Flooding and rainfall can have a big impact on the longevity of soil applied termiticides. The latest research highlights some of the factors to consider in the choice of chemical.

The impact of rainfall on soil-applied termiticides is a key consideration in choosing an appropriate product, or indeed deciding whether a soil-applied termiticide is a suitable treatment at all. What level of rainfall does the area typically receive? What is the soil type being treated? Is it a sloping block? Is the area close to a river? All of these questions come into sharper focus when a major rain event hits or during periods of flooding. What happens to the treatment and is a re-treatment necessary?

Researchers from Louisiana State University Agricultural Center in the US set out to investigate the impact of flooding on termiticide residues. Their study focused on four formulated termiticides based on fipronil, imidacloprid, chlorantraniliprole or bifenthrin as the active ingredient. Dosing both sand and soil substrates at 1, 10 and 25 ppm, the treated soils were allowed to fully dry for two days before being exposed to a flooding methodology for one week. Untreated sand and soil samples were exposed to the same regime. The substrate samples were then analysed for termiticide residues and also assessed for efficacy by introducing Coptotermes formosanus workers and soldiers to the substrates.

The behaviour of termiticides in the soil is largely dependent on their intrinsic solubility in water, their ability to bind to the soil (KoC) and their stability (rate of hydrolysis).

The results demonstrated that the residues of all the termiticides were reduced on flooding and that the reduction in termiticide residues was greater in sand than in soil for all the termiticides tested. This observation is consistent with previous studies that have indicated that the level of organic material in the soil has a significant influence on the rate of leaching – termiticides showed lower levels of leaching in soils with higher levels of organic content.

Liquid termiticide treatments will be compromised through flooding, so be sure to discuss potential actions with your supplier and insurer

In ranking the termiticides, the authors concluded that imidacloprid was the most leachable insecticide, bifenthrin was the least leachable insecticide, and fipronil and chlorantraniliprole were somewhere in between. (Due to the lack of replications in the chemical analysis the authors were not comfortable in drawing more precise ranking conclusions.)

It is of course important to understand the efficacy of any treatment after a flooding event. In soil treatments in the study, although there was clearly a drop in the level of actives, there was no difference in the level of mortality between flooded and un-flooded samples at all application rates for all actives, except the 1 ppm bifenthrin treatment where there was a significant drop in efficacy.

However, with the sand treatments, a number of significant drops in efficacy were recorded. Fipronil originally dosed at 1 ppm was no longer efficacious after flooding and imidacloprid showed significant reductions in efficacy at all doses. Interestingly, chlorantraniliprole showed significant increases in mortality at the 10 ppm and 25 ppm levels. The authors hypothesised that this increase may be due to increased bioavailability after flooding, noting that the mortality was lower in soil than in sand – suggesting that the organic content in soil impacts chlorantraniliprole efficacy. Bifenthrin showed no drop in efficacy in any of the treatment levels.

This particular study did not assess the impact of hydrolysis, which can also occur as a result of flooding. Fipronil, imidacloprid and chlorantraniliprole hydrolysis is high under basic conditions but they tend to be stable under acidic and neutral soil conditions.

The results of this study suggest that the more water soluble termiticides are unlikely to provide protection after a flooding event, and that even the least soluble termiticides will have a reduced duration of protection.
The study provides a good understanding of what happens to the various chemicals under flood conditions, but the reality of making a decision of whether and when to re-treat after a flooding event is a little more complex. There is no way of knowing the actual level of active remaining in the soil without taking samples for chemical analysis. With the need to take numerous samples from around the site, the cost to do this is likely to be prohibitive. The other key issue with flooding that this trial does not address, is the reality that additional soil and silt is brought in with the flood water, thus creating an untreated zone on top of the treated zone.

In deciding what to do, it is also important for pest managers to consider the insurance situation, both for themselves and their customers. Home and contents insurance policies would generally accept a claim if the homeowner had flood cover selected and had a termite barrier in place. Insurance companies would consider the barrier as structural improvement – and it is in their best interest to ensure the home is protected.

Regarding any warranties that pest managers have offered their customers, it really is necessary to review each situation on its merit and get input from both their product supplier and insurer before recommending a course of action.

Further reading: Sapkota et al. (2020). Residual Effects of Termiticides on Mortality of Formosan Subterranean Termite (Isoptera: Rhinotermitidae) on Substrates Subjected to Flooding. Journal of Economic Entomology, 113 (1): 367-374.