A common pest in agriculture all over the world, leaf-mining insects are a diverse array of species including flies, moths, sawflies, or beetles which lay their eggs on the surface of leaves. Vegetable gardeners may be familiar with common leaf-mining pests that damage edible crops such as the spinach leafminer or the tomato leafminer. While leaf-mining infestations don’t necessarily kill the plants or disrupt growth on their own, the presence of scars on leaves is obviously an undesirable aesthetic concern. More importantly, any open wounds on leaves can invite infection into the plant or at least disrupt photosynthesis. There is also the possibility that leaf-mining insects can transmit fungal or viral pathogens while feeding.
While there are too many species to address comprehensively, some of the most common leaf mining insects are small black or yellow flies belonging to the genera Liriomyzia or Pegomya. These flies are tiny (1 to 3 mm in length) and have incredibly short life cycles. For the cannabis grower, this means that many generations can occur within one growing or flowering cycle.
Leaf mining bugs typically lay their eggs on the bottom surface of a leaf. Eggs can be hard to see with the naked eye but can sometimes be observed in clusters along the veins on the underside of leaves. When the eggs hatch, the larvae burrow through the leaf tissue, resulting in trails of dead, yellowing tissue. Most growers will recognize the serpentine-like pattern associated with leafminers as a sure indication, but blotches, spots, or tent-like bulges on leaves can also be signs of infestation.
The presence of leaf-mining insects can be verified if growers see adult flies around the plants or frass (insect droppings) near or inside the scars. Leaves can also be isolated in a plastic bag or other container and observed over several days to see if any adults develop. However, many of the methods of control for leaf-mining insects overlap with other pest control practices which are commonly employed for thrips or spider mites.
Unfortunately, because leaf-mining insects burrow inside of the leaves, spray application pesticides may not always come into contact with the larvae. Additionally, pesticides like Spinosad which are used for leafminer control in other agricultural applications may be banned for cannabis use depending on the state. Neem oil can be applied to keep eggs from hatching as a measure for slowing the life cycle, but any spray application pesticides need to be timed when eggs are first laid to be effective. Many leafminer species have also been shown to have a high pesticide resistance, so this method is not always the best practice.
Pruning leaves infested with leafminers will at the very least reduce the population and keep damage to a minimum. Any fan leaves showing signs of leafminer infestation should be removed from the grow immediately, which is generally good practice for any damaged leaves in a cannabis grow regardless. Sticky traps can also be used to trap the adults of leafminer insects.
Releasing natural predators of leaf-mining insects is one of the best controls for these types of infestations. There are several species of small parasitic wasps which target different species of leaf-mining flies and can be purchased from reputable biological control dealers. Diglyphus isaea and Dacnusa sibirica are small, non-stinging wasps which paralyze and lay their eggs in leafminer larvae, feeding on the host when they hatch. Steinernema roundworm nematodes, which are also used as biological controls for thrips and other pests, may also prey on some leaf mining insect pupae which drop into the soil.
While leaf-mining insects are certainly a pest of concern for cannabis growers, cannabis is not necessarily the favorite plant of these pests. Another option for managing leafminer infestations is to plant “trap crops” of plants that will draw the attention of these species. Common plants which attract leafminers are lamb’s quarters, columbine, and velvetleaf, among several others. These plants can be placed near your cannabis plants to encourage leafminers to move to the other crop.
As with many pests, it may not be possible to clear an infestation out entirely within one grow cycle. Often the goal is management and reducing the population of the pest as much as possible and decreasing maximum damage to the plants. If you find that your garden has been overwhelmed with leafminer infestation, employing several of these methods will keep the population as low as possible. Prevention is always the best cure, so it is important for cannabis growers to be prudent about how pests are introduced in the first place.