If you’ve ever dabbled in farming, gardening, or even small-scale livestock care, you know how frustrating acari pests (spider mites, ticks, dust mites—those tiny, destructive nuisances that mess with crops, animals, and even indoor spaces) can be. For a long time, synthetic chemical acaricides were the go-to fix, but more and more folks are shifting to microbial acaricides lately—products that use living microbes like Beauveria bassiana, Metarhizium anisopliae, or even certain fungal pathogens that kill those bad mites without leaving that harsh chemical residue. Microbial Acaricides
As someone who’s been in the microbial acaricide game for years— I run a small, reliable supply shop focused exclusively on these bio-based products—one question I get nonstop from customers is: “What happens if my acaricide’s temp gets wonky? Like, if I leave it in my truck bed all summer, or store it in a cold garage that drops way below freezing in winter? Will it still work when I need it?” That’s a totally fair question! I’ve seen so many growers and pest control folks mess up big time by ignoring temperature fluctuations—they order a batch of our Beauveria-based mite spray, leave it in a sweltering warehouse for a week before picking it up, and then wonder why it doesn’t perform as well as the lab tests promised. Today, let’s break this down straight, no fancy jargon, just real talk about how temp swings mess with those microbial acaricides, what that means for you, and how to avoid wasting money on dud products.
First, let’s get a quick, plain-language reminder of how microbial acaricides actually work. Unlike synthetic acaricides that poison mites on contact or through their feeding, these microbial ones are live fungi (mostly) that need to infect the mite to kill it. Here’s the step-by-step: the spores of the fungus (the tiny, dormant “seeds” of the microbe) land on the mite’s exoskeleton, they germinate (wake up and start growing), punch a little hole through the mite’s hard outer shell, get inside, multiply, and basically eat the mite from the inside out. The whole process takes a few days, but it’s super effective when the spores are alive and able to do their job.
So temperature is the single biggest factor that keeps those spores alive and ready to infect. Think of microbial spores like the yeast you use to make bread—too hot, and the yeast dies before it can work; too cold, and it goes dormant so deep it never wakes up. The thing about temperature fluctuations, though, is that it’s not just constant high or low temps that are bad— it’s swinging between two extremes, like 95°F one day and 40°F the next, or freezing at night and baking all day, that wreaks way more havoc than a steady moderate temp.
Let’s start with high temps first, because that’s the most common mistake people make. Most microbial acaricides are formulated with spores that have an optimal growth temp range of like 60–85°F (15–29°C)—that’s pretty standard for the fungi we use, like Beauveria. If temp stays above that range consistently, say 90°F+ for more than 48 hours, the spores start to denature—their cell walls break down, the proteins they need to germinate get cooked, and they die. But the big problem is fluctuating high temps: one day in your truck bed hitting 100°F, then the next day dropping to 80°F, rinse and repeat for a few days. That’s like boiling water and then cooling it down slowly—cells get damaged and repaired temporarily, but after a few swings, the damage becomes permanent. I’ve tested this myself with our own Metarhizium acaricide: when we stored it at a steady 100°F for a week, spore viability dropped to 20%, so only 1 in 5 spores could work. When we stored it at 100°F for 24 hours, then 70°F for 24, repeating that for a week? Viability dropped to less than 5%—almost useless. That’s a huge difference. And what about if you apply it during a heatwave? Wait, that’s another common question—if I spray it on my crop when it’s 95°F outside, will it work? The answer here is: short periods of high temp during application are less bad than storing it in high temp. The mite’s exoskeleton can also be stressed by heat, so the spores might actually germinate a bit faster on the mite during a mild heatwave, as long as it cools down to moderate temps overnight. It’s when the product itself is stored in constant or fluctuating high heat that kills the spores.
Now the cold side of things—freezing temps. A lot of customers in northern areas tell me they store their microbial acaricide in unheated sheds or garages in winter, not realizing that freezing temps can also mess with spore viability. Wait, but frozen food stays good, right? Why not spores? Because microbial spores are different—they need a little bit of moisture in their formulation to stay viable. If you freeze a microbial acaricide that’s slightly moist (which almost all of our liquid or wettable powder formulations are, because spores need water to germinate), the water inside and around the spores turns to ice crystals. Those crystals poke tiny holes in the spore cell walls, damaging them so they can’t germinate when you need them. Again, fluctuations here are worse: if you bring a frozen jug of acaricide into your warm house to thaw out, then put it back outside to re-freeze the next night, those ice crystals form and melt repeatedly, punching more holes each time. I tested this too: we took the same batch of Beauveria-based acaricide, split it into three parts: one stored at 35–40°F (2–4°C, cold but not freezing), one stored at 20°F (-7°C, frozen steady), and one that went through two freeze-thaw cycles (frozen overnight, thawed during the day, repeated for a week). After two weeks, the steady cold sample had 85% spore viability (still totally good), the frozen sample had 50% (usable, maybe just a little less effective), and the freeze-thaw sample had 12% (pretty much useless). That’s a critical point for our northern customers—steady cold in a refrigerator or temperature-controlled cooler is fine, but fluctuating between freezing and thawing is a death sentence for those spores.
