Figure 1: Hemp russet mites developing on new leaves around the flower
All Photos by Whitney Cranshaw, Colorado State University, Bugwood.org

The hemp russet mite, Aculops cannibicola, is an eriophyid mite that can be a serious pest of hemp and cannabis crops. It is small and difficult to detect. Extensive populations of the hemp russet mite can quickly develop on indoor- or greenhouse-grown cannabis if temperatures are conducive for development and reproduction. The level of development and reproduction will also be contingent on temperature (both inside and outdoors). The higher the temperature, the faster development, which will lead to more generations and consequently more individuals that can reproduce within a growing season.

The hemp russet mite is closely related to the tomato russet mite, Aculops lycopersici, which is a pest of greenhouse-grown tomatoes.

Biology

Hemp russet mites are about 0.2 mm (0.0078 inches) long, elongate and pale (Fig. 1). Like most eriophyid mites, they have two legs on each side of the body (a total of four legs instead of eight like most mite species).

The life cycle (egg to adult) takes 30 days at 80°F (27°C) and 70% relative humidity. Adults can live for approximately three weeks. Each adult female can lay 10 to 50 eggs during her 20- to 40-day lifespan.

Immatures (nymphs) crawl short distances but are readily carried on air currents and fans (e.g., Horizontal Air Flow), which can quickly spread hemp russet mites in a greenhouse. Moreover, if plants are spaced closely enough so their leaves touch, it can increase the distribution of hemp russet mites and lead to the establishment of new populations. In indoor environments, hemp russet mites may be present all year.

The presence and extent of the populations in outdoor environments will be based on temperature. Seasonality is related to when hemp russet mites are initially present and how long they may be a problem during the growing season. For example, if temperatures are cool (between 50°F and 60°F) in mid-spring, populations may not be present in sufficient numbers to cause damage during early production. However, if temperatures are higher (more than 70°F) in early spring, populations may occur earlier and at higher numbers. Consequently, there is a greater probability of damage occurring to the crop early in production.

A dissecting microscope with high magnification (100X) is required to observe mites because they are nearly invisible to the naked eye.

Figure 2: Hemp russet mites can distort flowers and new leaves.

Damage

The hemp russet mite feeds on leaflets, petioles (leafstalk), flowers and within meristematic plant tissues containing undifferentiated cells capable of division. The initial symptom of hemp russet mite feeding is the upward curling of leaf edges or margins (Fig. 2, above); however, this depends on the cultivar. Later symptoms include: leaf bronzing, leaf yellowing (chlorosis), and brown or necrotic spots on leaves. Petioles can become brittle, leading to leaflets falling off plants. Leaflets heavily infested with hemp russet mites are gray or bronzed.

Extensive hemp russet mite populations can reduce the size of flower buds, influencing the production of biologically active compounds in plants (phytochemicals) and resulting in reduced yields of extractable cannabinoids. Hemp russet mite damage can substantially impact maturing buds/flowers of female clones grown for CBD production, and possibly THC production, though there is no research, thus no data, to confirm this.

Management

To prevent hemp russet mite populations, first and foremost, start with non-infested or “clean” plants. Avoid introducing any infested plant material such as live plants and cuttings. Cuttings should be quarantined until they have been determined to be “free” of hemp russet mites. So long as plants are grown from seed and not exposed to other plants (e.g., mother plants) during production, then the likelihood of being “free” of hemp russet mites could be for the entire growing season.

For ongoing prevention, scout crops regularly using a 16X hand lens and by occasionally removing leaves to check for hemp russet mites under a dissecting microscope where they are easier to see. Flag certain plants and use them as “indicator plants,” which will help focus your scouting efforts. Immediately dispose of heavily infested plants and those adjacent to infested plants. Workers should wear rubber gloves and change them routinely throughout the day when handling plants to avoid spreading hemp russet mites.

Horticultural oils (petroleum-, mineral-, or neem-based) may be effective in suppressing hemp russet mite populations. However, it is important to be aware that these oils can cause phytotoxicity (plant damage). Consequently, water all plants before applying a horticultural oil to ensure that plants are turgid (full of water), which will reduce the possibility of plant injury (phytotoxicity). A plant experiencing water stress is much more susceptible to plant injury from a pesticide application, and this is especially true for oils.

Sulfur is a miticide that can be effective against hemp russet mite. In addition, certain entomopathogenic fungal-based pesticides (e.g., Beauveria bassiana and Isaria fumosorosea) can be an option in managing hemp russet mite populations. However, sulfur and entomopathogenic fungi must contact the mites to be effective. In addition, phytotoxicity can be a concern associated with sulfur; therefore, ensure that all plants are watered prior to any sulfur applications.

The commercially available predatory mites (Neoseiulus californicus, Galendromus occidentalis, and Amblyseius andersoni) used against greenhouse insect and/or mite pests may not be effective in regulating hemp russet mite populations because the environmental conditions (temperature and relative humidity) associated with growing cannabis and hemp may not be conducive for the development and reproduction of the predatory mites.

Dr. Raymond A. Cloyd is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. Reach him at rcloyd@ksu.edu.