Photos courtesy of Raymond Cloyd
Weeds can serve as alternate hosts for insects such as aphids, which can migrate to hemp and cause damage.

Hemp crops grown outdoors are susceptible to a wide range of insect and mite pests. These pests can have either chewing mouthparts, in the case of caterpillars and beetles, or piercing-sucking mouthparts, in the case of aphids, spider mites and plant bugs. If not managed early in the growing season, insect and mite pests can cause substantial damage to hemp crops. Below are five tips that will help you to manage these pests effectively and prevent damage to your crops.

1. Properly identify the pest.

The first item that requires your attention is correctly identifying the insect or mite pest that is feeding on your hemp crop so you can implement the appropriate plant protection strategies. Resources such as books, fact sheets and online sources will help you correctly identify the insect or mite pest. In addition, you can contact a state extension entomologist for identification assistance.

2. Know what plant parts insect and mite pests feed on.

Insect and mite pests will feed on different parts of the plant during the growing season. Below are plant parts with examples of insect and mite pests that feed on them.

  • Leaves: twospotted spider mite, aphids, leafhoppers, spotted cucumber beetle (Diabrotica undecimpunctata), flea beetles, grasshoppers, hemp russet mite and caterpillars
  • Stem: Eurasian hemp borer (Grapholita delineana), European corn borer (Ostrinia nubilalis), aphids and grasshoppers
  • Buds/seeds: corn earworm (Helicoverpa zea), Eurasian hemp borer, stink bugs and lygus bug/tarnished plant bug (Lygus hesperus)
3. Scout regularly during the growing season.

Scout your hemp crop at least once a week during the growing season using one or a combination of the following methods:

  • Randomly select plants and check leaf undersides, which is where aphids and spider mites are typically located. You can use a 10- to 16-power hand lens to look for the twospotted spider mite (Tetranychus urticae) and hemp russet mite (Aculops cannabicola), which are commonly found on hemp.
  • You can use the beat method, which involves placing a white piece of paper attached to a clipboard under branches or leaves, and then shaking the plant. Any insects or mites you dislodge from the branches or leaves can be seen crawling on the white paper.
  • You can randomly place yellow sticky cards or pheromone traps among the crop. The yellow sticky cards will capture adult moths and beetles, as well as certain insects in their adult stages, including thrips, leafhoppers and stink bugs. Pheromone traps attract male moths, which are then captured in the sticky substance in the trap.

All these scouting methods will detect insect and mite pests early, which will avoid having to deal with insect or mite pest outbreaks.

The beat method helps detect insect and mite infestations early.
4. Know when insect and mite pests are a problem.

Insect and mite pests will appear during certain times of the growing season. However, not all of them will be present simultaneously. It is important to know when insect and mite pests are active during the growing season so you can determine the most appropriate scouting method. For example, corn earworm larvae (caterpillars) will be present later in the growing season because they feed on the buds. Be sure to keep detailed records on the month that pests are present during the growing season, along with the crop growth stage—whether vegetative or reproductive—so you can know what to expect.

5. Remove weeds from the growing area.

Weeds can serve as reservoirs or alternate hosts for many insect pests, including aphids, leafhoppers, leaf miners and thrips. The presence of weeds in the growing area can lead to increases in insect pest populations that can migrate onto the hemp crop and cause plant damage. Therefore, removing weeds from the growing area eliminates alternate hosts for insect pests. Strategies that can be implemented to minimize problems with weeds include installing geotextile or fabric barriers, hand removal, mowing or weed eating, and applying herbicides.

Raymond A. Cloyd, Ph.D., is a professor and extension specialist in horticultural entomology/plant protection at Kansas State University. Reach him at rcloyd@ksu.edu.