Grow Opportunity

The problem with pests

Early intervention key to successful control

October 23, 2020  By Peter Mitham & Greta Chiu

Fungus gnats are one of the most common pests affecting cannabis crops. (Photo: Amelia/Adobe stock)

With few effective pest management options in their arsenal, cannabis producers in Canada need to emphasize preventative measures and target their resources towards the right pests.

While greenhouses offer a controlled environment for crop production, insects and other pests eventually find their way into the structures. Being in a closed environment can make it even more challenging to manage them.

This is why Suzanne Wainwright-Evans told growers attending the CannaTech West program, alongside the Pacific Agriculture Show in Abbotsford last winter, that prevention is key. A well-travelled speaker on pest management and principal of Buglady Consulting Inc. in Slatington, Pennsylvania, she’s seen some spectacular infestations, underscoring the need for proper sanitation, scouting, identification and management.

To head off infestations, one of the first things that growers should do is to implement a stringent quarantine process. ”When you’re designing a facility, it’s important to include a quarantine area. But if you don’t currently have one, look for an area at your facility that you can retrofit or use as a substitute,” says Wainwright-Evans.


New plants, need to be quarantined for a few weeks before they are put into production, giving any unknown pests enough time to develop and appear. For early detection and monitoring, sticky cards placed in quarantine and high-traffic areas are a great way to check for adult flying insects such as thrips, fungus gnats and whiteflies, but they’re less effective on crawling pests such as mites. “That’s why you have to physically look at the leaves, do the bang board test and scout the plants really well,” she says. Large mass trapping cards should also be placed as a preventative measure.

Because of the strict requirements set out by Health Canada, insects and mites cannot be found in the bud, regardless of whether they’re pests or beneficials. The presence of thrips, for instance, have not been correlated with a reduction in bud yield, but it’s still important to manage them for a better quality product.

Fighting off fungus gnats
“One of the more common pests we’ve been seeing in cannabis are fungus gnats, which feed on the roots of plants,” says Wainwright-Evans. What’s more, research has shown that this pest can vector plant pathogens such as Fusarium, a key problem in cannabis crops.

For control, the beneficial nematode (Steinernema feltiae) has been highly effective in managing fungus gnat larvae, in addition to a few other pests, and should be a routine preventative step in any pest management program. While they’re compatible with many crop protection products, the key to effective nematode use is knowing how to properly store, handle and apply them. “You have to make sure you read and follow the labelled directions,” she says, as she’s seen growers putting out dead nematodes before.

Which Phorodon is it?
The cannabis aphid (Phorodon cannabis) is a growing concern, in part because many producers confuse it with the hops aphid (Phorodon humuli). But it’s not, and hops aphids won’t infest cannabis even when given the chance. Because P. cannabis acts differently from other members of the same genus, producers cannot simply apply the same management techniques and expect them to work.

What is known about the cannabis aphid is that it is highly reproductive, giving birth to live young, and in some circumstances, even laying eggs. However, notes Wainwright-Evans, “For indoor cannabis production, eggs are less of a concern.” Up until now, she hasn’t seen any evidence of eggs hatching in greenhouses or indoor grow operations, likely because the environment isn’t quite right for it.

“Cannabis aphids can be challenging to manage,” says Wainwright-Evans. Any operations currently infested with the pest will likely need to apply an initial knockdown spray. This will reduce the number of cannabis aphids to a more manageable level, giving beneficials a chance to get ahead.

“Biocontrol agents are not going to control an infestation of this pest. You have to start when the pest numbers are extremely low,” she says, explaining that biocontrols are not meant to be a rescue treatment. “Once your numbers are down, then you can come in and release parasitic wasps, like Aphidius colemani, A. ervi, and A. matricariae in addition to using more generalist predators like green lacewing larvae. Managing this pest is a multi-pronged approach. There’s no silver bullet.”

Wainwright-Evans also called out rice root aphids (Rhopalosiphum rufiabdominalis) as a particular concern. She’s seen this one in many cannabis production facilities throughout the US and Canada. “The rice root aphid is a classic example of why you need a good quarantine area because most infestations come from people bringing in infected plant material. Since the aphids are in the media, people often miss the pests that are there. You have to start managing this pest at the propagation stage.”

Start from the beginning
Targeting major pest issues at the young plant or clone stage essentially gives the crop, and beneficials, a head start. “If you don’t have a strong program in prop, you’re going to be fighting the pest uphill the whole time.” It’s also less expensive to treat for pests early on. Not only is there less plant material in a smaller area to manage, producers can stop problems before they start, and overall, that means less product needed.

Since cannabis doesn’t have access to many of the traditional agrochemical products used in other greenhouse crops, growers really need to focus on sanitation, good-growing strategies and incorporating a strong preventative biocontrol program to head off any issues.

With commercial cannabis production in its infancy, not only are there limited resources to help producers identify and manage pests, but the information is changing at a rapid pace. Wainwright-Evans cautions that there is a lot of misinformation online, and even the right information can become outdated quickly as the results of new studies emerge. Producers need to make sure that they’ve found the most recent information and from a trusted scientific source.

She also underscores the importance of working with a qualified crop consultant or supplier to help put together the right preventative pest management program for the operation. “Every cannabis facility is different,” she says. “You have to look at what the key pests are at the facility and which products are going to work in your environment.”

Limited options
While many crops enjoy an established set of tools for managing pests, the relatively new cannabis sector isn’t so lucky. Unlike in the U.S., all pest control products – even minimum risk pesticides – must be registered with Health Canada prior to use in their intended crops. There is no list of exempt “minimum risk” materials in Canada.

This means biopesticides are the default backup plan for cannabis growers, says Caroline Bédard, provincial minor use coordinator with the BC Ministry of Agriculture. She points growers seeking pest control options to Integrated Pest Management for Commercial Cannabis in BC, which lists more than 14 approved biopesticides, including canola oil, garlic powder, hydrogen peroxide, and strains of the beneficial fungus Trichoderma harzianum and Beauveria bassiana.

This article was originally published in Greenhouse Canada, a sister publication of Grow Opportunity.

Peter Mitham is a freelance writer and photographer in British Columbia.
Greta Chiu is the editor Greenhouse Canada magazine.

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