About Russet Mites
The hemp russet mite is a member of the Eriophyoid mite family, and has emerged as a devastating pest in cannabis cultivation. The eriophyoids mite are unique among the acari: they have worm-like bodies (known as fusiform) and larvae and adults have 4 legs, not 8 like other mites. Eriophyoid mites are also microscopic; too small to be seen with the naked eye.
Little is known about many members of the eriophyoid mite family, and the hemp russet mite is no exception. This mite was first identified and described on hemp in Hungary by HK Farkas, in 1960. Further morphological elaboration of hemp russet mites discovered in Serbia was provided by Petanović et al, in 2007. Nothing else has been published.
At the time of this writing, no applied research has been performed on the hemp russet mite. We do not know the specifics of its life cycle, optimum environmental conditions, how they overwinter, if its host range extends beyond cannabis, etc.. Furthermore, we feel borrowing critical details of the hemp russet mites cousin from its cousin, the tomato russet mite (Aculops lycopersici) is inappropriate. There are over 200 species in the genus Aculops; the fact that these mites share the same genus proves only that they look similar.
We can, however, gain a rather accurate portrait of this pest by comparing years of observations in the field with what we know to be true about other eriophyoid pests.
What to look for:
Russet mites cause leaf yellowing (russeting), leaf margin curling, brown pistils, ‘sawdust’ on stems
Russet Mite Life cycle
The life history of the hemp russet mite remains unknown at this time.
– The hemp russet mite feeds on the epidermal cells of leaves, not the vascular system.
– With only 4 legs, hemp russet mites do not travel very efficiently, on their own.
– Transported on clothing or with the help of the wind, they spread easily and infest nearby plant material rapidly.
– For mites that are blown onto the plant by the wind, it may seem as it the infestation started in the middle of the plant. In fact, the mites arrive weeks prior, slowly building up numbers as the plants grow larger.
– They have a strong vertical migration habit, slowly working their way up the plant.
– Once at the top, the russet mites form ‘chains’ on top of one another, seeking to be blown in the wind.
– When observing an infestation, it is important to understand that the russet mite population typically extends beyond the areas where damage is visible. There is normally a large number of mites in the areas directly above the visible damage.
– Similar to the tomato russet mite, the hemp russet mite will seek shelter from predators behind glandular trichomes.
– Moisture stress may exacerbate the damage caused by the hemp russet mite.
I see russet mites, now what?
By the time symptoms are visible, russet mite populations may be in the thousands, or more; early detection is critical. Magnification is required to see these mites; 90x is sufficient. A stereo microscope, or high-quality USB digital microscope, should be used for best results. Russet mites may start low on the plant, or may already be on the upper leaves; a wide variety of samples from different parts of the plant may be necessary. To reiterate: it is vitally important to detect the presence of russet mites as early as possible, which is why regular and meticulous scouting is crucial in using biocontrols successfully to defend against these pests. Once identified, there are a number of species of predatory mites that have been documented to feed on them, including Andersoni , Swirskii , Californicus , and Cucumeris . However, to combat active infestations, it is best to use the liter containers of active adults and apply them in successive weeks until the issue is under control.
I don’t see any russet mites, but I’d like to prevent problems.
For preventative treatments, introducing predatory mites in sachets has resulted in the best outcomes. The sachets can be used from the beginning of the veg cycle, all the way through the midway point of the flower cycle, when the forming trichomes tend to deter and inhibit the mobility of the predatory mites. The sachets are designed to slowly release the predators, as they reproduce inside the sachet, over the course of 3-5 weeks, depending on the environmental conditions of your facility or farm.
Russet Mite Predators
Amblyseius swirskii is primarily known as a thrips predator and is ideal for warmer climates, as it is native to the Mediterranean. It feeds not only on thrips, but also broad mites and russet mites, as well as whitefly eggs. Optimal results are seen when adults in 1 liter containers are used in conjunction with slow-release sachets. Feedback from growers for broad mite control has been very positive.
Amblyseius californicus is a generalist predatory mite that primarily attacks spider mites, but will also feed on many other leaf inhabiting mites (even some microscopic species, like broad mites), other small insects and pollen.
Californicus is tolerant of various temperatures and lower humidity, but works best under warm to hot conditions.
Amblyseius cucumeris feeds on the larvae of several species of thrips, including the common variety: Western Flower thrips. It will also feed on broad mites, cyclamen mites, and, to a lesser extent, two spot spider mites.
Their relatively low cost makes them an attractive option against thrips in operations when temperatures are below 85˚F.
