Research Highlights Four Main Pests Outdoor Cannabis Growers Should Be Aware Of

Ask any farmer, and they’ll tell you how crucial it is to protect their fields. But with cannabis, you’ve got to be extra careful.

As a valuable crop that’s just opening up into the medical industry, crop protection and safety is an important endeavor. By upholding good farming practices and hygiene protocol, you can greatly reduce the risks of losing the crop due to diseases and infestations. Moreover, with cannabis, you need to produce a healthy yield consistently to ensure that consumers receive a high-quality end product.

The problem is that many insects love cannabis. So Whitney Cranshaw, a professor at Colorado State University, led a team of researchers. They identified 150 genera of insects, mites, and spiders that thrice in hemp fields. The field samplings were taken during 2016, 2017, and 2018 growing seasons in Colorado.

Researchers did not study marijuana fields because it’s federally illegal. But since hemp and marijuana are similar, many of the findings from hemp research are being used for marijuana crops.

Researchers say that even though there was an overwhelming amount of insects discovered in the study, only four of them contribute to most damage.

An article published in Hemp Industry Daily looks closely at these four main pests.

Cannabis Aphids

The cannabis aphid is a new pest to North America. It was first seen in Colorado in 2016 and is now widespread throughout other states and parts of Canada.

Adult cannabis aphids are about 1.8 to 2.7 mm long. They are generally colorless to pale yellow in the summer. But as fall approaches and the days get shorter, their color changes to light green or brown with dark green stripes that run the length of the body.

Multiple plants bunched together.

Like all aphids, cannabis aphids also suck sap. Affected plants get damaged due to the loss of vital fluid, resulting in slowed plant growth and wilted, yellowing leaves. 

Additionally, when excess sap is excreted on leaves and flowers, a sticky fluid called honeydew forms. This promotes the growth of black fungal molds on the buds.

Cannabis aphids usually reproduce asexually. That means that females are born pregnant. They give birth to about 1-5 young per day. Each female adult will live about 3-4 weeks.

But another problem arises as winter approaches. The shortening day length signals the males and females to mate and produce eggs. The eggs are laid on flowers and leaves and stay dormant throughout the winter. In spring, the cycle begins all over again.  

In high populations, cannabis aphids can cause serious problems for outdoor growers. 

Russet Mites

Hemp russet mites are some of the most damaging and difficult-to-control pests. They are hard to spot and reproduce quickly, so build a large population before symptoms of an infestation are evident.

Hemp russet mites are about 0.2 mm long, elongate and pale. A russet mite’s life cycle (from egg to adult) takes about 30. Adult mites can live for approximately three weeks. Each female lays 10 to 50 eggs during her lifespan.

Often cultivators have to look for russet mites with hand tools and microscopes. And because they are so small, they can be blown around with air currents.

The initial symptom of hemp russet mites is the upward curling of leaf edges or margins. But this doesn’t necessarily happen all the time. Crops heavily infested with these pests are gray or bronzed. This is because russet mites feed on the epidermal cells, killing the first top layer of the leaves. This creates a grayish hue. The mites then continue to move on to newer growth and buds, eventually damaging the flowering parts.

Unfortunately, little is known about dealing with them. So Colorado State University is focusing on these insects more than others. In a trial last year, Cranshaw’s research team used sulfur as a treatment. 

Eurasian Hemp Borers

The Eurasian hemp borer is currently found east of the Rockies. It’s become a prevalent pest in eastern Colorado and outdoor cannabis cultivation sites.

This pest attacks cannabis flowers. It causes stem girdling at the base of buds and the leaves to wilt. 

The Eurasian hemp borer has several generations in a season. The first generation can be found in the stems, appearing early in the season by June. When they’re in the stem, you may see wilting near the point of entry. The best way to check is to cut at the point where you see some wilting leaves. If you find the tunnels within, you’ve identified the pests.

The next generation can be completed later in the season, possibly by July. This is when flowers are in production. If left untreated, the tunneling might extend into the bud. Besides damaging the plant, this insect leaves it vulnerable to other insects.

For cannabis growers that focus on flower buds and seed development, the presence of Eurasian hemp borers can seriously stunt production. One study found that Eurasian hemp borers caused a 28% reduction in seed production.

Corn Earworms

Corn earworms are one of the most widespread agricultural pests in America. They’ve attacked everything, from corn to cotton to tomatoes.

These pests cause a great deal of damage, posing a severe problem for outdoor cannabis producers. Corn earworms are caterpillars that raid the buds late in the season. They’re attracted to the plants when they start budding flowers. So they don’t lay eggs on the leaves till you get flowers.

Cannabis plant leaves.

Young corn earworms don’t do much damage. But once they molt and get bigger, they do a lot of chewing and destroy the bud. And they can bore through multiple buds during their lifespan.

Furthermore, the caterpillars defecate and leave pellets everywhere, allowing the rot to start. But because they are commonly found in other crops, cultivators learn ways to deal with the corn earworm.

Wrapping Up

Outdoor cannabis production is a challenging responsibility, especially when it comes to pest insect control. But when plants are being grown for medicinal purposes, extra care needs to be taken.

Researchers have identified four key pests that are causing the most damage. It’s a problem that every cannabis grower needs to consider when growing medical cannabis.

Everyone who grows marijuana needs to follow strict protocols to produce the best and safest cannabis for their consumers. Many are looking into natural insecticides derived from organic compounds or plant extracts. 

They carry less risk of harm to the end consumer and the environment. Others are interested in beneficial insects that feed on agricultural pests but do not disturb the marijuana plants.

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