Contrary to what some people may believe, hemp is not a pest-free crop.
I like to remind prospective growers that hemp is a plant, and plants get eaten by lots of different organisms that can be nuisances and cause economic or aesthetic damage.
For now, let’s focus on those pests that could or are causing injury to the hemp plant, either directly by feeding on the marketable portion of it or indirectly by feeding on other plant structures that could cause a reduction in the plant’s productivity.
More than 300 arthropods, e.g., insects, spiders and mites, are associated with the cannabis complex worldwide, according to the article “Cannabis Pests.” However, not all of these are considered pests, as only a small portion of them are likely to cause injury to the plants. In fact, some of these arthropods are beneficial to our environment, including predators like lady beetles and parasitic wasps. We even find that hemp grown for grain or fiber fosters a diverse population of bees (Biomass & Bioenergy 2019; Environmental Entomology 2019).
Some insects only feed on leaves, like flea beetles, and the damage is relatively minor. Because beneficial arthropods exist in the hemp complex, we have to be mindful of them when we manage pests.
Identifying Pests and Damage
Identifying pests can be difficult, but having the right tools can help. For small insects and mites, growers should have a hand lens or jewelers’ loop for in-field or greenhouse identification. Pests may not always be present when you scout plants, so taking photos of the type of damage is one way to determine what may have caused it.
Familiarizing yourself with the different groups of insects is another important step to figuring out if what you are observing is causing damage or just hanging out. Some insects that hang around hemp plants are beneficial, so it’s important to research before taking action.
Several resources for arthropod identification exist. Field guides, county extension offices and university fact sheets and bulletins can be helpful tools. They are low-cost or free for growers and often come in printed and online versions. Smartphone apps, such as iNaturalist and Purdue University’s Plant Doctor, can be used to identify insects and mites as well.
Some of our growers opt to post photos on social media for identification. This can be a great resource, but make sure those giving feedback have pest identification experience. When in doubt, try to get a hold of an extension educator or researcher with entomology experience at a university.
Management Options and Biological Control
So how do growers manage pests once they identify them? The type of pest dictates what action is necessary and the options available.
It is important to note that many states do not allow application of pesticides in hemp. Some states do have long lists of products that can be used for pest control; however, most Midwestern states only allow the minimum risk pesticides listed under Section 25(b) of the Environmental Protection Agency’s Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). Hemp growers should always check with their state’s pesticide office on what products they can or cannot use.
Also, remember, not all insects that feed on hemp cause damage that would warrant control measures. Because we have limited research on economic loss due to pests and have not yet developed these thresholds for hemp, making the decision to spend money on management tactics can be difficult.
Here are a few pests that are raising concern for growers in multiple states based on preliminary survey data from growers and feedback from educators.
Corn earworms can vary greatly in color from brown to bright green. This pest has a wide range of host crops, but corn, cotton and tomatoes are three of the most notable. They can also cause a lot of damage to hemp plants, most significantly to female flowers and seed heads, where larvae feed for the duration of their immature stages. This feeding can lead to mold growth, and many growers refer to this as bud blast or bud rot.
Depending on the state you grow in, you may have the option of applying a pesticide. For example, the state of Kentucky recently got approval from the U.S. Environmental Protection Agency to allow growers to use a nucleopolyhedrovirus (NPV) for corn earworm under a 24(c) special local needs packet of FIFRA. In Colorado, some Bacillus thuringiensis (Bt) products can be used for corn earworm suppression.
Eurasian Hemp Borer (Grapholita delineana)
Eurasian hemp borer larvae have a pinkish to orange hue and are found in small diameter stems. These pests cause the most damage later in the growing season when larvae feed on the flowers and seed heads. The difficulty with this pest is the boring behavior.
When an insect bores into plant material, it is protected from any kind of chemical application. Early in the season, this pest stays in a small area of stalk or stem. Later in the season, you can see more damage toward the tops of the plants, where the larvae feed from the inside of flowers.
Control for this pest is difficult. We need to develop monitoring tools and explore what kind of parasitic wasps may attack Eurasian hemp borer larvae.
Potato Leafhopper (Empoasca fabae)
Potato leafhoppers are bright yellow/green insects that suck fluid out of plants. Alfalfa and potato producers are familiar with this pest and the characteristic scorching appearance, called hopperburn, to the leaves. I have heard few reports of hopperburn in hemp, but the potato leafhopper damage at one field site in Indiana was extensive. The hopperburn appeared to be worse on certain cultivars of hemp grown for cannabidiol (CBD), so cultivar selection may become an important management decision to reduce potato leafhopper damage.
Some pests tend to attack hemp grown indoors. The following can be found in outdoor hemp production, but more complaints have come from indoor producers. Many complaints started at the beginning of the 2019 growing season for Indiana hemp producers, largely due to plant material arriving already infested with aphids and mites. Growers should follow good sanitation practices and start with healthy plant material, especially since there are so few, if any, products they are permitted to use, and many farmers producing floral material do not want to apply anything. There is increasing concern about pesticides concentrating to harmful levels through the extraction process and pesticides creating toxic by-products when treated floral material is heated or vaporized.
Cannabis Aphids (Phorodon cannabis)
Cannabis aphids are closely related to hops aphids, and for a while, many thought they were the same species. However, cannabis aphids have hornlike projects near their antennae, a morphological difference between cannabis and hops aphids.
Aphids can be found under leaves and on stems. They can also be identified by cast skins and shiny leaves caused by their waste. It is unclear how heavy of an infestation will cause yield loss. Some growers use commercially available biological control options to mitigate aphids, including parasitic wasps and lacewings.
Spider Mites (Tetranychus urticae)
Spider mites are a common greenhouse pest and can be found outdoors when conditions get very dry. They are easy to identify by the webbing they cause on plants. Some growers will purchase predatory mites for biological control.
Hemp Russet Mite (Aculops cannibicola)
Hemp russet mites are much smaller than spider mites and have to be identified using a hand lens or microscope. These tiny mites do not cause webbing like spider mites, but they can cause a lot of headaches for growers. They can cause leaf curl and russeting on hemp. Sometimes leaf curl can appear without any russet mites and could be genetic or caused by a pathogen. This is where a hand lens is essential to see if mites are on the leaves for proper identification. Biological control options for hemp russet mites have not been widely researched.
This is by no means an exhaustive list of outdoor and indoor pests you may encounter, but it should give you a taste of what can attack hemp, as well as some options for pest management. Brushing up on identifying pests and the damage they cause can mitigate potential problems during production—especially if they have a known presence in your state—and help prevent plant loss.