While most equestrians living in temperate regions of the U.S. look forward to the pleasant weather of late spring and summer, the humidity and warm-but-not-scorching temperatures are also ideal conditions for grain mites. These tiny members of the Acaridae family are only between 100th to 300th of an inch in length and are relatives of spiders and ticks. When conditions are ideal, these common pests can occur in large numbers in grain, hay and straw, happily eating their way through your horse’s dinner.
“Grain mites eat the germ out of any kind of stored grain products and proteins,” explains Jessica Starcevich, M.S., staff entomologist with Spalding Labs. “They thrive in high humidity. Depending on the species, this means relative humidity over 70%.”
But a mite infestation is more than just a nuisance—exposure to grain mites can cause allergic reactions in several species. Grain mites are known human allergens, and ingestion of large quantities has caused lesions in the stomach lining of cattle. While more research is needed, there is increasing evidence that grain mites, which also feed on molds and fungus and spread fungal spores throughout their environment, may play a role in triggering equine respiratory syndromes.
Because mites are so small, managers might not notice their presence—at first. But with a fairly long lifespan (mites can live up to fifty-five days) and prolific reproductive capacity (a female will lay 600-800 eggs during her lifetime), it won’t take long for their numbers to compound. An active mite infestation looks as if the grain bin (or bag) has been coated in a light brown dust. The grain itself may appear dusty, and if you begin moving bags around, the mites’ crushed exoskeletons can give off a “minty” odor.
“They are most common in regular whole grains, like oats, corn and barely,” says Starcevich. “But they can infest pelleted feed and certainly sweet feed that has oats and things mixed into it.”
When it comes to grain mites, the best defense is a good offense. Many mite-prevention techniques should already be best practices in terms of grain storage. Strategies such as never pouring new grain over old, thoroughly cleaning out and scrubbing bins in between fill ups, storing grain in airtight containers and religiously cleaning up hay chaff, mold and dust, can all help eliminate the conditions that favor grain mites.
Additionally, take steps to reduce the humidity in your grain storage area. Consider installing a fan, using a dehumidifier or even desiccant packets (keep these away from farm pets). Grain moisture meters can be purchased from agricultural supply companies; stored grain should absolutely not read at more than 16%, and ideally will be much lower than that.
“Make sure there is no place hospitable for them,” says Starcevich. “If you can dry them out, they will die very quickly.”
During humid months, consider getting smaller quantities of grain delivered more frequently, so fewer bags are stored. With each delivery, do a careful inspection of each bag before opening it. If there is any evidence of mite activity, get that bag out of your storage area immediately—and contact your supplier.
“Suppliers get anxious about mites, but it’s usually not their fault, as mites could have been picked up anywhere along the process,” says Starcevich. “Suppliers actively watch for mites and try to avoid getting them. But if they do, they likely have an action plan for how to take care of it.”
If, despite your best efforts, grain mites appear, don’t panic—but do act efficiently, as it is far easier to control an outbreak when the numbers are small. The most important step is to remove the infested grain from your bin, and if possible, remove the bin itself from your storage area. Contaminated grain should be thrown out, buried in an active compost pile or spread in an extremely thin layer where it will be exposed to the sun. Removable bins should be left in the sun for several days, then treated with an acaricide such as pyrethrin. If possible, do not return grain to the bin for several weeks.
“The biggest thing is to get things dry,” says Starcevich. “And keep food sources away from the mites.”
If removing the bin is not possible, thoroughly clean the entire area, paying close attention to corners, cracks and crevices, then treat it with an acaricide. Wait a week, then treat again, then wait another week before use. While this may seem like overkill (if you will forgive the pun), juvenile grain mites living in high concentrations can morph into a phase called the hypopus, which has a sucker that they attach to animals to help them disperse to new areas. During this stage, they are highly resistant to pesticides.
Finally, reach out to your county extension agent if you have further questions. Usually, consultation and even testings are free, and they will know if there is a specific outbreak of any pests going on in your area.