A guide to understanding pests, pesticides and the rampant misinformation circulating around the cannabis industry
By Suzanne Wainwright-Evans
Controlling pests on plants is a complicated business.
Look at crops such as corn or soybeans, which have been grown on a large scale for many years. Millions of dollars have been spent on research, and growers have a wide range of clearly labeled pest control options for specific crops and conditions, yet growers still struggle to control their pest issues.
This is the first in a series of articles addressing pest issues of marijuana. Articles in future issues will cover specific pests and their recommended management techniques with a focus on biological control.
I have worked with commercial growers cultivating a wide variety of traditional nursery, greenhouse and agricultural crops for 20 years now. When I began to get calls asking for help with marijuana, I did some searching to see what sort of information was offered already. I was horrified by some of the recommendations being made.
Two of the biggest shocks were the disregard for human safety and the lack of understanding about how the chemicals work. To top that off, many recommendations simply would not work for the pest they were trying to control —assuming the pest was even identified correctly. Other recommendations would just create more problems for the grower.
This is a very touchy subject because of federal regulations on pesticide labeling. It is a violation of the law to apply a pesticide that is inconsistent with the label (including the specific type of crop and growing method, such as greenhouse, field growing, indoor facility, etc.). Since marijuana is still federally illegal, this crop cannot be specifically listed on pesticide labels approved by the Environmental Protection Agency.
Pesticides (including insecticides, fungicides and herbicides) are regulated by the EPA, and it is a long process to get a product registered. So from a federal standpoint, there are no approved pesticides for marijuana in the U.S. This leaves many growers guessing what to do, often relying on the advice of other growers or information found online. This is less than ideal in terms of making sound decisions on which control method and products to use.
Keys to remember:
– With today’s technologies, labs can test any plant material for any pesticide residue down to the parts per billion. Keep that in mind when selecting a product to use.
– Prevention is key. When bringing in plants from outside of your growing facility, there should be a quarantine area where new plants can be held for observation.
To get started, I can tell you what not to do.
It may seem obvious, but do not believe everything you read on the Internet — or even in print. A lot of the information I have found is either inaccurate or just not good information. One recommendation I see regularly is the use of imidacloprid (sold as Merit, Marathon, Bayer Tree & Shrub Insect Control, Bayer Fruit, Citrus & Vegetable Insect Control and many other labels). This product is in the class neonicotinoid, named because the compounds in it are similar to nicotine. I see imidacloprid recommended for controlling pests such as root aphids, whiteflies, fungus gnats and other pest insects.
The first issue of concern is that imidacloprid is a systemic pesticide. When it is applied, it will move upward inside the plant, throughout the xylem tissue, where it stays for a long period of time. It cannot be washed off the plant. Imidacloprid is registered for some food crops, but marijuana is not always consumed by the human body in the same methods that you would a tomato or other traditional food crops.
The second glaring issue is that the use of imidacloprid is known to cause plants to have spider mite flare-ups. Two-spotted spider mites (Tetranychus urticae Koch) are one of the biggest pests of marijuana. Why on earth would you treat this crop with a compound that could make your mite problem even worse?
Imidacloprid offers no control for spider mites, but how does it create a mite problem? Research by David G. James and Tanya S. Price at Washington State University found that two-spotted spider mites that feed on plants treated with imidacloprid produced 10-26% more eggs during the first 12 days of adult life. (More information can be found in the 2002 Journal of Economic Entomology article “Fecundity in Twospotted Spider Mite (Acari: Tetranychidae) is Increased by Direct and Systemic Exposure to Imidacloprid.”)
So how does one go about dealing with pest issues? First and foremost: identification, identification, identification.
No matter what crop you are growing, you must know your enemy so you can understand how to prevent or manage it. Sometimes you can find out what pest you’re dealing with by simply matching it to photos online. But beware: just because there is a photo online does not mean that it was identified correctly by the original poster. I have seen many insects misidentified online. Also, look at the source; try to stick with university websites, or reputable resources like Bugguide.com (which is continually vetted by entomologists). Avoid forums or sales websites where anyone can post anything.
Once your pest insect or mite has a positive ID, a control option will need to be selected.
Start by determining if the problem can be mechanically removed. Sometimes this can be done by washing the plant. Washing plants raises the humidity, something two-spotted spider mites do not like. They prefer hot and dry environments, but the flip side is that wet leaves can lead to more foliar pathogens like powdery mildew.
If the pest cannot be washed off or removed, see if it’s something that can be managed using biological control. The bio-controls of today are much more developed than the bio-controls of 10 years ago. Many advancements have been made in application methods and the quality of the beneficials (including insects, mites and nematodes). A great deal of research has been done regarding the compatibility of specific bio-control options and various pesticides. Pests that have been controlled successfully using biological control include fungus gnats, two-spotted spider mites, broad mites, western flower thrips, leafminers, many aphid species, two species of whitefly and others.
