Just like any other crop cultivated, cannabis is subject to a wide range of pests and diseases that can distress your plants, even kill them if left untreated.
Before a grower can make an informed decision about how best to solve a problem, it is necessary to be able to identify the cause of the plant stressor by its symptoms.
Many of the same pest species that are the bane of many greenhouse and field growers of traditional crops can affect cannabis crops just as easily.
When it comes to identifying pests, there are two main categories of insect pests: sucking insects that feed by pulling nutrients out of a plant, and chewing insects that have moving mouth parts and eat sections or even entire leaves or other plant parts.
Sucking Insects Cannabis Growers Need to Look Out For
These guys are not actually true insects, but are arachnids. Control of mites by way of insecticides will not always work as their biology is different than that of insects.
Spider mites feed on the chlorophyll of leaves, leaving a speckled or mottled appearance on the leaves. They are very difficult to see without magnification, but like spiders, will spin webs and quite often other than the damage to the leaves, this is the best clue that what you have is a mite infestation.
(Above: Spider Mites. Photo by Green Box Grown)
Tapping a leaf that is suspected to have mites on it onto a white sheet of paper and seeing if anything is scurrying about (that has eight legs) with a hand lens can help to make a positive identification.
Aphids are large enough to be seen without magnification. There are several different types of aphids and they may be yellow, green or black in appearance, but each is about the size of the tip of a pencil. As aphids start to feed on cannabis leaves, the leaves will start to pucker and appear distorted.
Before the population of aphids explodes, they are usually found on the undersides of leaves and therefore often go undetected until either their damage is noted or they are seen crawling along stems or even on the upper surface of leaves in search of food.
(Above: Aphids. Photo by Green Box Grown)
These flying insects are easy to spot, but it is not the adults that do so much harm; it is the larvae of fungus gnats that feed on the roots of your plants in the soil. Adult gnats can spread disease in the grow room, hence their name, but they do not feed on the plant.
Damage from fungus gnats on cannabis plants may appear as stunted growth, if at all. They do, however, stress the plant and make it more susceptible to other diseases.
Whiteflies quickly make their presence known. Brushing up against a marijuana plant inhabited with whitefly will produce a cluster of them flying about that looks like a puff of smoke. They will feed on the nutrients of the leaf and leave mottled spots much like the mite damage, but are easily seen in large numbers making their identification rather easy.
(Above: Whiteflies on a tomato leaf)
Thrips are much a much smaller flying insect than a whitefly and are harder to see. Their damage is usually seen as silver marks on the cannabis leaf, sometimes with “dots”. Leaves that have been attacked by thrips are also often brittle. The damage of thrips will often be seen before the insect can be seen or identified unless using monitoring traps.
Chewing Insects Cannabis Growers Need to Look Out For
These are the larvae of several different moth and butterfly species. Their sizes range from nearly invisible to several inches long and they may be camouflaged to blend in with the leaf or may be ostentatiously colored.
Either way, their damage is quickly noticed either as holes in the leaves or from leaf margins that have been chewed.
Snails and slugs
These are more problematic for outdoor cannabis crops but can be a real threat. They feed on the leaves of cannabis and often leave a slimy, silvery, mucous-like trail making their identification easy.
Leaf miners are very tiny maggots that are rarely seen on marijuana plants until the damage is done. They feed on cannabis leaf tissue between the upper and lower portions of the leaf, essentially “mining” tunnels in the leaf. The leaf will appear as though squiggly lines have been drawn on it, but the damage is in fact inside the leaf.
Borers are a generic term for any number of larvae or maggots that instead of chewing on leaves, chew into and burrow into the trunk or stems of a cannabis plant.
As the name implies, root maggots are larvae that feed on your plant’s roots. Like fungal gnat larvae, you may not notice the pest until you see evidence of its damage which will be a weakening of the plant and it may appear as nutrient deficiencies or too much water.
A little digging in the soil at the base of your cannabis plants could reveal the presence of these damaging pests.
Learning the basic appearance of common pest damage and deficiencies will help cannabis growers make informed and proper decisions when it comes time to take action or prevent the return of unwanted pests in the grow room.