Soil treatments are a proven way to protect buildings from termite attack. However, there is debate over whether they eliminate termite colonies.
Non-repellent termiticides are undoubtedly good products for creating a treated zone around a building to protect the property from termite attack. It is also proven that these termiticides are capable of being transferred between termites during their normal foraging and grooming behaviours. However, the amount of secondary transfer from soil treatments and its impact on termite populations in the field, is a topic of debate. This is particularly the case when claims are made around colony elimination. Researchers from the University of Florida have taken a closer look at termiticide transfer in soil treatments and their impact on termite populations.
Many of the transfer studies on non-repellents have focused on fipronil. The time to mortality for an individual termite is dose dependent – the higher the dose the termite receives, the shorter the time it takes to die. The dose therefore impacts how far a termite can travel after being exposed to termiticide. A high dose allows for more transfer, but the termite dies quicker; a low dose will allow the termite to live longer, but deliver minimal transfer. When foraging sites can be up to 100 m from the colony, one of the key questions regarding termiticide transfer is, how far can the termiticide be transferred from the treatment area while still delivering a lethal effect? Several studies both in the laboratory and in the field have suggested that the impact of any transfer effect is limited to a distance of around the 5-metre mark.
Apart from understanding the limits to the transfer of fipronil, studies have also established the concept of secondary repellency. When applied as a soil treatment the resulting termite death in the treatment area causes surviving termites to seal off foraging tunnels within 3-5 m of the treatment. This latest laboratory study aimed to characterise this “death zone” and understand the impacts of this behaviour on the termite population and colony.
To create a laboratory set up, approximately a field situation, the researchers connected whole colonies of Coptotermes gestroi of approximately 50,000 termites to a wood structure (foraging site) by 3 x 15 m tunnels (tubes). Fipronil soil treatments where then applied by replacing a 50 cm section of tubing in each of the tunnel, with tubing containing treated sand. The treated inserts were placed at either 1.5 m, 7.5 m or 12.5 m from the nest. All three tunnels for each nest had the treatment placed at the same distance, with different nests having the treatments placed at one of the set distances. Observations were made daily for 60 days after treatment.
For the nests where the treated zone was 7.5 or 12.5 m from the nest, the death zone on the nest side of the treated zone stabilised on average around 2.56 m away from the treatment. When the treatment was only 1.5 m from the colony, the death zone on the nest site of the treatment was smaller, only 1.1 m on average from the treatment.
The majority of termite death occurred within the first seven days after contacting the treatment, until the death zone was sealed off. Colonies with a treatment 1.5 m away suffered mortality between 23.5-65.9%, colonies 7.5 m away from a treatment suffered mortality between 8.3-25.4% and colonies 12.5 m away from a treatment suffered mortality between 1.6-20.2%. Despite the loss of termites, all the colonies survived. Although wood consumption dropped after the initial exposure to the treatment and loss of termites, by 200 days after treatment the rate of wood consumption had increased as the population rebounded.
The researchers noted that in a number of the replicates, the foragers trapped in the structure on the other side of the treated zone survived and continued to consume wood for the duration of the experiment.
The researchers concluded that the study confirmed previous findings: that the transfer of fipronil only occurs over short distances and that although it kills a proportion of the termites, primarily foragers, it is unlikely to eliminate the colony even when the treated zone is close by (at least for Coptotermes gestroi). The mechanism by which the colony protects itself is through blocking off the treated zone, which happens within a week of exposure.
Whilst the treated zone proved an effective ‘barrier’ to protect the structure, the authors also pointed out two additional implications for best practice application in the field. As with repellent termiticides, non-repellent termiticides also have the potential to trap termite populations within the structure, unless the active termites are eliminated from the building before applying the treated zone. Secondly, for those pest managers who may use in-ground baits in conjunction with soil treatments, it is important not to place the bait stations near the treated zone, as the termites may not find bait stations within any “death zone” that develops.
Further reading: Chouvenc, T (2024). Death zone minimizes the impact of fipronil-treated soils on subterranean termite colonies by negating transfer effects. Journal of Economic Entomology, 15 July 2024, 1–14. https://doi.org/10.1093/jee/toae150
