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Marijuana Growers Need To Beware Of These Pesky Insects

In places where it's now legal to grow cannabis, you don't have to worry about the Feds destroying your crops anymore. But you do need to worry about bugs.

Cannabis cultivation is becoming a big part of the North American agricultural landscape, but it's still a relatively young industry. With that comes a host of problems that growers have to face without being able to rely on generations upon generations of experience. Among the biggest issues for marijuana farmers are crop-destroying bugs.

"This is a crop that doesn't have much history being grown outdoors," Dr. Whitney Cranshaw - an entomologist who has spent years researching insects that feed on hemp - told Civilized. "And I think as we have people growing it year after year after year - particularly if they don't do much rotation - then we're going to see more problems."

Cranshaw operates a website called Hemp Insects, which is loaded with information on the kinds of pests hemp farmers can be expecting to deal with. The website has entries on dozens of potentially detrimental insects, but Cranshaw says there are 4 that will be of particular concern to cannabis growers - whether they're cultivating hemp or marijuana.

The Corn Earworm

Corn Earworm on an onion

Corn Earworm on an onion

The corn earworm - also sometimes known as the tomato fruitworm, or the cotton bollworm - is one of the most widespread agricultural pests in America. But don't let the names fool you. They're actually the larval stage of a moth known as Helicoverpa zea. These larvae can borrow into seeds and buds, making cannabis crops unsuitable for harvesting.

"Corn Earworm is a caterpillar that will raid the buds late in the season and if we have an outbreak year there's gonna be quite a bit of damage," Dr. Cranshaw told Civilized. "Corn Earworms are the one that first became obvious to me when I've been looking at these hemp fields for the last four years."

What can you do about it?

Because of their prevalence in other crops, ways of dealing with the corn earworm are well established.

"That one you can track using a trap and that will give you an idea of how abundant the insect is."

Once you know you've got earworms, you can simply hit them with a pesticide.

"For the corn earworm it's basically monitoring it and if you have evidence that there's a big flight of the adult moth at a critical time in the crop - usually September around here - then there's two products that we suggest you could treat them with: ones a Bacillus Thuringiensis product and the other is a virus."

Second In Command: Hemp Russet Mite

A Scanning Electron Microscope Image of an Eriophyid

While not a whole lot is known about the hemp russet mite right now, it is assumed that they are very similar to related species of eriophyid mites, which includes the tomato russet mites. These microscopic bugs show up later in the season, after buds have begun to develop. They feed on nearly every part of the cannabis plant and can seriously affect both the quality and yield size of your harvest.

"I'd say that the hemp russet mite is right up there with the Corn Earworm in terms of being the two that are most capable of causing significant plant damage in this field growth," Dr. Cranshaw told Civilized.

The worst part is that they're tough to detect. 

"The mite, you can't see it - it makes a spider mite look like an elephant. I mean they're really tiny," but they can cause huge problems, according to Cranshaw.

What can you do about it?

The best tools to combat the hemp russet mite are vigilance and tidiness.

"The big thing that I'd say now is clean up," Cranshaw said. "The big thing for hemp russet mites is to learn about it and whack it in the greenhouse so you don't have it [in the field]. Treat your cuttings, make sure you're not blowing them all over your greenhouse. These things blow in the air."

And if one plant is infested with them, you'd be wise to sacrifice it rather than risk contaminating the entire crop.

"Don't put out plants that are infested. Hit this in the greenhouse production phase," he urged. "If you don't transplant plants that have this, you should be in pretty good shape in the first place. Some growers don't have any hemp russet mites in their field because they didn't have it in their greenhouse. And others have some and then they build up big populations four months later."

If you do end up with a large infestation of hemp russet mites, there isn't much you can do to get rid of them, though pesticides are currently in development.

"There are various oils that are probably going to be pretty good and we're looking at this in terms of a research project. Perhaps we could introduce certain predator mites. There's one in particular we're looking at that could colonize the plant and suppress the russet mites after it's put out in the field."

The Cannabis Aphid

Aphids on a plant stock

Aphids are another one of those pests that are well known to anyone with previous knowledge of plant cultivation. The cannabis aphid is like a leech because it sucks fluid out of the plant, slowing its growth and causing wilting.

"There can be huge, huge numbers of cannabis aphids, particularly late in the season developing on some fields," Cranshaw told Civilized.

What can you do about it?

In most cases, the cannabis aphid isn't much of a concern. While they can develop in huge numbers there are plenty of natural predators that will take care of them. When it comes to indoor cultivation, the aphid population is mostly controlled by parasites that devour the bug internally and leave remains that resemble a mummified corpse.

"We're trying to figure out which parasite is the one that's attacking it," Cranshaw noted. "We see mummies but we don't know what species of parasite attacks it, so once we get that identified then we'll have a specific one to recommend."

Outside, there are no shortage of takers for an aphid buffet.

"Even though they build huge populations outdoors, there's natural enemies that will crush them. I would see huge numbers building on in fields last year in September. And then three weeks later, they were toast. I mean, everything on the planet was eating them: lady beetles, lacewings, flower flies - everything."

So unless you have a major infestation, you can probably let nature takes its course with aphids.

The Newcomer: Eurasian Hemp Borer

GettyImages 984977852

One of the potential threats that Cranshaw says he's watching out for is the Eurasian hemp borer, which is known for riddling cannabis leaves with holes (much like the very hungry caterpillars in the picture above). While it has yet to become common west of the Rockies, the Eurasian hemp borer is already proving to be an issue for cannabis cultivators on the east coast.

"It's a tiny little moth, really tiny. Develops in the stalks and stems and has multiple generations. Later ones can get into the the heads - this one could also be an issue in seed production, too."

What can you do about it?

As a relatively new threat, there isn't much in the way of established pesticides or predators for the Eurasian Hemp Borer just yet.

"Eurasian hemp borer - still trying to figure that one out," Cranshaw told Civilized. "I'm not sure how that's going to play out."


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