Hemp Russet Mite
There are several thousand species of russet mites estimated, sometimes called rust mites due to the coloration of their damage or their own bodies on leaves or blister mite due to the ability of some to modify plant tissue into extremely odd shapes for shelter.
However, many people are not aware that despite their ominous reputation in Cannabis, the vast majority of russet mites are innocuous, barely causing any damage to their host. It’s also the case that most russet mites are highly specialized on one or a few related plant species, which may account for their inoffensive lifestyle.
Only a fraction of species are major agricultural pests, and the specialist Hemp Russet Mite, Aculops cannabicola has become prominent as its only source of sustenance has expanded across earth with intense alacrity.
These pestiferous species like Citrus Russet Mite, Aloe Russet Mite, and Hemp Russet Mite tend to share a common trait: substances in their saliva warp and gnarl plant tissue during feeding, providing a sort of pseudo-gall of protection by corrugating and twisting leaves and likely severely disrupting plant immune response at the site of feeding.
This is also similar to the Broad Mite which has similarly phytotoxic saliva and damage profile but look very different examined under magnification.
Hemp Russet Mites are ~200 micrometers long, with a short worm-like body that tapers at the distal end. The head is the widest region where four short legs can be seen trundling its unwieldy body.
Interestingly, while some arachnids such as mites often have six legs in juvenile stages and eight legs for walking as adults, ancestral russet mites lost several groups of genes over time that influenced this and other traits. Many parasites that specialize on hosts often develop in this way, as the tools needed to subsist tend to narrow and mutations chisel away at the less important physiology.
Due to their small size, it is their damage that is most noticeable and distortions develop soon after feeding. Vigilant plant appraisal allows the savvy, proactive grower to catch these colonies before they bloom.
Cuttings are a common vector, and mites can be spread to new plants if transferred quickly (or unscrupulously) without the typical symptoms. In natural settings, russet mites adopt a special pose to catch wind or possibly nearby host, a behavior known as questing.
Wind can carry them great distances as their sheer weightlessness facilitates travel as aeroplankton, the collective name for living organisms like viruses attached to particles, spores, small arthropods, and pollen that transport on wind currents without much influence over their direction.
Despite 99.999% of plants being completely unsuitable on which they would die, this strategy seems to have worked for tens of millions of years much to the chagrin of growers everywhere.
Wind, cuttings and other fomites like equipment or clothing are the most common russet mite vectors. Preventative measures must account for this and there is a clear advantage in the physical separation of greenhouse and indoor contexts compared to outdoor conditions.
While research directly focusing on russet mites in general and Hemp Russet Mite in particular are lacking, there are some nominal, conserved traits to inform growers.
Ostensibly, outdoor areas of denser Cannabis presence (cultivated and uncultivated) are the most vulnerable to windborne mite movement whether urban or rural. Locations where there is less dense cultivation may be less likely to encounter these mites from the wind, though many other factors likely influence this avenue such as mite population presence, environmental and topographical conditions.
Physical barriers like screen or windbreaks help limit direct contact with the host since russet mites that descend and deposit onto unsuitable surfaces are not known for their dexterity or movement speed and entail substantially increased mortality without food.
Humans often vector these mites to Cannabis either on plant material like cuttings, shared equipment that isn’t properly cleaned prior, or on their attire after interacting with infested plants elsewhere.
Cuttings from growers at various scales of cultivation unfamiliar with or ambivalent towards the mites and their damage continues to be a major vector. Biosecurity stronger than a reliance on the diligence of others may necessitate quarantine of some sort. Unlike some other pests that can potentially take longer time to show symptoms, the relatively fast life cycle of 1-2 weeks, high reproductive rate, and contorted leaf growth soon after feeding makes recognition straightforward.
A popular chemical treatment is micronized sulfur, which is economical and highly effective when used appropriately on contact. Some products that are also effective but have compatibility issues when applied around the same time include horticultural oils which rely on suffocating and sometimes also toxic effects that are mite-specific.
Academic institutions like Louisiana State University have suggested several predatory mites such as Californicus (Neoseiulus californicus) and Swirskii (Amblyseius swirskii) Mites have proven successful against Hemp Russet Mite in extension publications, while Colorado State University has shared media that affirms and more recently denies proof of predatory mite efficacy.
In my experience, the Cucumeris (Neoseiulus cucumeris) and Swirskii Mites are effective biocontrols and I’ve both guided and encountered their use in Cannabis cultivation, particularly effective in proactive applications or for flowering crops.
Several well-known biocontrol companies have recommended generalist species like Cucumeris and Swirskii for the treatment of Hemp Russet Mite. Despite this experience, further biocontrol research should be carried out to understand predator-prey dynamics of various predator mites as well as the investigation of additional biocontrols.
It can be difficult to ascertain exactly how a population of pests establish, particularly if there are several plausible options. While it does happen, the spread of Hemp Russet Mite is not necessarily due to malicious negligence as the increase of new growers unaware of various pests has probably contributed greatly. The best defense is knowing these pests and safe strategies against them, and sharing that information as much as possible.
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