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.
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.