Wilting and leaf yellowing
Greenhouse and field
Aculops cannabicola or hemp russet mite is an understudied pest in industrial hemp. Lab studies indicate that its host range is limited to Cannabis sativa L, since establishment on other related plants in the Cannabaceae family has been unsuccessful. Hemp russet mites are small. Females range in size from 170-210 microns, and males are 160-165 microns, or about twice the width of a human hair. They are pale in color and have just two pairs of legs. The lack of webbing makes this mite is easily distinguished from other mite pests. The size of the hemp russet mites limits its ability to move off the host plant on its own. It can, however, move to the edge of leaves and be carried by wind to other parts of the field.
Though this pest has not been thoroughly studied, it is believed that it has a similar life cycle to other eriophyoid mites. It is believed to have anywhere from a 7-10 day to 30-day life cycle depending on environmental conditions. Since hemp is the only known host, and the life cycle has not been well studied, it is uncertain how the hemp russet mite survives between plantings in the field. It can, however, survive year-round in greenhouse settings with a constant supply of new plants.
Scouting and Thresholds
Because injury from hemp russet mites may look similar to disease symptoms or abiotic injury, observation under a microscope is necessary to confirm infestation. Leaves can be collected from plants and either observed directly or washed in soapy water or 70% ethanol. Mites washed off leaves can then be observed in solution. Placing the wash solution in a dark container or above a dark surface can aid in observation. It may also be possible to use a mite brush to assess hemp russet mite infestations.
As industrial hemp is a relatively new crop in North Carolina, no research has been done to determine the threshold for concern. Observation in field grown plants suggest that fairly high levels of infestation do not necessarily result in obvious injury. Further investigation is needed since the hemp crop is grown for different markets (i.e. fiber, seed, CBD).
Damage in Industrial Hemp
Hemp russet mite develops in the stems and petioles of the hemp plant and can cause stunting. In high populations, the mites can also be found on the underside of leaves. This results in smaller leaf size and suppressed bud growth. Other signs of damage include dull, grayish, or bronzed leaves. Bud growth can also be suppressed resulting in a reduction in the number of buds and dwarfed buds. This damage can reduce yield and quality.
Disclaimer: The following recommendations have been known to work in other plant systems, however, their effectiveness in hemp needs further investigation. We are developing a table for industrial hemp materials in the North Carolina Agricultural Chemicals Manual, but for the time being the Industrial Hemp portal is the best resource for up to date information. As laws and regulations are constantly changing in regard to hemp check with your local extension agent with any label or regulatory concerns.
Preventing Infestation During Propagation
Planting from seeds is the best way to prevent an infestation of hemp russet mite as they can not be sustained on seeds. If planting from cuttings, be sure to thoroughly inspect and disinfest prior to planting. Also, ensuring the mother the cutting came from is not infested. Lastly, the control of personnel can reduce the spread. Restrict the movement of people and supplies between infested and uninfested areas.
Highly refined mineral and seed oils from neem, soybean, cottonseed, or canola have been used against hemp russet mites. Oils have no residual activity and kill mites by coating and suffocating them, which means they must be applied thoroughly to ensure adequate suppression. Some materials have recently been evaluated for activity against hemp russet mite, but none are currently registered in North Carolina.
Amblysieus andersoni is a predatory mite that has been known to feed on other russet mites in greenhouse setting, but its potential in field grown plants in unclear.
Publication date: Aug. 4, 2020
Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by NC State University or N.C. A&T State University nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your local N.C. Cooperative Extension county center.
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