The third of a three-part series looking at the science behind non-repellent chemistry.
Last issue, we looked at the evidence behind the horizontal transfer of non-repellent termiticides. In this article, we will explore some common myths about insecticide repellency.
When the first non-repellent termiticides – imidacloprid and fipronil – came to the market, they revolutionised termite treatments. As a consequence, pyrethroid termiticides were promoted as repellent chemistry. This concept was also linked to the fact that when alphacyano pyrethroids are sprayed on cockroaches, they flee from their harbourage; leaving many to consider they repel all pests. This is not true.
Independent, peer-reviewed studies have repeatedly shown many insect species, including ants and bed bugs, don’t avoid surfaces treated with dried deposits of pyrethroid insecticides. This lack of repellency is observed even more when non-alpha cyano pyrethroids such as bifenthrin or permethrin are applied.
A study by Dr Phil Koehler from the University of Florida, showed that Argentine ants (Linepithema humile) do not avoid pyrethroid treated surfaces when foraging for food. Dr Koehler set up a bridge for ants to cross, with half the bridge treated with a pyrethroid. The ants showed no preference for avoiding the treated side and, as a consequence, died. However, a foraging ant would sometimes lay a trail on the other side, and other ants would follow it. This could have been interpreted as ants being repelled, but in reality, they simply missed the applied pyrethroid, and a trail was established on the untreated side.
The speed of action of a product can also lead people to assume it’s a repellent. Pyrethroids are fast acting and can affect ants before they lay a trail. Each time an ant forages from the colony, if it is not following an established trail; it either dies from contact with the pyrethroid, or by chance, finds an untreated path. And because it didn’t die, it creates a trail that others can follow. This explains why the ants sometimes appear to follow a trail and avoid the treatment.
Insecticides such as fipronil or chlorfenapyr are slow acting, so ants can walk over the treatment and establish a trail before they die. The difference here is not repellence, but speed of action. Dr Koehler concludes, “In the end, there probably is no such thing as a repellent insecticide to ants. What you see after treatment is deceiving.” Ants appear to avoid a treatment because the speed of action is too fast, or because they establish trails by accident on the non-treated surfaces.
Similarly, in trials against bed bugs, Dr Dini Miller, from Virginia Tech, showed there was no preference between untreated surfaces and surfaces treated with pyrethroids, concluding that, “Pyrethroids were not repellent to bed bugs and would not cause bed bug aggregations to scatter or avoid treated surfaces.” This also applied to bed bug strains known to be resistant to pyrethroids.
Even with cockroaches, the spastic movements and crazed running seen at time of application is a consequence of the rapid penetration of the pyrethroid into the cockroach nervous system. This behaviour is generally not displayed on aged deposits.
What about termites?
When we consider termites, repellent chemistry works differently. The termites are tunnelling through the soil, putting particles in their mouth and moving them out of the way. We are now looking at a very different concept in terms of exposure to the insecticide. The termite has to determine whether to place the treated soil in its mouth or move elsewhere.
Usually they will back-up and move laterally to avoid the treated area. In the end, the termite tunnel doesn’t penetrate the treated soil, and the termites don’t die. If the treatment is thorough, they will not find any gaps in the treatment and will avoid the area, leaving the building protected. Hence the importance of creating a complete and continuous chemical barrier in the soil.
When we consider non-repellent termite chemistry, we can see how it has led to a different myth – that the termiticide can be relied upon to eliminate the termite colony. In a peer-reviewed study published in June in the Journal of Economic Entomology, Dr Thomas Chouvenc, from the University of Florida, has amplified the data from previous studies.
Working with complete colonies of Coptotermes gestroi (a species native to SE Asia and now an invasive pest of Florida and the east coast of South America), the effects of fipronil were observed over a 12-metre foraging pattern. The treatment was intended to represent a typical soil applied remedial treatment around a building.
Within two weeks, all termites within a 1.5 metre radius of the fipronil treatment zone had died. The accumulation of large numbers of termite cadavers in proximity to the fipronil zone led to the appearance of secondary repellency, with termite workers avoiding the treated area as a consequence. So here we see a secondary effect, where the fipronil is not repellent, but the consequences of the presence of the fipronil led to avoidance of the area, as observed with repellent chemistry.
At the end of three months, termite colonies exposed to the fipronil treatments showed no variance in size to those of the untreated control colonies. This confirmed that the large numbers of dead workers and soldiers that arose as a result of the exposure to the fipronil, were quickly replaced by the colony. The treatment zone was then avoided, with the colonies seeking new foraging pathways. Importantly, this means the termites were only locally excluded in proximity to the treatment zone, while the colony retained its potential risk for structural damage. This is in contrast to termite toxic bait treatments, where some systems have been consistently proven to eliminate the entire colony.
As I’ve said before, US studies performed with their native Reticulitermes and Heterotermes spp, are of little relevance to the termite fauna of the Australasian region. These species generally reside in small colonies with limited foraging areas. Recent studies from the USA are now showing the challenges presented by Coptotermes spp.
It’s interesting to note that since studies commenced against Coptotermes spp, numerous papers have shown an exclusion zone of termites in close proximity to the treatment zone, yet none have demonstrated transfer and mortality from non-repellent liquid termiticides on termites in the soil beyond six metres from treatment zones.
The consequence of this is that subterranean termites may still be able to bypass a treatment or find a point of entry that was untreated. This emphasises the importance of creating a complete and continuous chemical barrier, whether using non-repellent or repellent termiticide chemistry.
The extended foraging abilities of Coptotermes species makes them particularly challenging. Recent studies suggest their foraging territory may extend well beyond previous estimates and reach up to 100 metres.
With confirmation that liquid termiticides are likely to impact a subterranean colony for only a limited distance from the treated zone, it raises questions on the impact of such treatments on the population of termites in an area, suggesting termite challenges will persist even after treatment.
Steve Broadbent, Regional Director, Ensystex Australasia