Russet Mites: A Need-to-Know Guide for a Microscopic Enemy

Aculops cannabicola, or the hemp russet mite, is striking Washington cannabis growers hard this year. Numerous growers are fighting off these tiny pests, and they are spreading like a plague of locusts.

The most frightening fact about these pests is that they are not visible to the naked eye. The size of these guys ranges from 20 to 170 µm long and up to 65 µm wide. To give you some perspective on that, they are smaller than dust mites and thinner than human hair. Russet mites are so small they can actually be distributed to neighboring areas via wind- which is a big concern in Eastern Washington! They’ve even been known to hitch rides on other insects to spread to new areas!
Russet mites can hide in contaminated potting soils as well, or can be introduced by bringing in clippings and clones of infected plants. Don’t assume you are safe from this pest just because you are operating an indoor grow.

Being microscopic means that many farmers don’t know they have a problem until their crop has been affected. Suddenly your bacteria counts start spiking and your potency starts dropping, but you haven’t changed anything and don’t see any obvious issues. You don’t know why or what’s going on until the mites get so bad the plants themselves start telling you something is wrong, and even then the issue might not be obvious- you start wondering if you have a mosaic virus, a nutrient deficiency, or other form of plant stress.

The visible symptoms created by the russet mite are very similar to other common problems. Fan leaves might curl a bit at the edges and have a glossy wet look- similar to heat stress. Leaves may have yellow or bronze spotting – sometimes also a symptom of mosaic viruses. New growth may come in twisted and seem stunted/limited or leaves might droop – as if the plant is suffering from environmental stresses or nutrient deficiency.

The females over winter inside plant stems, where branches join main plants, and sometimes in root structures. The mites are sap suckers. They start by feeding on the lower structures of the plant, then work their way up as the food supply is depleted. Russet mites are especially attracted to flower resins and can go unnoticed while feeding in flower structures.

To check for mites, you’ll need to purchase a magnifier that is at least 14x, preferably slightly higher than that.
Most jeweler’s loups will do the trick. Next find an area slightly above where the plant is showing stress. If the plants are not yet showing stress, start checking towards the lower portions of your plants. Be sure to check in several places if you don’t see them at first. The insects don’t stay in the same area after they’ve fed and will move to other areas on the plant. Be sure to check thoroughly before you decide you are in the clear.

So you found mites, now what? There are a variety of pest management strategies available. Regardless of what strategy or combination of strategies, be sure to maintain it long enough to eliminate the pests. Given the russet mite life cycle, treatments can take several weeks before the infestation is eliminated, and, because the mites easily spread through wind, vigilance is required to prevent re-infestation. Under ideal conditions, a single mite may live a month (or more) and lay many eggs during that time period. When the eggs hatch, the cycle starts all over again.

Some of the pest management options available include:

• Predatory mites and beneficial nematodes
• Insecticidal soaps
• Neem oil (if applied at the first signs of infestation)
• Pyrethrum based sprays (but be wary of how much you use to ensure you don’t exceed WSLCB limits for pyrethrins!)
• Diatomaceious Earth (won’t get rid of an infestation but will slow down the spread of the mites)
• Sulphur dust/wettable Sulphur (not to be used on flowering plants!)
• Pruning infected branches, removing infected plants, and quarantining infected areas.
• Removing weeds and other vegetation near your crop that could harbor mites.
• For indoor grows, adjusting environmental factors, such as temperature and humidity outside of optimal range for mite reproduction (remember changes to environmental factors will affect your plants too!). Cooler temperatures will slow reproduction, but not kill mites. Mites don’t survive well in temperatures over 105-115 F. Russet mites prefer drier climates and breed more slowly in higher humidity.

This list is by no means exhaustive. There are numerous brands and types of products out there that are effective against mites. Regardless of what method you choose, there are two important things to remember:

1. Make sure your pest management strategy is in line with state law and only includes pesticides approved on the PICOL database

2. Apply your pest management strategy long enough to eliminate infestation and stay vigilant watching for any future infestations and getting treatment started as early as possible.