There is a prevalent myth that Cannabis is pest and disease free. In fact, nearly 100 fungal pathogens, numerous bacteria, viruses, and parasites, and 300 insect pests have the potential to interfere with your healthy Vermont hemp crop. This is true whether you're growing for seed, fiber, or CBD.
Fortunately, taken together very few hemp pests and diseases cause economic damage. It would be more accurate to say that Cannabis is pest tolerant, given its ability to stand up to natural predators.
What to do though to ensure your Vermont organic hemp crop triumphs over these insurgents?
Prevention is key and early detection essential. You can stay ahead of most of these problems with daily crop monitoring; scouting for evidence of pest or disease damage, nutrient deficiencies, or signs of over- or underwatering.
But one day, sooner or later, you're almost sure to encounter plant stress indicating you have a pest or fungal outbreak on your hands.
The good news is, there is an abundance of online resources to help you. Among them, the most authoritative guide to identify and counteract hemp pest and disease pressures is Hemp Diseases and Pests Management and Biological Control, written in 1996 by one of Vermont's own, J.M. McPartland Ph.D. and his colleagues R.C. Clarke, and D.P. Watson.
McPartland and his colleagues' body of work on the topic is unsurpassed. Their book, and the voluminous research it pulls from serves as the primary source material for this article. We encourage every enterprising hemp farmer to own a copy and have it on the shelf as your primary reference when nature strikes.
If your daily practice of scouting your crop reveals a pest or disease problem, our advice: keep calm, and thumb (or scroll) to the back of McPartland, Clarke, and Watson’s masterpiece to Appendix 2. This is the diagnostic section titled, "Dichotomous Key of Diseases and Pests". Step-by-step, this key takes you through a symptom identification procedure that covers over 90 common cannabis problems, pests, nutritional deficiencies, and diseases.
For an excellent pictorial guide to hemp insect pests, also check out Colorado State University's Hemp Insect Factsheets.
As you read on, use our image below to help identify the parts of the plant where the most common predators are likely to attack. Then, what follows are brief descriptions and recommendations on some of the best cultural and mechanical methods used to control them.
Top Common Hemp (Cannabis) Pests
There are a few important hemp pests quite capable of inflicting economic damage. Among them, two standouts are the European corn borer (Ostrinia nubilalis) and the Hemp borer (Grapholita delineana).
European Corn Borers - The ECB has an incredible appetite and is one of the most wide-ranging and destructive pests that can impact a hemp crop. ECBs can bore into and infest any plant with a large enough stalk. For this reason, planting a field of hemp next to a corn crop should be avoided.
McPartland writes, "(ECB) Caterpillars are light brown with dark brown heads (see photo layout below). Along the length of their bodies are found several rows of brown spot-like plates, each sprouting setae (short hairs). Mature caterpillars may grow to 15-25 mm (5-9 in.) long."
For a full description and explanation of the ECB lifecycle, see pg. 45 of Diseases and Pests Management and Biological Control.
As young caterpillars, they munch on the leaves, then when half-grown the ECB larvae will bore their way into small branches and stems. The bore holes they leave behind create entry points for other insects or fungal infection.
And so begins the real damage.
These areas where the larvae infiltrate become structurally weak, but it's the infections that cause wilting in the smaller branches or stem cankers in the stalk. The weakened stems can break under the weight of full-flowering tops, and whole plants can collapse under heavy infestations. Not a pretty picture.
Hemp Borers - Everything in nature has a niche, and in the case of the Hemp borer (HB), researchers have identified it as one of the few host-specific pests to Cannabis.
HBs are smaller than ECBs, and the HB can cause similar stem damage (see photo layout below). The HB is much more destructive in the flowering tops, however, because of the larvae's late-season life cycle. McPartland's full description starts on pg. 48 of Diseases and Pests Management and Biological Control.
Flea Beetles - Flea beetles and the hemp flea beetle (Psylliodes attenuata) in particular, can be a serious pest for outdoor crops. The adults have large hind legs and this beetle leaps like fleas when disrupted (see photo layout below).
Adult flea beetles feed on the leaves and flowers, while their larvae (grubs) feed on the roots. Flea beetles are visible to the naked eye, and the first signs of plant injuries are small pits in the leaves produced by chewing. As leaves develop, these pits usually form shotholes in the interior of the leaf (see photo layout below).
There are a few other odious creatures worthy of mention. According to McPartland et al., these are listed below in descending order of importance (in terms of the potential for economic loss).
- Human thieves,
- Cutworms and Budworms,
- Leaf-eating caterpillars,
- Aphids and Slugs,
- Crickets, Weevils, and other beetles.
See Chapter 4 of Diseases and Pests Management and Biological Control, which covers identification and symptoms of these common pests in detail.
Recommendations For Hemp Pest Prevention And Control:
- Destruction, burial, or removal of crop residues after harvest will deter many pests and keep them from becoming a problem. Roots can be plowed in.
- Deep autumn plowing and spring tillage done correctly expose pests and pathogens to their natural enemies and the weather.
- Cultivate well or use cover crops to keep weeds in check (see our previous article, here.)
- Use crop rotation, optimize soil structure, and introduce beneficial predators like ladybeetles, green lacewings, and Trichogramma minutum wasps. These and other organic farming best practices will lead to a more pest and disease resistant crop.
- Transplanting in June, the early part of the "planting window", helps seedlings to harden before pests start to arrive. If faced with a late-season planting, and problem pests have arrived, be extra vigilant.
- Remove pests in the early stages by hand picking. Neem oil containing azadirachtin can be effective (but don't spray neem on hemp flowers!). Or at any stage, try soap sprays or applications of Bacillus thuringiensis (BT).
For a full account of useful countermeasures, see McPartland et al., Chapter 9: “Cultural & Mechanical Methods of Controlling Diseases and Pests”.
Always keep in mind, your best defense against pests and disease is:
- Build healthy soil with lots of organic matter,
- Use beneficial inoculants, and
- Grow vigorous plants.
Then, as the crop matures, hemp's natural defenses (and higher numbers of natural pest predators) make an economically damaging event less likely.
This article is the first of a 2-part series covering the most common hemp pests and diseases likely to affect a high-CBD crop. In Part II, we'll cover hemp diseases and controls. See you next week!
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