(This story has been updated to include additional information about sulfur applications.)

A relatively unknown predatory mite can stave off potentially disastrous infestations of hemp russet mites in marijuana and hemp production when used in the right conditions.

Hemp russet mites (aculops cannibicola) were virtually unheard of in marijuana and hemp production until widespread outbreaks in 2017, said Andrew Maltby, president of Biotactics, a Southern California company growing predatory mites to control pests in cannabis and other high-value crops.

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One longtime outside grower contacted Biotactics in desperation. The grower hadn’t seen hemp russet mites in his 10-foot-tall cannabis plants but faced an outbreak that was so bad “he was about to walk away from the grow,” Maltby said.

“It was like the plague. Nothing at the time worked on them, and people didn’t know they had them until it was too late.”

What we know about russet mites

The hemp russet mite is too small to be seen by the naked eye, and cannabis is its only reported plant host, according to the Colorado State University Hemp Insect Website.

The russet-mite attacks marijuana and hemp plants, and the level of THC does not appear to make a difference in the severity of attacks, Maltby said.

Little is known about the hemp russet mite, though it is considered one of the most severe pests for cannabis growers, said Eze Pojmann-Ezeonyilo, a graduate student at Purdue University studying cannabis pests.

Entire plantings have been lost to russet mites when infestations are severe, he said.

Symptoms of an infestation of hemp russet mites include:

  • Yellowing on leaf edges and between veins.
  • Stunted, dry or wrinkled leaves.
  • Leaves curling upward at the base (also known as tacoing or canoeing).
  • Flower pistols that dry up prematurely.
  • A light brown powder on the plant.

Since that initial “plague” year of 2017, cannabis growers have learned more about russet mites and turned to using micronized sulfur and horticultural oil products for control.

But those techniques have their limits, Maltby said.

Using sulfur once flowering has begun risks buds smelling like rotten eggs at harvest.

Plus, while sulfur does kill russet mites (and natural predators), it doesn’t kill spider mites, leaving many growers with a spider-mite problem after dealing with their russet-mite outbreak.

Predatory mites

Instead, Maltby encourages cannabis growers to look to a more natural control – predatory mite galendromus occidentalis – as part of their integrated pest management plan.

Also known as the western predatory mite, galendromus occidentalis is a voracious consumer of russet mites as well as spotted spider mites and European red spider mites.

However, the western predatory mite is effective only in the right conditions. Western predatory mites need high temperatures (over 75 degrees) and relatively low humidity (35%-65%), Maltby said.

Producers in low-humidity, outdoor environments and indoor grows where temperatures and humidity can be controlled are good candidates for adding western predatory mites to their pest-management regime, Maltby said.

He has seen successful control of russet mites using western predatory mites even in significant outbreaks.

“We’ve saved a couple of grows, one that had 10- and 14-foot plants he was about to walk away from at the beginning of September,” Maltby said.

“We sent him 4,000 to 6,000 predators per plant, and he ended up getting a full harvest in October.”

Maltby sees predatory mites used effectively for russet mites in indoor and outdoor marijuana and hemp production, although different deployment strategies might be used. Large-acreage outdoor hemp growers are using drones to release predatory mites, he said.

Not always perfect

Predatory mites might work for controlling russet-mite outbreaks, but they can come with risks growers need to consider beyond matching their release to the correct temperature and humidity, said Raymond A. Cloyd, entomology professor at Kansas State University.

Once flowering has started, predatory mites such as galendromus occidentalis don’t do well in the sticky resin produced in cannabis buds, Cloyd said.

“If you have hemp russet mites at the time the plants are in bud, you have a problem,” Cloyd said.

He recommends either committing to using horticultural oils, making sure to get complete plant coverage, or using predatory mites combined with aggressive scouting so sulfur can be applied if needed before flowering.

However, sulfur applications require a 24-hour re-entry period for worker safety.

Growers should scout regularly, using handheld magnifying glasses and occasionally removing leaves to inspect under a high-magnification dissecting microscope, Cloyd said.

Maltby disagrees that galendromus occidentalis can’t control russet mites once cannabis plants have started flowering. Some growers are using micronized sulfur in the vegetative stage and then turning to predatory mites at flowering, he said.

It all depends on how the predatory mites were raised, Maltby said.

Traditionally raised predatory mites are grown on artificial food sources and don’t have a high pest-consumption rate when released, he said.

His predatory mites are raised using a proprietary method that leaves them unfed and hungry when released into a target crop, making them very good at their job, even when plants are in flower.

Biotactics provides growers with quick-release bags designed like a “doorknob hanger” meant to be hung directly from a plant, so they release the predatory mites close to flowering buds.

Maltby recommends 10,000-20,000 predatory mites per acre for preventative management, at a cost of $250-$500. Prices do go down if growers purchase 500,000 predatory mites or more.

“If you’re one to two weeks from harvest and you need to load up your plants with predatory mites because you’re infested, the motility of your predators is the least of your problems,” Maltby said.

Environmental controls

Maltby and Cloyd agree that keeping your crop clean – especially for indoor growers – is crucial to controlling russet mites.

Buy clones only from reputable growers or start plants from seed. Maltby recommends taking a microscope to inspect new plants before you bring them to your operation.

If you do have an outbreak indoors, consider a full reset, Maltby said.

“If you’re seeing tacoing leaves and leaf torsion, they are everywhere,” he said. “They are in the vents, your fans, your clothes. They are everywhere.”

In that case, Maltby advises, raise the temperature to 140 degrees for eight hours, clean everything with bleach water and clean up all residue, then raise temperatures again.

When new, clean plants are introduced, release predators immediately, Maltby said.