Basic information on russet Mites and russet mite control in commercial cannabis production
Kentucky: Hemp Russet Mite Management
During the last 2 weeks of July, several hemp samples were received and evaluated by specialists at the University of Kentucky (Lexington and Princeton). Hemp russet mites (Aculops cannabicola, Acari: Eriophyidae) (Figure 1) were observed on nearly all of the samples. Mite infestations originate in the greenhouse and outdoors and can become a serious problem for hemp growers when population outbreaks occur.
Description of feeding damage by hemp russet mite
In greenhouse settings, small populations of hemp russet mites can go unnoticed. However, when populations increase, curling leaf margins can easily be identified (Figure 2, see older leaves) – similar injury can also be caused by broad mites (Tarsonemidae), which will be discussed at another opportunity.
When population outbreaks of russet mites occur in shoots, and developing leaves, they appear brown or golden colored (Figure 2). Later, the foliage may become brittle and break easily at the petiole. Hemp russet mites can also affect stems, causing bronze discoloration.
Dr. Whitney Cranshaw, from the Department of Bioagricultural Sciences and Pest Management, at Colorado State University wrote that “the most serious damage by hemp russet mites occurs to maturing buds/flowers of all-female clones grown for CBD production. Extremely high populations of mites may build in late summer which damage these tissues and reduce yield and quality”. The latter may happen in Kentucky.
Description of hemp russet mite
Hemp russet mites are quasi-microscopic organisms that belong to the Eriophyoidea. They are not visible to the naked eye unless you use lenses with high magnification, and even with stereomicroscopes they are hard to separate from the secretory glandular hairs on hemp leaves (Figure 1).
Hemp russet mites have an elongate/oblong shape, and females can be approximately 195 to 210 µm long and 62-70 µm wide (Figure 1). Within the eriophyid group there are several plant feeders in different commodities.
They include the russet mites affecting pepper and tomatoes, the apple rust mite, the cereal rust mite and the citrus rust mites. To our knowledge, hemp russet mites feed only on hemp plants, no other hosts have been reported.
Figure 1. Female hemp russet mite in the lower leaf surface of a hemp leaf. Approximate length and width measurements are shown in microns (0.0026 by 0.008 inches). (Photo: Raul T. Villanueva, UK)
Figure 2. Damage caused by hemp russet mites on leaves and shoots of industrial hemp. (Photo: Raul T. Villanueva, UK)
Biology of hemp russet mite
Although its life cycle is not completely studied, the life cycle of hemp russet mites can be similar to other eriophyoids, and may complete its life cycle in 7 to 10 days depending on environmental conditions. Eggs hatch in 2 days, and a larvae occur 3 days later; they may molt into nymphs.
Both larvae and nymphs are similar to adults, but smaller. Instead of having eight pairs of legs as most of the Acarine (spider mites or ticks), all immature and adult stages of the hemp russet mite are four-legged. Females may live up to 3 weeks and lay 1 to 2 dozen eggs.
Management of hemp russet mites
Villanueva (one of the authors of this note) had worked with the apple rust mite and citrus rust mites in Ontario, Canada and Florida, USA, respectively. In both cases (apples and citrus) predation of rust mites by phytsoeiid mites was difficult to detect. Some of the phytoseiid species tested did not prey on citrus rust mites even when they were starved. Other studies had shown similar results.
AgFax Weed Solutions
In the majority of cases, phytoseiid mites are released to control spider mites. Also, it is important to notice that there is a fine balance to manage insects and mites in many crop systems. Hemp farmers need to have this understanding when considering mangement strategies.
For example, when some neonicotinoids and pyrethroids insecticides are used to control aphids or whiteflies in greenhouses or outdoors, several studies showed that spider mites increase their fertility (a phenomenom called hormoligosis).
Studies on citrus rust mites in Florida and Israel shown that the interruption of broad spectrum pesticide applications for more than 3 years may increase populations of predacious mites. In industrial hemp, making judisciuos use of pesticides may be an effective tool to reduce hemp russet mites.
As industrial hemp is a relatively new legalized crop, many miticides utilized for specialty crops or field crops have not been registered in Kentucky. Currently the Kentucky Department of Agriculture (KDA) has a list of pesticides available for control of insects, mites, weeds, and diseases. This list can be found on the 2019 Pesticide Products Registered for Industrial Hemp in Kentucky.
Currently, there are only five miticides registered in this list (Table 1). All of them are products based on oil extracted from some plants. The effectiveness of this product for hemp russet mite has not been tested by the authors; however oils are used to reduce eggs and spider mites. These products should be used following the recommendations and rates indicated in by the manufacturers.
Table 1. Miticide availables for industrial hemp growers in Kentucky. (2019 Pesticide Products Registered for Industrial Hemp in Kentucky). Click Image to Enlarge
Kentucky: Hemp Russet Mite Management During the last 2 weeks of July, several hemp samples were received and evaluated by specialists at the University of Kentucky (Lexington and Princeton).