Part 4 in our Active Insight series looking at how to get the best results with cockroach baiting.
Currently there are no reports of control failure of bait products against Blatella germanica due to physiological insecticide resistance.
Evidence of genetically inherited bait aversion behaviour has been well-studied and reported on. Consequently, the choice of active is less important than the nutritional profile of the bait and how the bait is placed. Maximising attraction and first-time consumption of a bait will prevent a sub-lethal dose and delay (or avoid) resistance.
Resistance to insecticides is a major concern in the management of B. germanica. Currently though there are no reports of control failure of bait products against B. germanica due to physiological insecticide resistance, with respect to the newer actives such as abamectin, fipronil, or indoxacarb. This is most likely due to the fact that they are present in a highly palatable food mixture, that is consumed by the cockroaches.
When cockroaches feed on a quality cockroach bait, the active constituent is usually ingested at a significantly higher dose than that required to kill the cockroach. For example, in one study, German cockroaches consumed 150-1300 times the lethal dose of fipronil and 218-441 times the lethal dose of indoxacarb.1
In practice, it is thought that this will likely mean that, due to the consumption of large amounts of the active ingredient, the bait will eventually kill all of the cockroaches and development of resistance may not occur, or the onset may be significantly delayed in cockroach populations exposed to baiting programs.2 The risk remains though, if cockroaches are subject to sub-lethal exposure to a bait, such as might occur if aggression behaviour led to cockroaches abandoning a bait point. This is one of many reasons, as discussed previously, why it is important to make lots of small bait placements when baiting for cockroaches.
Whilst it may be possible to enhance resistance to baits in the field, for example through poor placement or use of lower palatability baits, whether it would lead to sufficient pressure to result in control failure has yet to be evidenced. Although there is no evidence of physiological resistance in the field to modern insecticidal cockroach baits, the rotation of bait actives with different modes of action may still be considered to further limit the risk of this occurring.
In contrast to the lack of physiological resistance in field populations, bait aversion resistance in German cockroaches has been well documented. This was primarily associated with the presence of glucose in baits. Feeding stimulants such as glucose were added to bait formulations to promote attraction and consumption. Glucose is a low-cost sugar and many lesser quality or cheap baits still use glucose. Glucose bait aversion was first reported in cockroaches in 1993 in Florida.3 It has subsequently been well documented in many other parts of the USA and around the world and, since glucose aversion is an inherited genetic trait, it is almost certainly present in Australia.
What this means is that glucose averse cockroaches are present naturally in most cockroach aggregates. This is a genetic factor. The cockroaches that carry the gene that makes them glucose averse are less viable than other cockroaches though. They have smaller and fewer numbers of eggs in their oothecae and they have lower population growth rates than non-averse cockroaches. So normally they are only present in very small numbers in a cockroach population.
It is only when toxic baits containing glucose are introduced into the cockroach environment that they gain an advantage. This is not behavioural resistance; the trait is in fact controlled by a single major gene affecting a single site within a taste receptor. The mutation is present in a sensory receptor that normally binds deterrent molecules, typically bitter agents. This has led to the terminology of Bitter Gustatory Neurone Resistance, which better describes the process than bait aversion behaviour, since the latter term suggest behavioural resistance, rather than genetic resistance.
What happens with the resistant individuals is that the glucose binds to the deterrent receptor site. Consequently, the cockroach considers that glucose, which in a normal cockroach is a good food type, is a bitter agent that should not be consumed.
In a given field population, subjected to regular baiting with baits containing glucose, what happens is that the normal cockroaches consume the bait and die out. Those with the Bitter Gustatory Neurone Resistance gene avoid the bait, as they see it as being unpalatable. They remain behind to breed, and the density of the resistant gene increases in the population, leading to control failure.4
Paracelsus wrote in the fourteenth century, “The dose makes the poison”, and this remains true today. Control of a pest population is a factor of time and dose, so with the high dose delivered through cockroach baits, resistance is not a primary concern; the method of application and choice of feeding substrates to induce feeding are more important. This means that the choice of active is less important than the nutritional profile of the bait, and how the bait is placed in the cockroach environment. Maximising attraction and first-time consumption of a bait will prevent a sub-lethal dose and delay (or avoid) resistance.5
Hopefully this series of four articles has shed light on the importance of correct bait placement, and provided knowledge for technicians to enhance their bait placement strategies to achieve optimal results. By selecting a high performance bait based on its nutritional profile, rather than the price, and spending the time to place baits correctly, will further the goal of achieving excellence in cockroach baiting.
Steve Broadbent, Ensystex, Regional Director
1 Bayer, B.E., Pereira, R.M and Koehler, P.G. (2012) Differential consumption of baits by pest blattid and blattellid cockroaches and resulting direct and secondary effects. Entomologia Experimentalis et Applicata 145, 250-259.
2 Gondhalekar, A.D. and Scharf, M.E. (2012) Mechanism underlying fipronil resistance in a multiresistant field strain of the German cockroach (Blattodea: Blattellidae). Journal of Medical Entomology 49, 122-1 31.
3 Silverman, J. and Bieman, D.N. (1993) Glucose aversion in the German cockroach, Blattella germanica. Journal of Insect Physiology 39, 925-933.
4 Silverman, J. (2005) The genetic basis of German cockroach bait aversion. In: Lee, C.Y. and Robinson, W.H. (eds) Proceedings of the Fifth International Conference on Urban Pests, Singapore, 10-13 July 2005. P & Y Design Network, Penang, Malaysia, pp. 425-426. Accessed from the Conference CD.
5 Kells, S. (2005) Bait aversion by German cockroaches (dictyoptera: blattellidae): The influence and Interference of nutrition. In: Lee, C.Y. and Robinson, W.H. (eds) Proceedings of the Fifth International Conference on Urban Pests, Singapore, 10-13 July 2005. P & Y Design Network, Penang, Malaysia, pp. 419-422. Accessed from the Conference CD.