Volatile Essential Oils Can Be Used to Improve the Efficacy of Heat Treatments Targeting the Western Drywood Termite: Evidence from Simulated Whole House Heat Treatment Trials

Colonies of western drywood termites, Incisitermes minor (Hagen) (Blattodea: Kalotermitidae), are difficult to detect and treat due to their cryptic nature. The use of heated air to create lethal temperatures within infested wood serves as a nonchemical treatment option targeting whole structure or large portions of the structure. However, the presence of hard-to-heat areas and potential risk of damage for heat-sensitive items are recognized as important challenges. Here, we tested if a localized injection of volatile essential oil could be utilized to address the heat sink issue, potentially increasing the overall efficiency of heat treatments against drywood termites. Artificially infested wooden blocks were placed in several locations of the test building, and heat treatments were conducted. For the treatment group, a small amount of essential oil (methyl salicylate) was added in the blocks prior to the heat treatment. All blocks placed in uninsulated wall voids had 92–100% termite mortality by day 7. However, the presence of a large concrete wall in the subarea hindered heating of blocks therein, resulting 36–44% mortality by day 7 when there was no essential oil treatment. Incorporation of the essential oil substantially increased the control efficacy for the subarea, resulting in more than 90% mortality. This approach might also be helpful in reducing the risk of potential heat damage during heat treatment without compromising its control efficacy.

Volatile Essential Oils Can Be Used to Improve the Efficacy of Heat Treatments Targeting the Western Drywood Termite: Evidence from a Laboratory Study

Use of heated air to create lethal temperatures within infested wood serves as a nonchemical treatment option against western drywood termites, Incisitermes minor (Hagen). When treating a whole or large portion of the structure, however, the presence of hard-to-heat areas (structural heat sinks) and potential risk of damaging heat-sensitive items are recognized as important challenges. To address these challenges, we tested if the incorporation of a volatile essential oil would increase the overall efficacy of heat treatments against the drywood termites. To choose an essential oil for use, we tested the volatile action of several candidate compounds against individual termites using a fumigant toxicity assay. As a proof-of-concept experiment, field-collected termites were housed in small wooden arenas and subsequently subjected to 2-h heat treatment at various air temperatures within a gas chromatography oven. A simulated heat sink and essential oil treatment was also included in the experimental design. Analyses of lethal temperatures (LTemp50 and LTemp99 values), probabilities of mortality, and survivorship data over time suggested that 1) the presence of a heat sink significantly increased the minimum air temperature needed for complete kill of the termites and 2) the volatile essential oil added at the site of a heat sink effectively counteracted the impact of the heat sink. The use of volatile essential oils makes it possible to effectively kill drywood termites even in areas which might not reach lethal temperatures (~50°C), potentially improving the overall efficacy of heat treatments while reducing the risk of heat damage.

Quantitative observation of the foraging tunnels in Sitka spruce and Japanese cypress caused by the drywood termite Incisitermes minor (Hagen) by 2D and 3D X-ray computer tomography (CT)

The cryptic lifestyle of drywood termites makes it difficult to study their foraging behaviour and to detect infestations in hidden regions. In the present research, computer tomography (CT) scanning images were collected over a 1-year observation period of drywood termite Incisitermes minor (Hagen) in order to understand its foraging behaviour and, perhaps, to be able to detect the infestation as early as possible. Sitka spruce and Japanese cypress woods were the test objects. As CT scanning results show, foragers caused distinguished tunnelling patterns based on the physical properties of wood during gallery construction. The foragers primarily excavate along the earlywood in radial direction towards the outer surface of the wood to establish the primary and satellite chambers, followed by mainly superficial (0.3–3.0 cm under the surface) longitudinal expansion with occasional lateral deviations. The development of tunnelling is affected by the physical constraints of knots with their high density, high extractive and lignin contents, reaction wood (compression wood) and wood damage caused by wind.

Aspergillus sclerotiorum fungus is lethal to both Western drywood (Incisitermes minor) and Western subterranean (Reticulitermes hesperus)termites

Termite control costs $1.5 billion per year in the United States alone, and methods for termite control usually consist of chemical pesticides. However, these methods have their drawbacks, which include the development of resistance, environmental pollution, and toxicity to other organisms. Biological termite control, which employs the use of living organisms to combat pests, offers an alternative to chemical pesticides. This study highlights the discovery of a fungus, termed “APU strain,” that was hypothesized to be pathogenic to termites. Phylogenetic and morphological analysis showed that the fungus is a strain of Aspergillus sclerotiorum, andexperiments showed that both western drywood (Incisitermes minor) and western subterranean (Reticulitermes hesperus) termites die in a dose-dependent manner exposed to fungal spores of A. sclerotiorum APU strain. In addition, exposure to the A. sclerotiorum Huber strain elicited death in a similar manner as the APU strain. The mechanism by which the fungus caused termite death is still unknown and warrants further investigation. While these results support that A. sclerotiorum is a termite pathogen, further studies are needed to determine whether the fungal species has potential as a biological control agent.