Hemp russet mite (Aculops cannibicola)
Pest description and crop damage Hemp russet mites (Order: Acari; Family: Eriophyidae) were first discovered in Europe in 1960 and they are currently known to be present in different regions of the US including the PNW. These mites are extremely small and can only be seen with a microscope and not with the naked eye. Similar to other eriophyid mites, they have an elongated body and are pale in color. Hemp russet mites' range in size from 160 to 210 microns or less than the half the size of two-spotted spider mites; they have only two pairs of legs; and females are typically larger in size than males.
Hemp russet mites are one of the most damaging pests for hemp plants grown under greenhouse conditions. There is no confirmed report of infestation to outdoor grown hemp in Oregon or in the PNW, but they have been reported in outdoor conditions in other states such as Colorado, Utah and Virginia. These mites feed on fluids from the outer plant surface cells on leaves, petioles and shoot tips. Compared with other mite species such as two-spotted spider mites, they do not produce webbing on plants and their damage is often unnoticed in low populations. When there is a heavy infestation, leaf damage symptoms apparent on hemp plants include upward curling along leaf edges, russeting leaf tissue, and brown or powdery appearance on leaf edges. Feeding symptoms on the petioles consist of slight bronzing or a golden color. Severe infestations on developing flower buds can reduce bud growth and size that will eventually impact yield and quality.
Biology and life history The hemp russet mite biology and life history have not been well-studied, but they are believed to be similar to tomato russet mite. The life cycle can be completed very rapidly (7-10 days) under optimal conditions. These mites most likely survive year-round on hemp plants in the greenhouse environment. However, it is not clear yet how hemp russet mites survive among plants in the field since hemp is the only known host for this mite species.
Scouting and thresholds Regular scouting of plants is required. Since mites are not visible to the naked eye and the symptoms may resemble disease and/or abiotic stress, it is recommended to collect leaves and check for russet mites under a microscope to verify infestation. Also, the mite brushing technique can be used to check the infestation level. There are no established treatment thresholds for hemp russet mite.
No research information is available on which biological agents are effective against hemp russet mites. The generalist predator, Amblyseius swirskii, may have some potential to control hemp russet mites; this agent has been shown to control tomato russet mites which belong to the same genus as the hemp russet mite.
Because there are few management options available, preventive tactics are the best way to minimize hemp russet mite problems in greenhouse production. If you suspect or see signs and symptoms of hemp russet mite, remove the suspected plants immediately and isolate until a precise diagnosis can be done. Be mindful while transporting or moving plants from one place to another, they can easily spread mites and other pests throughout the plant population.