Supplier selection is critical when it comes to beneficials. You really want beneficials to come straight from the insectary, where they raise these beneficial insects, mites and nematodes. Internet searches for bio-control suppliers often bring up distributors that purchase beneficials from insectaries. The beneficials are delivered to the distributor’s facility before being repacked and shipped. This can add a few days to the transportation time, and time is critical when you are talking about shipping live beneficials. You will get a much better product and much better control if you order direct from the insectaries or through a distributor that drop-ships directly from the insectary. Some insectaries that are excellent suppliers include Beneficial Insectary, Syngenta Bioline, BioBest and Koppert.
When biological control programs fail, it can often be attributed to the wrong beneficial being applied. For example, when dealing with mites, knowing which specific species is causing the problem is critical to selecting the right beneficial.
Another reason programs can fail is due to pesticide residue. Products sprayed on plants can sometimes have a surprisingly long residual against predatory mites and beneficial insects. Just because it is listed by the Organic Materials Review Institute (OMRI), does not mean it is safe for beneficials. When someone asks about applying beneficials, I am going to ask what has been sprayed on the crop for at least the last 60 days. Some products can leave a foliar residue on the plant for two months or longer, and they can impact your beneficials. These can include fungicides and plant growth regulators (PGRs).
Using bio-controls has so many benefits. You do not have to worry about which pesticides are allowed. There are no concerns with phytotoxicity (burning/damage of plant tissue) and there will be no pesticide residue left on the crop. The key is to start your bio-control program early. Do not wait until you have a problem.
Mite spray banned, pulled from shelves
By Garrett Rudolph
Just days after publishing a list of chemicals approved for use on cannabis, the Oregon Department of Agriculture placed a “temporary” ban on Guardian Mite Spray after a chemical analysis revealed inconsistencies between the product’s ingredients and label.
The insecticide was also removed from Washington and Colorado’s lists of approved chemicals following reports that it contains abamectin, which was not included on the product’s label.
The Oregonian reported that the inconsistency was uncovered by Rodger Voelker, a chemist with OG Analytical in Eugene, Oregon. Several cannabis samples analyzed by Voelker had tested positive for abamectin, which he eventually traced to Guardian Mite Spray.
The Washington State Department of Agriculture issued a statement that marijuana producers must “immediately discontinue use of Guardian and remove the product from the licensed premises” and “advise all processors to whom you have sold marijuana treated with Guardian that the marijuana may have been treated with an unapproved pesticide.”
Processors must provide a notice about the possible use of the unapproved pesticide to retailers carrying products that may have been treated with Guardian Mite Spray, and retailers must post that notice in a conspicuous location.
The Colorado Department of Agriculture issued a similar statement: “Growers and retailers are cautioned against using or selling Guardian Mite Spray until further notice.”
If there isn’t a “bug to eat your bug,” then maybe a softer spray can do the trick. Products like SuffOil-X from BioWorks is an OMRI Listed oil that has been very effective and safe for pest control in vegetable and ornamental crops. There has even been research conducted in Canada by the Vineland Research & Innovations Center about using this oil as a dip for ornamental plants. The results are promising, but cultivators are still waiting for the dip to be added to the SuffOil-X label in the U.S.
Keep in mind that dip rates are much lower than spray rates for some products. SuffOil-X is on Colorado’s list of approved pesticides for use in marijuana production, which can be found at www.colorado.gov.
Another softer active ingredient is Bacillus thuringiensis ssp. kurstaki (Btk for short). What’s great about this is that it only kills caterpillars once they ingest it. Btk is specific to caterpillars and does not affect any other pests or beneficials. There is also a Bacillus thuringiensis ssp. israelensis for fungus gnats. Do not get these two products confused because they control completely different insect pests. This mistake has been made in some recommendations that have been printed or posted online.
It is helpful to know if the product is registered with the EPA when making pesticide selections.
EPA-approved pesticides “are required to perform their intended function when used according to manufacturer’s directions, without posing ‘unreasonable risks of adverse effects on human health or the environment.’” Granted, these products have not been tested on marijuana, but at least it will provide a starting point.
There are many products on the market today that fall under the “minimum risk pesticide” category that are not EPA-registered. The EPA has determined that there are some inputs that pose little or no risk to human health or the environment, so they are exempt from EPA registration. This only means these products are generally safe, but there is no one vetting the claims made regarding these products, so buyer beware. Lists of these active ingredients can be found on the EPA website under FIFRA 25(b) Pesticide Products.
When deciding to use a pesticide, do your homework. Just because someone recommends it does not mean you should use it. Different states have different laws regarding pesticides. Some states, like Colorado and Washington, provide a list of approved pesticides to help guide growers, so make sure you really think before you apply.
Suzanne Wainwright-Evans has been in the horticulture industry for more than 25 years, focusing on biological control and proper use of pesticides. She has worked throughout the U.S. and internationally as a consultant for greenhouses, nurseries and gardens. She is the owner of Buglady Consulting, now in business 15 years.