Mite diversity, abundance, and potential impacts on hemp

Observations of industrial hemp, defined as < 0.3% total THC, during the North Carolina 2019 and 2020 seasons suggest that Tetranychus urticae (two-spotted spider mite), Aculops cannabicola (hemp russet mite), and Polyphagotarsonemus latus (broad mites) are primarily greenhouse pests but have also been observed in field production. Spider, broad, and russet mite injury can cause cupping and yellowing of leaves, resulting in leaf drop, reduced flower and resin production, and plant death. Along with the physical damage caused by mite infestations, growers are concerned with how the feeding may affect the cannabinoid concentrations. Initial experiments evaluating the effects of different arthropod feeding modes on cannabinoid concentration showed that the change over time for THC and CBD under high infestation (~200) of T. urticae had a greater reduction in concentration than the Controls and low infestation (~50) T. urticae (THC: p=0.0003; CBD: p=0.00025). Results also showed that two-spotted spider mite density is strongly correlated with reduced CBD concentration.