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I am just wondering if it is really that the insects are becoming immune to the products or if it is a problem of owner compliance?[/b]
More likely a Urban Legend than anything based in fact. It is possible/probable that some feas have developed resistance to Frontline but frontline Plus contained another ingredient that prevents the feas from reproducing.

Flea Product Comparison
Frontline Plus includes a nontoxic flea sterilizer called methoprene, so any fleas resistant to Fipronil will not be able to pass on their resistance genes.

It is important to note that with the advent of popular topical treatments for fleas, special attention should be paid to the development of resistance to these products. Experience with other insects tells us that resistance can develop in 12-15 generations. In order to preserve these new insecticides, it is important to consider what is called “integrated pest management.” What this means is that insecticides should be rotated or combined with insect development inhibitors such as lufenuron or insect growth regulators like methoprene. Fleas that are resistant to the topical treatments must not be allowed to pass on their genes for resistance[/b]

Some studies of Fipronil
<a href="" target="_blank">Identification of the Rdl mutation in laboratory
and field strains of the cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae</a>
The A302S mutation was found
in eight of the nine strains analysed, although the relative frequency of the mutant allele varied between
strains. Only one strain (R6) was found to be homozygous for the S302 allele in all the individuals tested,
and this correlated with previous reports of low-level fipronil resistance in this strain.[/b]
Comparative Efficacy of a Combination of Fipronil/(S)-Methoprene, a Combination of Imidacloprid/Permethrin, and Imidacloprid Against Fleas and Ticks When Administered Topically to Dogs
Dogs treated with the fipronil/(S)-methoprene combination had significantly fewer fleas (p<0.05) than the dogs treated with the combination of imidacloprid/permethrin and significantly fewer fleas (p<0.05) than dogs treated with imidacloprid alone at each evaluation through Day 28. Dogs treated with imidacloprid had significantly fewer fleas (p<0.05) than dogs treated with imidacloprid/permethrin combination on Day 28 (Table 1, Figure 1).

Although dogs treated with fipronil/(S)-methoprene or imidacloprid/permethrin had significantly fewer ticks (p<0.05) than the controls at both Day 21 and Day 28, the dogs treated with fipronil/(S)-methoprene had significantly fewer ticks (p<0.05) than those treated with imidacloprid/permethrin at both of these time points (Table 2, Figure 2)[/b]
On The Rebound, Theories Abound Regarding The Resurgence Of Flea Calls
04 presentation at the Western Veterinary Conference, Dryden stated, “While insecticide resistance may cause organophosphate- and pyrethroid-based flea products to fail, no one knows the extent of insecticide resistance to fipronil, imidacloprid, lufenuron, methoprene, pyriproxyfen, or selamectin in cat flea populations. Even today, if a product containing fipronil, imidacloprid, lufenuron, methoprene, pyriproxyfen, or selamectin fails, some veterinarians cite resistance as the cause. But in my experience, true product failures are rare, and most problems stem from poor compliance, application or administration errors, and a lack of understanding of important biologic and epidemiologic parameters in the flea life cycle.” Dryden’s research has been recognized for its significance in modern veterinary dermatology.

...Hinkle cites a study by Dryden showing that raccoons, opossums and such feral animals in urban areas support more fleas than those in rural areas. The animals are attracted by the cities’ safe living conditions as well as accessibility of food, such as particles found in open dumpsters or pet bowls left outside, which results in more wildlife per area in the city than in rural areas. Thus, she says, “they have increased opportunity to interact with one another and share their fleas,’” and to interact – and share fleas – with house pets as well.

This interaction is, according to Dryden’s KSU biography, an area of cooperative research. “As a result of continual urbanization, displaced wildlife often finds refuge and seeks food in suburban areas. Interactions with some of the most common species of urban wildlife, such as raccoons, skunks or opossums may pose serious health risks to humans and their pets.”[/b]

and the latest about resistance to Advantix

Determining a diagnostic dose for imidacloprid susceptibility testing of field-collected isolates of cat fleas 2005
None of the larvae from the susceptible laboratory strains survived the DD. Eighteen field-collected isolates were evaluated for their susceptibility to imidacloprid and to validate a DD of 3 ppm. Probit lines from 18 field-collected isolates were very similar, with LC50 values ranging from 0.14 to 1.52 ppm. When exposed to the DD, between 3 and 10% of the exposed larvae emerged as adults from only three of the 18 isolates. All other field isolates gave 100% mortality at the DD. Under the criteria established (>5% survivorship at 3 ppm), two isolates would be established on mammalian hosts and more extensive tests conducted to exclude or confirm the presence of resistance. The DD of 3 ppm is robust enough to eliminate most of the susceptible isolates collected until today, yet low enough to identify possible isolates for further testing.[/b]
Pest Management Symposium 2007
An international effort to monitor for the susceptibility of cat fleas, Ctenocephalides felis (Bouché), for imidaclorpid is
currently ongoing with the support of Bayer Animal Health. The initial phase of the study conducted on laboratory strains and 18 field isolates of cat fleas was to develop a Diagnostic Dose (3 ppm imidacloprid) that was used in a larval
bioassay (Rust et. 2005). To date over 1,079 field isolates have been collected and 863 have been assayed for imidacloprid susceptibility. To date, none of the isolates have exhibited decreased sensitivity to imidacloprid.[/b]
Efficacy of a four registered spot-on products against fleas and ticks on dogs

Efficacies of a four registered spot-on products, Sergeants Silver Flea and Tick Squeezeon
for dogs (40% cyphenothrin), Meriel's Frontline for Dogs (9.8% fipronyl), Bayer’s Advantage
for Dogs (9.1% imidacloprid) and Advantix for Dogs (8.8% imidacloprid and 44% permethrin)
were evaluated on dogs against fleas and ticks (Rhipicephalus sanguineus and Dermacentor
variabilis). Flea and ticks counts were performed at 1, 4, 24 and 48 hours after treatment and
after each weekly re-infestation over the following 8 weeks. Based on speed of action/repellency
at 1 and 4 hours, attainment of 100% immediate efficacy, duration of efficacy values of 90% or
better and statistical analyses of differences in flea and tick burdens, comparing the four
treatments with the untreated controls and between the treated groups, the cyphenothrin product
performed best with data that would support a label claim exceeding 6 weeks residual efficacy at
values of 98 to 100%.[/b]
Efficacy of the combination fipronil/(S)-methoprene (FRONTLINE® Plus)
against flea eggs and larvae (Ctenocephalides felis) following weekly
infestations of treated cats with gravid female fleas.

...The same methodology was repeated weekly during 10 weeks. 24 hours
after the transfer on groups 1 and 2, the adult fleas were counted to assess the insecticide activity.
The efficacy ranged between 92.9 and 100% during 6 weeks following the treatment of the cats
(arithmetic means). At the same time, the flea eggs were collected from the cats of groups 1 and
2 and placed in a media for breeding. The number of newly emerged fleas were counted 30 days
later. It was then possible by comparison between groups 1 and 2 to calculate the efficacy of the
control of the evolution of the eggs&larvae. This effectiveness was 100% from D1 to Day56
(Week 8) and still 97.6% at the end of the study (Day76 – Week 10) (arithmetic mean). In
conclusion, throughout the 10 weeks of this study, the evolution of the flea eggs present on cats
treated at D0 with the combination fipronil-(S)-methoprene was blocked.[/b]
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