One of the most asked questions this fall as an entomologist was, "Why are there so many moths right now?" This fall in British Columbia there is an immense moth outbreak. The moth experiencing this population boom is the Western Hemlock Looper. Historically, every 10-15 years these moths will have a population explosion and then will fall back to typical numbers. This year happens to be big year for the moths, and it's expected that next year will be too. These particular moths generally infest Hemlock forests and should stay away from your cannabis crops, they are far more interested in destroying thousands of hectares of forest. However, if one of these bugs is spotted - quickly escort them out of the building - they might just feel like laying eggs in the nice warm grow room. Eggs will lead to hungry, hungry, caterpillars and they likely won't turn down fresh plant material especially if that's all that is available for food.
Both moths and butterflies lay eggs that hatch into caterpillars. Caterpillars on cannabis can be a large problem for outdoor grows, less of a problem for indoor grows. This doesn't mean that caterpillars don't make their way inside. Inchworms can come in stuck to clothes or cuttings could come with eggs attached to them. Sometimes eggs can be leftover in growing medium if not properly sterilized. As always, we recommend thoroughly inspecting any plant material brought into the grow space, in case of infestation by any insect pest. The best way to spot any insect eggs on plants is to look at the bottom of the leaves. If possible, remove these leaves and thoroughly inspect the other plants for caterpillars or more eggs.
Photo from www.growweedeasy.com
Another way to identify caterpillars as the pest bug on cannabis is the damage they leave in their wake. Wholes or chunks of missing leaf are good indicators. Yellowing leave can also be a sign if the variety of caterpillar is attacking the stems instead of the leaves. They can also affect the size and vigour of the plant. Additionally, caterpillars are easy to spy as they are large and visible to the naked eye.
Once the pest has been identified as a caterpillar it is vital to remove any that are found. They will rapidly eat your plants and pupate to moths, which will lay more eggs and exponentially the pest issue. Depending on what species of caterpillar is infesting your crop, it may be nocturnal, meaning they are most active at night. If caterpillar damage is present, but no caterpillars are visible during the day, chances are they are coming out at night. Once you have removed all visible caterpillars, defoliate the plants. This leaf removal will help by disposing of eggs that have been laid on older leaves, and minimizes leaf surface area to be sprayed when using nematodes or bacterial foliar sprays.
One common method is to spray with bacterial insecticide. The bacteria Bacillus thuringiensis (BT) is a microscopic organism that is produces a toxin fatal to caterpillars. After ingestion, the toxin interacts with their intestine, making them unable to eat any further This leads to starvation and ultimately death. A BT product registered for use on cannabis in Canada is Bioprotec CAF. Another excellent way to control caterpillars on crops is to apply nematodes to the crop. The nematode Steinernema carpocapse is the best type of nematode to use for caterpillars. Nematodes are microorganisms that have no eyes or legs. They wiggle around until they find their host insect, and are surprisingly effective considering their limited senses. The nematodes find their way inside the insect through their natural body openings. Once inside of their prey, they release the contents of their gut, which contains a toxic microorganism. These microbes digest the inside of the caterpillar, allowing the nematode to easily eat the liquified insect. Once fed, the nematodes will multiply quickly and in great numbers. They will continue to liquify and eat the insect until there is no more insect to feed on, then they will move on to the next prey. From initial contact to death of insect may take up to 3 days but they will stop feeding likely after the first day of initial contact. Their lack of sight and legs makes it extremely important to get full coverage of your plants so they can more easily find their prey.
Caterpillars can be mighty destructive, but monitoring and correctly administered pest solutions can save a crop.