There are a wide range of pests that can plague cannabis and interfere with optimal production. A few of the most common mite species found in cannabis are broad (Polyphagotarsonemus latus), cyclamen (Phytonemus pallidus), russet (Aculops cannibicola), and spider (Tetranychus utricae). Knowing how to properly identify them and the different symptoms from each of these mites are the first steps to controlling the problem.
Russet Mites
Hemp russet mites are pale green in color and have elongated bodies. They are extremely small mites, and at less than 1 millimeter long, they can only be observed with 80X to 100X magnification (Table 1, below).
The first sign of hemp russet mites is new leaves start to curl downward (Fig. 1, below). Advance damage to leaves and stems appear as an overall browning or “russeting,” which lends the name to the pest (Fig. 2, below). Damage is usually only apparent when populations explode on a crop late in the production season. Due to their small size and lack of plant symptomology with low populations, hemp russet mites will often go unnoticed. Thorough scouting is critical to detect populations before problems arise.
Figure 1: Hemp russet mite damage can often be observed as downward leaf curl and/or edges of leaves rolling in. (Photo: Patrick Veazie)
Inspect the lower leaf surfaces with a high-magnification microscope to determine if the plants are infected. Hemp russet mites, like other mite pests, can spread from clothing, leaves of infected plants touching, and wind currents or fan air. Some cultivars infected with russet mites will display a bear claw-like morphological change under extremely high population densities. Because cannabis has such variability in leaf morphology, look specifically for any stunted or distorted leaves, such as curling, cupping, bending, wrinkling, etc.
Figure 2: Advanced damage to leaves and stems appears as an overall browning or russeting. (Photo: Brian Whipker)
Broad and Cyclamen Mites
Curled and distorted leaf growth, especially on new and emerging leaves, is a typical symptom of a broad and/or cyclamen mite infestation. The initial symptom is a slight upward leaf curling (Fig. 3, below), which manifests into distorted growth (Fig. 4, below). That’s often how growers can distinguish broad and cyclamen with russet mites, which usually cause leaves to curl down. These mites, which belong to the Tarsonemidae family, are smaller than most two-spotted spider mites and cannot been seen with the naked eye.
Figure 3: Initial broad mite damage can often be observed as leaf crinkling. (Photo: Patrick Veazie)
Figure 4: Advanced broad mite damage results in distorted growth and severe leaf damage. (Photo: Patrick Veazie)
An 80x to 100x magnification is needed to adequately view these mites (Table 1, below). Populations are highest on the leaf underside and can quickly increase with indoor production.
Spider Mites
Common symptoms of spider mites are tiny white and yellow spots, giving the foliage a stippled or molted appearance (Fig. 5, below). Spider mites use their piercing mouth parts to feed on the sap from the underside of leaves. In severe cases, the damage can be confused with drought stress, and leaves may abscise (fall off) from the plant (Fig. 6, at top and below). One of the telltale signs of spider mite infestations is webbing; however, this may not be observed if populations are low (Fig. 7, below). Mite populations can proliferate quickly under high temperatures in a short amount of time. At (86 degrees F), the lifecycle for a spider mite to go from egg lay to laying their own eggs is only 7 days.
Ways To Prevent and Manage Mite Populations:
Proper scouting is crucial to an effective integrative pest management plan. Through scouting, problems can be identified early and limited to a small subsection of plants before large losses occur. Mites can flourish, especially in greenhouse or indoor settings because there are no natural environmental controls, such as high wind or heavy rainfall, to knock them off of the foliage. In outdoor crops, during drought or light rainfall conditions, mites can devastate a crop once the population is well established, and in many cases, it is hard to notice them until it is a major issue. In a greenhouse or indoor environment, with the addition of supplemental lighting, the additional heat, humidity, and lack of air movement can cause infestations to flourish. Lighting also emits heat that can encourage spider mites to endure over time even when greenhouse conditions aren’t entirely favorable to the species. Once a population is identified, determining the extent of the infestation is crucial in creating a management plant.
Figure 6: Spider mite damage can imitate drought stress when populations increase. (Photo: Brian Whipker)
Figure 7: Webbing is often the most common sign of high spider mite populations. (Photo: Patrick Veazie)
Ways to Prevent Mite Problems:
- Keep clean stock plants; this will address a majority of the problems.
- Remove dead vegetation from benches where mites can reside.
- Use high-pressured water to physically remove spider mites if populations are low.
- Remove weeds from the greenhouse to prevent mites from holding over between crops. This is a common problem for greenhouses using weed cloth or gravel.
- Scan vegetation regularly and remove problem plants to keep numbers low from the start.
The best way to control mite populations and prevent further spread once observed is to remove plants that are impacted. Once a population is observed, increased scouting should occur to ensure that the population is under control, and sanitizing the growing area between crops is essential.
