Spider Mites & Leafminer on Hemp:
The last 2 weeks have shown a definite increase in the spider mite damage to the crop. Pictures on the left are of typical damage. Two-spotted Spider Mites are what have been identified. The damage rages from light speckling to marked discoloration/bronzing. The hot/dry of last week spurred on populations and increased levels of damage. When the plant is stressed from lack of water and high temperatures the damage is more noticeable. To see the spider mites, you would likely need a lens. They are quite small but are identified by the 2 prominent spots on their back. Sometimes, when populations have decreased, you can still see webbing or frass (tiny black specks of excrement) or cast skins/carcasses on the undersides of leaves. The rain on Sunday/Monday decreased populations in many fields. The jostling and wetting dislodges them.
Spider Mite damage results in loss of photosynthetic area and could possibly lead to reduced production. Possibly, because the mites create wounds with their feeding, there would be increased disease infections from these wounds. Spider mites can come from weeds or neighboring crops such as corn or soybeans where they are a somewhat common pest.
Good factsheet on mites: http://www.greenhouse.cornell.edu/pests/pdfs/insects/TSSM.pdf This was written for greenhouse producers so it includes a section on the cyclamen mite which is not our concern in hemp in fields.