I recently got a good question from a pest control technician. Not something I would have ever thought to blog about, but very relevant to our work.
His question was: I’m using a pyrethroid for perimeter treatments, and I’ve been using it for the last few years. Do I need to rotate products? How long before the pests develop resistance?
The short answer: don’t worry about it, you’re fine.
Ok, I must admit it, I cringed as I wrote that. What I wrote is true, but it makes it sound like resistance isn’t something to worry about. Which is not true.
Your typical perimeter pests can develop resistance to insecticides. I’m talking about centipedes, crickets, ants, outdoor cockroaches, etc.
However, these pests are not just found around your house. They’re also found in a lot of other places, like any undeveloped land, around your neighbors’ houses (not all of whom have their yards sprayed), in parks, and so forth. Because many of these places are not treated, there’s not enough pressure from the insecticides for us to see resistance develop in them. Oh, it might be there in low levels, but not enough for us to notice.
This is different from a situation where you have, say, the small fruit fly, Drosophila melanogaster, in a commercial kitchen where sanitation is an issue. Here, you are treating the entire population of small fruit flies with your insecticide – unlike the perimeter pest example above, where you are only treating the population around one house. Many of the small fruit flies who survive will be resistant, and when they breed with one another, their young will be resistant. After many generations, your product will not be very effective on the small fruit flies. Compare this to the perimeter pests, where, if there are some resistant pests left after your treatment, non-resistant pests will come from adjoining, untreated properties, migrating, and can mate with any resistant pests. The result, is that the young may be resistant, or not. Often resistant pests aren’t able to compete as well for resources, so around the perimeter of the house, invaders from neighboring properties are also more likely to out-compete any pests that have resistance.
Back to the short answer: Feel free to keep on using your product of choice for perimeter treatments.