Aphids and whiteflies
Aphids, members of the family Aphidoidea, are small but extremely dangerous insects capable of destroying any kind of crop. With a size of 1-10 mm, their color varies widely depending on the species (over 4000 have been recorded): white, black, yellow, green, brownish… Visible to the naked eye, they love hiding on the underside of leaves and stems to avoid direct exposure.
Many signals will reveal the presence of these sap-sucking nasties. When they eat, they secrete a dense, sticky substance known as "honeydew" that favors the growth of a fungus called sooty mold. Ants can also be a telltale sign of aphids on the plant for they feed on such carbohydrate-rich excretion. A visual inspection of the leaves may be quite revealing too. When insects suck plant sap, leaves turn yellowish/wilt and dry out due to the leaf damage.
Whiteflies are 1 mm-long winged insects that weaken the plant by sucking sap, just like aphids. The best way to know whether your plants are being attacked by these white moth-looking insects is to shake the branches and see if any of them flies away. Another unmistakable symptom is the emergence of either yellow/translucent spots or some white residue on the underside of the leaves.
Like aphids, whiteflies produce a sticky substance similar to molasses (like fly droppings) that can bring about the development of sooty mold, a dangerous fungus that will considerably hamper both photosynthesis and the quality of your plants.
Caterpillars
Spotting them before it's too late could definitely save your crop. It's not easy, though, for they love playing hide-and-seek amidst the vegetation. Check the stems for the presence of large irregular holes with brownish edges. They're visible to the naked eye. If you find any, first thing would be to remove the entire part.
Look for eggs. Despite small and hard-to-spot, they are like small sets of white or yellow round-shaped dots. If carefully examined with the help of a magnifying glass or a pocket microscope, you may be able to see them.
Spider mites
Spider mites are not insects but arachnids that live on the undersides of plant leaves. Some telltale signs of their presence could be the small spots they leave with their mouths and the webbing that covers the branches and buds.
A colony of spider mites could accelerate the yellowing of the leaves, causing them to eventually die off. A long-running infection could stunt the plant's ability to grow and badly affect the resulting yield. Spider mites may also affect buds, preventing them from reaching full maturity. Worst-case scenario, a large enough colony could kill the plant, although this is rather uncommon.
Leaf miners
These larvae bite the internal tissue of plant leaves leaving small tunnels on their way. They just burrow into leaves and make a home between the layers. Despite not being the kind of pest that derails crops, it does affect plant yield. And, to top it off, the tunnels make it more likely for the internal tissues to become infected by fungi and bacteria.
The telltale signs and symptoms vary widely depending on how advanced the infection is. If eggs have just been deposited, there will be some small spots on the leaves. If the invasion moves forward, the signs of munching and mining will become far more evident.
Fungus gnats
This insect presents a significant challenge to any cannabis grower. With a size of 2 mm tops, it can lay 200 eggs per week. Their primary target: the rooting system. Something that should definitely pull the alarm is the presence of flying insects around your crop. However, it may be necessary to closely examine every plant for they are too tiny to spot with ease.
Check for pale leaves with brown edges and dark spots. Also for yellowish and twisted leaves. A fungus gnat invasion could affect bud and root growth for good and cause the wilting of the whole plant.
Thrips
The most common thrips attacking cannabis plants are known as Frankliniella occidentalis. These small invaders are basically yellowish-whitey flying bugs that hop from plant to plant. Although they are visible to the human eye, they love staying near the leaf veins so nobody can spot them.
What thrips do is pierce the tissue and suck the plant sap or, in the case of females, lay eggs. Hence, look out for slightly shiny stains on the surface of the leaves. If the invasion is well advanced, the signs will be far more obvious: the leaves will become brittle due to the loss of chlorophyll and many small dots will appear on them.
Snails and slugs
These mollusks can pose a real threat to cannabis plants, particularly if they're located outdoors, for they are voracious animals determined to wreak havoc. They generally appear in spring, when our plants are most vulnerable, that is, during the vegging phase.
Snails and slugs sleep during daytime and come out to eat at night, so it's more difficult to see them. And, to make matters worse, they tend to hide out in the shadows: under stones, trunks, tiles… any dark and humid surrounding will do. Telling whether your plants are being attacked during nighttime is easy, though.
These small creatures feed on any kind of plant matter, including the roots. However, it is on the leaves that the attacks are more evident: they end up entirely full of bites, normally in the shape of spider webs. If, besides these, a trail of mucus appears on them, no doubt there are snails or slugs nearby. In order to be 100% sure, you can look for them at night using a flashlight.
If you happen to find any of these signs or other strange symptoms, take action right away if you don't want the plague to destroy your plants. There are indeed many solutions to the attack of pests, but early detection is key.