Cannabis Pests - Leaf Miners

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Added 29 December 2020

Cannabis Pests Leaf Miners

Are leaf miners the be all and end all of your cannabis plants? You'll be glad to hear leaf miners aren't a serious problem and we're here to show you why. This article teaches you how to identify leaf miners and how to easily get rid of them.

What Are Leaf Miners?

Liriomyza trifolii

Leaf miners are in fact, not all from the same species of bug. Leaf miners are the larvae of different insects that hatch and burrow themselves inside the leaves of plants, whereupon they begin to feed by digging away between the leaf tissues. This leaves behind a distinct type of marking that has a swirly, discoloured, tunnel formation.

Leaf mining insects that affect plants can be anything from caterpillars to moths, or even beetles, although the most common in cannabis is a type of fly (Liriomyza trifolii). It goes through 4 stages in its life but the larvae stage is where they become most apparent, and destructive.

A female adult lays its eggs inside the leaves, where they hatch into larvae after about 10 days. The larvae feed for a few days and eventually turn into a pupa, which emerge from the leaf as adults. Living their early stages inside the leaves provides them protection from predators while also providing them the perfect food for development.

Leaf Miner Cannabis Leaf Damage

The female adults can also cause some damage as they continue to feed from the leaves by piercing the tissues and sucking out the sap with their mouth parts. Luckily, their populations do not grow heavily and pose little threat to your plants if taken care of.

The Causes of Leaf Miners

Warm Greenhouses Are Susceptible To Leaf Miner Infestations

Leaf miners can affect many different kinds of plant, not just cannabis, including citrus trees, blackberries, tomatoes, peppers, beans, lettuce, celery and many others. They are most commonly found outdoors but can also sometimes find their way into indoor spaces.

Warm conditions are favourable for leaf miners and it has been shown that they thrive best in temperatures around 30°C and are found less in colder climates. Liriomyza trifolii is native to the Caribbean and United states, but can also be found in many other places across the world such as Europe or Asia.

Greenhouses are particularly susceptible to leaf miners and as an invasive species, they are quite difficult to avoid. The causes are nature simply doing its thing. However, we must step in to stop them otherwise they can munch away a large portion of our cannabis plants, which as you can imagine, can lead to a number of problems.

Identifying Leaf Miners

Identifying Leaf Miners

Discovering leaf miners before the eggs have hatched is very difficult because they are so small. Around 35-40 tiny eggs can be laid per day. Each is about 1mm long and transparent in the beginning so they can be extremely hard to see unless you are checking thoroughly.

Most growers spot the problem quickly, meaning they can provide treatment before the leaf miners eat away too much of the leaves.

Caterpillar Leaf Miners

Symptoms of a leaf miner infestation:

  • Discoloured tunnels or trails clearly visible, mainly on fan leaves.
  • Faeces (black spots).
  • Holes in leaves.
  • Small, white or yellow larvae.
  • Transparent or cream coloured eggs buried in underside of leaves, giving a rough, 'bumpy' texture.
  • Slow plant growth.
  • Bite marks from female adults.

Moths or caterpillars sometimes lay their eggs in the leaves of cannabis plants and the larvae also dig tunnels. The tunnel formation looks similar, perhaps a little blotchier, and can often come with holes as parts of the leaf are completely eaten away.

Leaf Miner Treatment

Pruning To Remove Leaf Miners

Once you've identified which bug is living in your plants, providing the right treatment is straightforward. Let's take a look at some of the options for eliminating leaf miners from your cannabis crop.

  • Remove affected leaves - One of the quickest ways to control a leaf miner problem is to remove the leaves or parts of the leaves that have tunnels on them. Killing as many larvae as possible before they grow into adults reduces their reproduction rates, giving you time to apply further treatments to kill off any left over eggs or adults potentially still living in the plant.
  • Kill them by hand - If you look very closely, you can often find where the miner is inside the tunnel and squash it using your fingers. Although this might seem like a tedious task, it stops you from removing leaves that could be vital for the health of your cannabis plant. Further treatments are recommended to completely eradicate the leaf miners.
  • Predators - Parasitic wasps (Diglyphus isaea) do a good job of killing a lot of the eggs laid by the leaf miner fly. They lay their eggs alongside the leaf miner eggs and when they hatch, the wasp larvae proceed to feed off the leaf miner eggs. The adult wasps also attack the adult leaf miners, laying more eggs in their corpses which act as food once they hatch. 

Diglyphus isaea Predator

  • Neem Oil - Most organic gardeners have neem oil available for controlling a wide number of pests, including leaf miners. The neem oil is applied as a foliar spray and gets absorbed by the plant, which the leaf miners eventually consume. Although not recommended for use during flowering (stays on the plant for a while and can affect the taste of buds), neem oil works very well as a preventative pest spray during vegetation. Mix with water and potassium soap and spray all the leaves 2 times per week.
  • Sapinosad - This product works well for killing leaf miners, as well as a number of other pests. It's organic and isn't harmful for humans animals, and most beneficial insects, so it can be used outdoors without worry. However, it only stays on the plants for around 24 hours and the leaf miners need to consume it in order for it to work, so you may need to apply multiple times. Also, leaf miners can develop a resistance to it so it may not work for long. 
  • Blue Adhesive Traps - These may not completely get rid of the leaf miners and are targeted at the adults to prevent them from mating with each other. The flies get attracted to the blue colour and will stick to the trap upon landing. Although leaf miners do not spread too heavily, the blue adhesive traps can help you keep the populations down.

Leaf miner damage on outdoor plant by IlGreco from GrowDiaries.

Tips For Preventing Leaf Miners

Preventing Leaf Miners

To end this post, it might help to have some extra tips on preventing leaf miners from ever being a problem. There is not really much we can do to prevent them completely, but as always, the conditions of your grow space matter.

Like we previously mentioned, leaf miners populate faster in warm weather, specifically at around 30°C. That means the lower you keep the temperatures, the less risk there is of them feasting on your plants. Try to maintain the environment at roughly 20-25°C.

Nets designed to protect against pests can also be placed over outdoor plants. These can be trickier to use on big plants or large gardens, but work very well for small cannabis plants. Alternatively, you can always build a frame with a protective netting that allows your plants plenty of light but keeps them safe from leaf miners. 

Have you had problems with leaf miners before? If you have any tips on the topic of leaf miners and cannabis plants, we'd love to hear from you! Please feel free to share your thoughts with fellow growers down in the comments section!

External References

Comparative Study on the Effectiveness of Chemical Versus Biological Control Against Liriomyza trifolii (Diptera: Agromyzidae) under Field and Greenhouse Conditions. - Morsy, Amany & El-Shewy, A & Elgizawy, Karam. (2019).

Influence of Temperature on Development and Fecundity of Liriomyza trifolii (Burgess) (Diptera: Agromyzidae) on Celery , Environmental Entomology. - G. L. Leibee (1984)

Development and Stability of Insecticide Resistance in the Leafminer Liriomyza trifolii to Cyromazine, Abamectin, and Spinosad, Journal of Economic Entomology. - J. Scott Ferguson (2004)

Evaluation of Exotic parasitoid, Diglyphus isaea against Liriomyza trifolii on Tomato in Protected Cultivation in India. - Sabir, Naved. (2018). 

Leaf puncturing, feeding and oviposition behavior of Liriomyza trifolii. - J. A. Bethke, M. P. Parella (1985)

This article was updated December 2020.


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