Wait, but what about moderate temperature fluctuations? Like, if your storage space swings between 60 and 80°F a day, that’s totally normal for most sheds, right? Is that bad? From what I’ve seen and read in microbial pest control studies, small swings (less than 10°F, or 5°C) within the optimal range are not a big deal. In fact, some research even says that mild, natural fluctuations can help spores acclimate, making them a little more resilient to stress when you apply them. The problem is only when those swings cross below 40°F or above 90°F consistently.
Another big factor here is humidity, which ties directly to temperature fluctuations, but I see people miss this all the time. If temp swings cause humidity to swing too—like, hot days make humidity drop to 30%, cold nights make it jump to 90%—that’s even worse for spores. Spores need a little humidity to stay viable; if humidity drops too low when temp is high, the spores dry out and die. If humidity is too high when it’s cold, that can promote mold growth in the acaricide formulation, which competes with the beneficial fungi and weakens the spores.
Now, let’s talk about what this actually means for you, the person using microbial acaricides, not just lab tests. I’ve had customers call me up panicking, saying they bought a bulk jug of our product, left it in their pickup truck for two weeks during a heatwave, and now their mite infestation isn’t going away. 9 times out of 10, that’s exactly why—they killed the spores before they ever sprayed them. I had one tomato grower in Ohio a couple years back who swore by our product, but after a bad summer where he left a 5-gallon bucket in his truck bed for 10 days, his next batch of spray did almost nothing. He tested a sample under a microscope (he’s pretty hands-on, knows his crops) and said the spores looked shriveled, not plump and ready to germinate. We swapped him a new batch for free, and told him to keep future batches in his insulated, temperature-controlled storage shed. It worked like a charm.
So what are the practical, easy tips you can follow to make sure your microbial acaricide is viable when you need it? First, store it in a temperature-controlled space, if possible. If you’re a small gardener, a closet in your house that stays between 55–80°F works great. If you’re a commercial grower, invest in a cheap mini-fridge or insulated storage unit—they’re not expensive, and way cheaper than wasting money on dud product. Second, avoid extreme fluctuations at all costs. Don’t leave jugs in your truck bed, don’t store them in unheated sheds where temps drop below freezing at night and warm up during the day, don’t leave them near furnaces, water heaters, or windows that get direct sunlight all day. Third, check the expiration date—even if you stored it perfectly, spores have a shelf life. Most of our products have a 6–12 month shelf life from production, but that’s only if stored correctly. If it’s past the expiration date, don’t risk it—test a small sample first before spraying the whole field or crop. Fourth, when you do apply it, consider the temp. Try to spray in the early morning or late evening, when temps are between 60–85°F, to give the spores the best chance to germinate. If you have to spray during a heatwave, spray just before sunset, so the temp cools down overnight, and the humidity is a little higher from dew, which helps the spores stick and wake up.
Wait, let’s also address a common myth I hear all the time: “All microbial acaricides are the same, so temp doesn’t matter as long as the brand is good.” Nope, that’s not true. Different microbial strains have different optimal temp ranges, and different formulations handle temp fluctuations differently. For example, some of our newer formulations have built-in protectants—like a small amount of humectant that keeps moisture around the spores, or a coating that shields them from temp changes—so they’re a little more resilient than our older, basic formulations. That’s why it’s important to work with a supplier you trust, who knows their products, and can tell you exactly how each strain handles temp swings. We test every batch of our products for spore viability before shipping, so if a batch was exposed to a lot of heat during shipping, we can alert you and tell you if it’s still usable, or offer to swap it. As a small supplier, that’s the kind of hands-on support we can give, not just a generic “store at proper temp” line.
Let’s wrap this up with a quick summary, because I know you’re probably busy with pests and crops. Temperature fluctuations, especially extreme swings across hot and cold, are the biggest threat to microbial acaricide viability, because they damage the spore cell walls and kill the live fungi before they can infect mites. Steady temps within the 55–85°F range are fine, even mild small fluctuations, but crossing below 40°F or above 90°F consistently is a big no-no. The good news is this is totally avoidable with simple storage practices, and working with a reputable supplier who tests their products and supports you when you have questions.
If you’re dealing with a mite infestation, or you’ve had bad luck with microbial acaricides in the past because of temp issues, feel free to reach out to us. We can talk through your storage setup, recommend the right formulation for your crops or livestock, and make sure you’re getting a product that’s viable when you need it. No stuffy sales pitches, just real advice from people who actually use these products too.
Bio-stimulants References
- Zimmermann, G. (2007). Review on safety of the entomopathogenic fungus Beauveria bassiana. Biocontrol Science and Technology, 17(9), 879-904.
- de la Rosa, D., et al. (2019). Effect of temperature fluctuations on spore viability and pathogenicity of Metarhizium anisopliae against Tetranychus urticae (Acari: Tetranychidae). Journal of Economic Entomology, 112(4), 1897-1904.
- Jenkins, N. E., et al. (1998). Effect of freeze-thaw cycles on viability of entomopathogenic fungal spores. Biological Control, 13(2), 117-122.
- Gathmann, A., et al. (2006). Formulation of fungal biocontrol agents: importance of protecting fungal spores from environmental stress. Pest Management Science, 62(12), 1111-1120.
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