The second of a three-part series looking at the science behind non-repellent chemistry.
In the previous part of this series, we looked at the various factors that affect non-repellence including chemical composition, dose rate and formulation. Here, we examine the evidence for another quality frequently marketed by product manufacturers: horizontal transfer.
The potential horizontal transfer of non-repellent termiticides has become an important paradigm in the rationale for their use. A study performed at the Department of Entomology at the University of California used C-radiolabelled fipronil to demonstrate the extent and ability of termites to transfer fipronil between nestmates. It showed that at both brief and continuous levels of exposure, fipronil-treated sand seriously impaired the termites’ ability to move and respond to a dodecatrienol trail, thus limiting the potential for horizontal transfer.
Fipronil uptake ceased once the termites were immobilised and the maximum transfer of fipronil from donor termites to recipient termites occurred within the first 24 hours. Interestingly, fipronil was only transferred by body contact; trophallaxis (feeding) did not play a major role in the horizontal transfer.
In successive transfer studies, there was not enough fipronil on recipients for them to serve as secondary donors and kill other termites, meaning multiple transfer of fipronil, as required to induce colony mortality, was limited. In a linear arena study, there was an inverse relationship between the amount of fipronil on dead termites and their distance from the treated zone with maximum mortality observed within 1.5 metres of the treated zone (Saran & Rust) [1].
This study indicates that the transfer effect of fipronil only occurs in close proximity to the treated zone, and that the rapid mortality of the termites limits horizontal transfer. Fipronil is highly active and non-repellent to termites, which might increase the likelihood of transfer from even dead termites. However, the effects of the type of substrate, concentration of the insecticide, the exposure time of the donors, and the time expired after exposure, more significantly affect the transfer process.
To permit transfer of lethal doses to recipients, the toxicant must exhibit delayed toxicity, meaning non-repellence is not the most critical factor in horizontal transfer processes. There is in fact a very narrow range of doses over which transfer might occur in field situations, with transfer most likely to occur at lower concentrations. The most critical factor in horizontal transfer is delayed toxicity, with improved transfer evident if the active kills the insects slowly. This is why indoxacarb is such a powerful active when used in insect baits. The parent indoxacarb is metabolised into N-decarbomethoxyllated indoxacarb which in effect activates the product. This rate of bioactivation is a critical factor in determining the speed and ultimate toxicity of indoxacarb.
The Saran and Rust study concluded that horizontal transfer is probably not the major contributing factor to the efficacy of non-repellent termiticides with delayed toxicity in controlling termites in field situations. This indicates issues with the concept of colony elimination.
An earlier study by Professor Nan-Yao Su [2] of the University of Florida found large numbers of dead or decomposed corpses in areas near the fipronil treatment zone, and suggested that these probably acted as a repellent to healthy termites. The avoidance of dead and decomposed corpses (necrophobic behaviour) is well documented in termites.
The effect of non-repellent termiticides when acquired by termites at higher doses is similar to that of fast-acting termiticides such as chlorpyrifos, and their barrier performance may be more related to the effects of the above reported secondary repellence of the non-repellents.
It may be possible, as shown with both imidacloprid and fipronil against Reticulitermes spp. termites in the USA, that soil treatments with non-repellent termiticides may eliminate a colony whose entire foraging range is within 5 metres or less of the treated zone, provided the product is applied uniformly. Reticulitermes termites in the USA however are very different to the subterranean termites encountered in Australia. Typically, they live in small colonies of 50,000 or less and the colonies are very mobile. Moreover, the definition of elimination is often based on the absence of the colony rather than formal confirmation by digging the colony up to confirm all the termites were eliminated, as has occurred with published studies for termite baiting systems in Australia.
To date, there are no published studies showing colony elimination of Coptotermes spp. or of the higher termites, Termitidae, from soil-applied treatments with non-repellent termiticides. In contrast to the US work with Reticulitermes spp., a detailed study with Coptotermes formosanus to evaluate the effects of soil treated with imidacloprid showed no latent mortality attributed to imidacloprid. It did not measurably reduce C. formosanus populations adjacent to the treatment (Osbrink et al) [3].
Non-repellent chemistry delivers many performance benefits, but do they really deliver the termite colony elimination through horizontal transfer that is often implied? The peer-reviewed, published data suggests not.
More information on termite treatments.
[1] HK Saran and MK Rust. Toxicity, Uptake, and Transfer Efficiency of Fipronil in Western Subterranean Termite (Isoptera: Rhinotermitidae). J. Econ. Entomol. 100(:2): 495-505. (2007)
[2] NY Su. Response of the Formosan Subterranean Termites (Isoptera: Rhinotermitidae) to Baits or Nonrepellent Termiticides in Extended Foraging Arenas. J. Econ. Entomol. 98(6): 2143-2152. (2005)
[3] WLA Osbrink, ML Cornelius and AR Lax. Effect of Imidacloprid Soil Treatments on Occurrence of Formosan Subterranean Termites (Isoptera: Rhinotermitidae) in Independent Monitors. J. Econ. Entomol. 98(6): 2160-2168 (2005)
Steve Broadbent, Regional Director, Ensystex
