In contrast, a 60-day-old deposit of 0.06% fipronil, another well-known insecticide, caused 94% mortality in just one day and 100% mortality by day 7.

Table 1: Mortality (%) of western drywood termites (Incisitermes minor) when exposed to orange oil and other insecticides of known age.

Conclusions Fresh deposits of orange oil are very effective at killing termites. However, orange oil is not an effective insecticide for long-term management of termites, simply because it has limited residual or long-term activity. Since drywood termite management is costly, often requiring insecticide injections into the wood, an ideal insecticide should remain active inside the wood for prolonged periods to prevent costly re-applications. Localized re-application of insecticides becomes necessary in two different scenarios:

First, when a localized treatment fails to kill all the termites in the structure, and the remaining termites manage to rebuild their colony. This is especially likely if the queen and other reproductives (called secondary reproductives) survive the termite control treatment. This can happen due to mistakes in mixing and applying insecticides. Other times, the failure of localized treatments is caused by limited treatment coverage as a result of a lack of physical access to wood members of a structure. This is very common in attics, where parts of the rafters and other wood members are not accessible for termiticide injections. Nevertheless, a residual insecticide application may partially solve this problem by remaining active inside the wood for extended periods so that if termites from inaccessible areas travel back to older or adjacent treated tunnels, they can still be exposed to the insecticide and die. Achieving this delayed kill is not possible with a short-lived or non-residual insecticide.

Second scenario involves when new termites enter a structure from adjacent properties. So, even when you successfully eliminate a whole colony of termites, new termites can always enter a structure and start new colonies. While the process of establishing and growing a new colony is extremely slow in drywood termites, every single wooden structure is prone to new termite invasions. As you can see, a residual insecticide can perform better than a non-residual insecticide in protecting structures in the long run. To conclude, orange oil should be a top choice for situations where sustainability and health risks are a priority. However, if cost and long-term protection of wood are desired, then a fipronil or borate product should be used for managing drywood termites.

As you can see, a residual insecticide can perform better than a non-residual insecticide in protecting structures in the long run. To conclude, orange oil should be a top choice for situations where sustainability and health risks are a priority. However, if cost and long-term protection of wood are desired, then a fipronil or borate product should be used for managing drywood termites.