Cannabis Spider Mites: IPM Solutions

The two-spotted spider mite is an extremely common pest and is recorded on over 1200 species of plants both cultivated and uncultivated, allowing for many hosts from which to colonize cultivation spaces. Cultivating cannabis in areas where other agricultural activities occur nearby naturally increases the chances of pest ingress, particularly where exposure to the elements is high. With the ubiquity of two-spotted spider mite populations, plants that are susceptible may face constant infestation pressure.

The ability to spin silk has also allowed the two-spotted spider mite to catch the wind and travel via ingenious silken balls that multiple individuals can use and construct [7]. They also spin protective webbed barriers to stop general predators from attacking them and their eggs [4]. Two-spotted spider mites can disperse both short and long distances using silk, and often do so when faced with dense colony conditions, detection of wind, or a decrease in plant host suitability.

Female spider mites can even produce male offspring when unmated, and female offspring when they mate, meaning that only a single adult female is necessary for a colony to develop. With the capacity to produce dozens of eggs in a short time span, a two-spotted spider mite infestation can quickly balloon out of control.

The two-spotted spider mite is so pervasive due to its genomes, which are geared towards overcoming the many plant immune responses. Some of these responses include plant toxins and even the suppression of the host immune response itself. Several of these genes first existed in bacteria [2] and were horizontally transferred to the spider mite’s ancestors, such as those that break down harmful chemical agents for both organisms. Some chemical compounds used against two-spotted spider mites in the past have become ineffective due to rapid onset of resistance, owing to their prodigious reproduction rate and innate chemical resistance. 

Most of these traditional chemistries, like imidacloprid, avermectin, and their ilk are not suitable for cannabis because of their systemic and toxic nature. However, the pest's resistance can cause incidental cross-resistance to other compounds because they target the same aspects of physiology or because they have a similar structure, among other reasons.

Some compounds have even caused a response called hormoligosis. This response stimulates spider mite reproduction by up to 30% [3]! A phenomenon called “mite flaring” is associated with the application of agents whereby said applications decrease two-spotted spider mite populations initially, but then rapid increases are observed. Part of this may be due to hormoligosis for select compounds, but the reduction in predatory biological pressures due to broad-spectrum applications has a cumulatively positive effect for the two-spotted spider mite. No matter if the source of the treatment compound is derived naturally or artificially, agents that affect a diverse array of organisms are often detrimental if improperly implemented in a pest management strategy.

While there are many clover mite species that appear almost identical, Bryobia praetiosa (Clover Mite) is one of the more prevalent cannabis spider mite types. It can also be a nuisance in residential settings due to the many individuals that become active in the spring and summer months. 

Unlike the two-spotted spider mite and others so-named due to their possession and application of silk, the clover mite does not utilize it, preferring to move quickly across the surface of plants, which may give them the appearance of beneficial predatory mites. 

An additional important difference between clover mites and other small mites is size and coloration: clover mites are about 0.7 mm in length and black with red legs, while other other spider mites are smaller and red or green usually. Predatory mites typically encountered are white or pale in coloration, or sometimes orange.

The clover mites are not well-studied cannabis spider mites, particularly when compared to the two-spotted spider mite.They are highly prolific in warmer seasons and lay small russet-colored eggs in clusters of two to four on average. Eponymously, the clover mite often favors various clover species, genus Trifolium which may be present on the property of cannabis cultivation spaces incidentally or as a part of a regenerative agricultural practices. Both white clover Trifolium repens and red clover, Trifolium pratense are common hosts, but so are agriculturally important plants like cedar, apple, iris, and strawberry.

One highly advantageous trait possessed by most clover mites is the ability to reproduce parthenogenetically, or asexual reproduction by females, that in this case, results in more females. This is not an endogenous effect borne from their own genetic code, but due to colonization by a bacterial species in the genus Wolbachia [6]. Wolbachia are actually extremely common in many insects and other arthropods, and they often alter the reproductive fitness of their hosts. In this case, it influences most clover mite populations to be able to reproduce asexually without overt detrimental effects, and this can lead to large populations under ideal conditions.

Clover mite damage is typically relatively innocuous, and presents as light scrape lines that move linearly across the surface of a leaf, or as smaller scrapes, stippling the tops and bottoms of leaves.

These pests still suffer from their prolific nature because, despite their advantages, many organisms still feed on them, and various naturally-occuring chemical compounds still damage them. Commercially, solutions are context-dependent: not all solutions are legal in all areas, and due diligence is required before any options should be considered. 

Generally speaking, biocontrol agents are popular and effective options if properly utilized. Predatory mites like Phytoseiulus persimilis and Neoseiulus californicus are specialist hunters of spider mites [4] and will readily feed on them. The predatory spider mite midge Feltiella acarisuga and spider mite ladybeetle Stethorus punctillum are also commercially available biocontrol agents, all having their specific advantages and disadvantages. No matter the agent, rapid deployment at the correct rate is critical, and preventative application is often superior.

Abiotic approaches like chemical agents are popular, and while several chemistries affect spider mites negatively, one of the least problematic to implement is micronized sulfur. When applied to non-flowering cannabis the sulfuric water binds with and destroys the exoskeleton of cannabis spider mites as well as many other animals with exoskeletons, including beneficial organisms, making a comprehensive integrated pest management strategy critical. Be sure to turn this strategy into accurate Cannabis Standard Operating Procedures (see the GrowerIQ Ultimate Guide to SOPs for more information). Fungi and other microbes can also be negatively affected by sulfur, so other biological agents as well as microbial pests in a cultivation area may be negatively affected.

Since spider mites and many other successful pests are transported long and short distances on wind currents or equipment, physical barriers to wind and pest movement can make a big difference to their passive ingress throughout the season and especially during their seasonal increases. Regular hygienic maintenance of cultivator equipment and behavior is paramount for preventing spider mite populations in particularly host-rich spaces where populations are constant and in close proximity to vegetation outside the control of the cultivator.

Spider mites are a common pest from the largest commercial cultivator to the smallest home cultivator. Over time, humans have collectively demystified them through focused research. Now solutions are available. Current cannabis cultivation IPM best practices offer several biological, physical, chemical, and cultural solutions, with preventative measures based on an understanding of spider mite dispersal. This includes discouraging wind-based, equipment-based, and plant-based travel, early detection through crop scouting, rapid management with biocontrols, and culling potential hosts are all strategies that reduce the rate of spider mite infestation.