A number of insects have been found feeding in hemp. To date none have been found at economic levels that would require control. As acreages increase and production becomes more intensive it is expected common pests of other crops may become a problem. No insecticides are registered for use on hemp field crops.
Grasshoppers have been a pest in isolated areas that have grasshopper populations in other crops mainly in dry or semi-arid regions. During the vegetative growing stage, hemp will generally outgrow the grasshopper infestation. The grasshopper will eat the large outside leaves on the hemp head first. High numbers cause damage to the bract around the seed preventing development of the seed and causing loss of yield. Damage to hemp has been observed to be limited in most years to the edge of fields in drier areas.
Grasshoppers can eat 30 to 100 mg of plant material (dry weight) each day. The Grasshopper life cycle starts in the fall with the female depositing eggs in an egg pod in the soil. Some development of the embryos starts in the fall but is completed in the spring after the soil warms up (over 10°C). Hatching begins in early May to mid-June to produce Nymphs which look like grasshoppers without wings. The Nymphs go through several stages to become adults with wings. The winged grasshoppers can move a distance to find food. The Nymph and winged generations as they grow bigger are able to do more damage.
Grasshopper feeding on Hemp
During years when Bertha Armyworm is a pest in Canola the larvae can be found feeding on hemp. Bertha Armyworms overwinter as a pupa in the soil. In mid-May the moths emerge and begin to lay eggs on preferred host plants including hemp. The eggs hatch to a larvae that feeds on the host plant and will do damage to the plant. The larvae when mature, drop to the soil and form a pupa to overwinter and complete the lifecycle. In most years, populations are kept low by unfavourable weather conditions such as cold winters and cool, wet weather, and by parasites, predators and diseases.
The fast growth, large plant and leaves help the hemp crop withstand relatively high populations of Bertha Army Worm. Economic thresholds need to be determined for hemp.
The European Corn Borer can be a potential problem in hemp in areas where there is a significant acreage of corn. The corn borer over winters as a caterpillar and changes to a pupa in the soil in the spring when conditions are ideal. The Corn Borer emerges as a moth in mid- May to June. The moth lays eggs on a suitable plant host which can include hemp. The eggs hatch to a larva which does the damage to the host plant. Borer larva will enter the hemp stalk through a pin hole and feed in the center of the stalk. This weakens the hemp stalk reducing the yield potential and leading to stalks breaking.No control or economic thresholds are available for hemp.
Corn Borer in Hemp stalk
Cutworms are a spring pest and they can occur in hemp production as in other crops. Spring scouting is recommended. Areas of dead plants or bare patches in the field are evident of cutworm activity. Cutworms rarely are found across the whole field.
Stink Bugs have a hard shell and are distinguishable by a distinctive smell. They appear in hemp near harvest time. Stink bugs feed by sucking juice from the plant which causes damage to the hemp seed head. This will damage the bracts around the seed and the seed will not develop. Stink bugs have been found in a few isolated areas in hemp fields. No economic damage has been reported to date.
Stink Bugs feeding on Hemp
Lygus Plant Bugs are common to crops like canola, alfalfa and vegetable crops. Lygus Bugs puncture the plant with their sucking mouth parts. In hemp, high numbers feeding on the bracts could cause significant loss in yield. The piercing of the plant with the mouth piece also can spread diseases like Aster Yellows.
Blister Beetle are common in crops such as fababeans. Damage is done by the insect chewing on the seed heads.
Blister Beetle defoliating Hemp
Birds such as sparrows, doves, starlings, blackbirds, etc. can be pests in a hemp crop. Hemp is often one of the later crops harvested which coincides with the time of year that birds begin to flock for southern migration. Migratory birds seek a source of food, water and somewhere to perch. Areas of field topography with these features encourage the migrating flocks to stay near the hemp field. The hemp seeds at harvest protrude from the seed head and are easy “pickings” for the birds.
The birds will leave droppings on the hemp head while feeding causing microbial contamination. This is a problem as the hemp grain is consumed without processing. Microbial contaminants could include molds, yeasts and bacterial coliforms, E. coli, Salmonella and Staphylococcus.. See Storage Conditioning for more details.
Bird droppings need to be monitored and eliminated from contaminating equipment, i.e., grain tank from parking the combine in the field for a few days, truck boxes, augers, etc..
Storage bins are also a source of contamination that need to be monitored and sealed from bird entry.
Deer and other grazing wild animals will feed on hemp when it is in the vegetative stage. While feeding, they will graze on the top portion leaving the root and bottom stalk. This stalk will branch from a node and produce a plant with two stalks. These stalks are smaller and will produce heads with viable grain at harvest time however maturity will be delayed.
Two stalks as a result of growing point damage