Leaf miners (Liriomyza huidobrensis) are like tiny insects that look like ordinary flies when they become adults. Their larvae are unique in that they damage cannabis plants in a very particular way: they sneak into the epidermal tissue of the leaves and eat their way through them.
The leafminer belongs to the order Diptera (one pair of wings, large compound eyes), and the family Agromyzidae consisting of over 2,500 species. Adult leaf miners don't cause damage to the plant, though. It's the larvae coming out of the eggs they lay that tunnel the leaves and suck up their lifeblood. Leaf miners are definitely the most common of all (speaking of cannabis plants, of course), yet there are many other types of miners derived from moths and beetles.
How to identify leaf miners
During their life cycle, leaf miners go through several stages we need to be aware of if we are to identify them correctly:
Egg
Be it for eating or for laying eggs, female insects pierce the upper surface of the leaves making a tiny oval-shaped hole that is yet visible to the human eye. As a matter of fact, it's to these punctures that you have to pay attention for they'll be the first signs revealing the presence of leaf miners in your cannabis plants. The eggs (from tens to hundreds, depending on the conditions) are inserted into the leave tissue, making them even more difficult to come across. Besides, they're about 0.25 mm long, meaning even the most skillful grower will struggle to spot them at first sight.
Larvae
It's in this stage that leafminers wreak havoc. The eggs hatch and the little larvae start to eat up the leaves' contents, tunneling them. The good news is that these tunnels can easily be identified. They are like twisty translucent paths with brownish edges. Larval worms are mostly white and yellow, although they could be greenish too. About 3-5 mm long, they can be detected with ease by holding up a leaf to the light. These larvae use the stems to move around the plant but never come out.
Pupa
Plentifully-fed fully-grown larvae break the leaf using their mouth hooks and fall to the substrate, where they bury themselves about an inch to give way to the pupa. There, or on the surface or the undersides of the leaves, the metamorphosis takes place. When larvae move to the pupal phase, they turn into slightly rounded and segmented brownish barrels of 2-5 mm.
Adult
Leaf miners emerge from the pupa like small winged 2 mm insects. Going from black to grey, they could show some yellow spots in various parts of their bodies. Their wings are small and translucent. The average lifespan of an individual is about 10-15 days and they spend the day mating, feeding, and laying.
How can a leaf miner invasion affect our cannabis plants?
The temperature and moisture levels are key to the development of leaf miners. Very humid and hot environments with temperatures exceeding 77°F are more likely to receive the visit of these damaging insects. At 60°F or below, females stop laying eggs.
If leaf miners have already infested your cannabis plants, the small tunnels will hamper their development for they take up part of the tissue plants need to perform photosynthesis. As a result, leaves become totally or partially dry and the yield decreases.
Another important issue worth bearing in mind is that the holes that leaf miners leave when laying the eggs or coming out could serve as entry points for multiple pathogens which would further damage the plant.
How to fight leaf miners
Despite not being the most deadly plague, leaf miners prove very hard to beat because they live inside the leaves. The winding path they create seems to have no end. That will help you figure out how devastating this pest can turn out to be. The best you can do is detect larvae when they're still at the initial stage and take action as soon as possible.
If you happen to come across a small tunnel or two, use a magnifying glass to find the worm somewhere near the wide part of the path and crush it with your fingers to kill it. This way you'll allow the rest of the leaf to keep working normally. If the entire surface of the leave is covered in tunnels, then there's no choice but to remove it completely and destroy it as quickly as you can.
Alternatively, you can set up traps. Now the market is full of yellow sticky traps which have proved to be very useful to trap adult individuals. Using predatory species is also a very interesting solution: contact insecticides aren't particularly effective against this plague for the larvae are protected inside the leaves. That's why the use of predatory species such as parasitic wasps (Dacnusa sibirica, Diglyphus, and Opius pallides) might be not only practical but incredibly effective too.
Neem oil is a great ally for preventing the appearance of leaf miners, maybe as practical as setting up anti-insect meshes to keep them out of your indoor grow or greenhouse. Leaf miners can be a bit of a nuisance, but they pose no actual threat to cannabis growers. If you manage to spot them early enough and handle the issue adequately, the yield will remain unaffected.