Insecticide Resistance in Fleas?

When a flea treatment fails, is it simply a case of needing to re-treat or could insecticide resistance be the underlying cause?  

A wide range of insecticides are used for flea control, whether for knockdown and residual performance on a surface, or for on-pet flea control. Pyrethroids have been used for flea control for decades and with the on-pet, spot flea treatments that use a range of non-pyrethroid insecticides also having been on the market for nearly 30 years now, there are regular reports of treatment failures, with the possibility of insecticide resistance being raised. But is there really insecticide resistance in flea populations? If so, which are the insecticides of concern, and what does it mean for flea treatments?

For the treatment of houses and outdoor areas for fleas (and their larvae), pyrethroids have been the insecticide of choice for decades. Not only are they the primary choice for pest managers, but pyrethroids are often a key component of the total release aerosols or ‘flea bombs’ that are readily available to homeowners to carry out their own flea treatment. Pyrethroids have been and continue to be used for flea control, so some level of resistance should probably be expected.

It’s therefore not surprising that there are occasional end user reports of products “not working” and indeed there is research data to demonstrate pyrethroid resistance in flea populations1 – resistance ratios of up to ten with some pyrethroids. Resistance ratio (RR50) is a measure of how much more insecticide is required to kill 50% of the insect population in resistant populations compared to a susceptible population.

But what about resistance to insecticides such as fipronil and imidacloprid, which are commonly used in the on-pet flea products?

Despite the regular reports of these on-pet products “not working”, since the arrival of on-pet flea treatments in 1994 there has been little evidence for resistance to fipronil, imidacloprid or lufenuron, common insecticides used in this type of veterinary flea treatment product.2 Indeed, an international team of scientists and veterinarians monitored cat flea susceptibility to imidacloprid between 2002 and 2017. Using field-collected samples from ten different locations around the world, the researchers found no change in the susceptibility of cat fleas to imidacloprid.3 That said, it’s important to note there have been no published studies on insecticide resistance in fleas for a number of years.


treating a dog for fleas
Despite reports of on-pet spot treatments ‘not working’, there is little evidence of resistance to non-pyrethroids


What does this mean for pest control treatments of houses and yards for fleas?

Pyrethroids still remain a go-to adulticide option for flea control. However, there is clearly a risk of encountering a pyrethroid-resistant population and of course, no-one likes a callback. The solution is to add an IGR (insect growth regulator) such as Sumilarv (pyriproxyfen). Pyriproxyfen acts on all life stages of the flea, and with its different mode of action acts as a good insurance policy in cases of pyrethroid resistance.

However, Sumilarv actually offers a significant performance boost to all flea treatments, including those that use non-pyrethroids as the adulticide. Trials on flea larvae using imidacloprid demonstrated that pyriproxyfen had a significant synergistic effect, delivering a significant performance boost.4

So, insecticide resistance does occur in fleas, but to date this has only been confirmed for pyrethroids (and some older chemistries). If using pyrethroids in flea treatments, an IGR such as Sumilarv is essential. The inclusion of Sumilarv in flea treatments using nonrepellents will also deliver performance benefits and indeed help in preventing potential insecticide resistance to these chemistries in the future.


1 Rust, M.K et al. (2015). Susceptibility of Adult Cat Fleas (Siphonaptera: Pulicidae) to Insecticides and Status of Insecticide Resistance Mutations at the Rdl and Knockdown Resistance Loci. Parasitology Research, 144: 7-15

2 Rust, M.K (2017). The Biology and Ecology of Cat Fleas and Advancements in Their Pest Management: A Review. Insects, 8 (118).

3 Rust, M.K et al. (2018). International Program to Monitor Cat Flea Populations for Susceptibility to Imidacloprid. Journal of Medical Entomology, 55 (5): 1245-1253

4 Rust M.K., Hemsarth W.L.H. (2019). Synergism of adulticides and insect growth regulators against larval cat fleas (Siphonaptera: Pulicidae) J. Med. Entomol. 2019;56:790–795. doi: 10.1093/jme/tjy239.