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  • Writer's pictureChelle Hartzer

Resistance is Futile (AKA: I wasn’t a math major)

There was a great article discussing a new paper on cockroaches recently. It talked about resistance to currently available pesticides. Let’s delve into resistance, why it is happening, and (maybe most importantly) what to do about it is important so these pests can continue to be managed.

"You think that will kill me? HA!"

First, think about this:

A German cockroach has between 30-40 eggs per egg case (AKA ootheca).

Once those eggs hatch, it takes them around 60 days to reach maturity and mate.

Take a pair of cockroaches at day zero and they are a happy little family with a bundle of joy on the way.

  • On day one that egg case hatches and 30 new little cockroaches start their new life.

  • So we have two parents, thirty kids, and that makes 32.

  • Since those kids take about 60 days until they are ready for their own families, we have to wait…

  • then at day sixty, they have now created kids of their own. Only half the population (the females) will create the next generation so we have 15 of the (original) kids, times 30 eggs, and mom and dad now have 480 grandkids. But wait! Mom is still laying eggs so there are an extra 30 kids plus the original mom and dad. So a grand total of 512. And it keeps going until at one year, there are over 500 million cockroaches.

Now before you criticize my math, I admit this is a bit simplified. This assumes no natural mortality and all the food, water, perfect temperatures, and other ideal conditions that the cockroaches could ever want. Development time could be longer based on food and temperature. Natural mortality and predation will obviously reduce the population. But if it makes you happy, go ahead and check my math. You can also play with the numbers: what if the development time is 90 days or the number of eggs is 40?

The point of this exercise is to illustrate just how quickly German cockroach problems can intensify. Which is why we need control methods like:

  • Reducing food so they can’t develop as quickly and may even die of malnutrition

  • Reducing water sources and moisture so they develop slower and may die from desiccation

  • Putting out traps so they get stuck and die

  • And putting out pesticides so they die

But what if pesticides don’t work? That means a valuable tool is no longer working to control the population and the population could spike (see table above!). This is what is happening with resistance. The pesticides no longer cause mortality, the cockroaches survive and procreate. Imagine it a bit like a bulletproof vest. The gun still fires and they can still get shot, but it just doesn’t hurt them anymore.

Going back to the first table, picture this:

You have 50 cockroaches and put down a pesticide. Instead of the pesticide killing all of them, 2 are “immune” and survive the treatment. Those two mate and produce their normal 30 kids. Since this is based on genetics, maybe not all of them have the immunity gene. Let’s say it’s only half of them. As those mate, more and more of the offspring are genetically immune until basically, the entire population survives every time you put down the pesticide.

Just because resistance has been found in a population, doesn’t mean every last cockroach will survive. Often it means that at lower doses the insects will survive as compared to a non-resistant colony. Studies are often done by comparing field collected insects with laboratory colonies. The lab colonies are pampered little insects that are pretty easy to kill. However, resistance will continue to get more severe the longer the insects have to be exposed to a particular pesticide, survive, and breed.

Back to the study in question

Resistance was detected compared to the lab strain. With the active ingredient fipronil, they reached 100% mortality with the lab strain, however the field strains only had between 50-80% mortality. As few as 50% were killed using the label rates of the pesticides! Other active ingredients also showed a range of mortality levels.

This was just one study done in one state in the US, other areas could show different results. Despite that, resistance should be anticipated and proactively managed. The pesticides being used to control cockroaches must be rotated and different active ingredients used. Ideally, they should be changed each time a service is done, but at a bare minimum, it should be done quarterly. It isn’t just for cockroaches either. Many of our pest insects can develop quickly and if the same active ingredients are used again, and again, and again, resistance can develop.

Ultimately, if we don’t stop resistance, our pesticides could become useless and we are left with fewer tools to manage pests. Consider your current pest management program and what products are being used: how often are they being used and how often are they rotated. If your program is static, maybe it’s time to have someone take a look at it and find what should be changed. Contact us to see how we can help!

Lagniappe: it's not just resistance...

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