For Leaf Miners Resistance is Not Futile: Game Over or Play On?
Mechanical and Organic Methods to the Rescue
Are your Cannabis leaves suddenly looking like the snaky maze aftermath of a Light Cycle tournament from Tron? Chances are pretty good that you’ve got hungry little Leaf Miner maggots tunneling through the juicy interiors to feed on plant cells like Ms. Pac Man chomping on a power pellet. Once sated, after a week or two of tunneling, the maggots will emerge from the leaf, drop to the ground and pupate, quickly metamorphosing into a small adult fly. The mated females will soon inject their eggs below the surface of a healthy mature leaf to start the cycle anew.
Every leaf is a food factory powered by the sun, manufacturing all a plant needs to grow utilizing raw materials gathered from the air and soil. When Leaf Miners are eating up that cellular factory material, your crop yield is going to suffer somewhat, but thanks to natural predators like parasitic wasps (so long as you don’t kill your beneficials with broad spectrum insecticides) it is unlikely that Leaf miners will do major damage. A worrisome secondary effect of Leaf Miners is that the egg insertion and pupa exit wounds at the two ends of the tunnels can serve as entryways to various pathogens, further weakening the plant. The egg end of the tunnel is understandably tiny, the path of destruction gets wider as the maggot grows, you can see clearly how quickly things progress. If upon inspection you find a leaf with just one or two short squiggly tracks, use your hand lens to identify the maggot at the wide end of the tunnel and simply squish it in place, allowing the healthy remainder of the leaf to continue its good work. Leaves laced with multiple serpentine tracks should be removed, quarantined and destroyed.
Leaf Miner maggots are very difficult to kill since they are shielded from soap sprays and suffocating oils by living underneath the leaf epidermis. Additionally, Leaf Miners have developed some resistance to some of the harsher insecticides which you shouldn’t be using anyway in this new age of residue testing. If allowed in your locale, Spinosad is very effective against the feeding maggots; they eat it, stop feeding and die. Spinosad is generally accepted as a safe organic, but please do take care to not use it around honeybees. Companion plantings of lambsquarter, columbine, and velvetleaf may reduce the amount of infestation on your Cannabis as the adult flies choose a more perfect host for their next generation of maggots to feed within.
Disclaimer: Any advice and opinions offered about the cultivation of cannabis by Bruce N. Goren are his own and do not represent the University of California or the Master Gardener Program.