Leafminers and How to Control Them

Marta Zwart

Ever seen little white or brown patches –  sometimes netting –  on the leaves of your plants? Or maybe there is a little, squiggly racetrack-esque design similar to the images found in this article by the University of Manitoba.

Yup, you have leafminers. 

While these little pests love leafy greens like lettuce, cabbage and Swiss chard, they can also be found on some of your other garden favourites, such as berries, pepper and bean plants.  

While a ‘leafminer’ doesn’t necessarily refer to a specific type of insect, it can be better associated to a behaviour that many types of larvae share. There are serpentine leafminers (Liriomyza brassicae), spinach leafminers (Pergomya hyoscyami), and the most commonly found, vegetable leafminers (Liriomyza sativae Blanchard.) 

Serpentine leafminers are most attracted to beet, cabbage, radish, spinach, and turnip leaves.

Spinach leafminers often infest beets, spinach, chard, and lambsquarter.

Vegetable leafminers go for soft-tissue plants such as beans, lettuce, Swiss chard, beets, spinach and several other crops.

Leafminers are grub-like larvae that may tunnel in plant leaves. They can mature into flies (Diptera), moths (Lepidoptera), or sawflies (Hymenoptera). Some larvae travel through the stems of plants, while others hatch from eggs laid on the leaves by their mature counterparts.

Okay, so how do I get rid of them?

Don’t fret too soon! If you’re lucky and catch them early, you might be able to stop them in their tracks by removing the damaged leaves and stems as close to the ground or the main stem as possible. However, leafminers are often difficult to catch in time. 

Prevention

A preventative option for avoiding and controlling pest outbreaks is to create a habitat that naturally encourages wild beneficial organisms. This is a long-term, preemptive move that can be inexpensive and be self-sustaining for years. However, it’s difficult to be sure that the wild beneficials will reduce the specific type of leafminer. This can be a viable option for gardens, greenhouses and fields. 

Crop rotation can play a critical role in reducing leafminers. Like many insects, leafminer larvae overwinter in the soil. You can drastically reduce their numbers by being mindful of the leafminer-infested areas from previous seasons and rotating out the susceptible crops with ones more resistant to the larvae, By screening your plants for larvae activity before they mature into flies (usually April-May), you can slow down their access to susceptible plants.To avoid future infestations, it’s best to squish the larvae before disposing of or composting the infected leaves. 

Control

To encourage ecosystem balance and avoid chemical use that kills beneficial insects, another option is to directly introduce predatory thrips that target leafminers, such as parasitic wasps like Miglyphus or Diglyphus isaea into your garden. These parasitic wasps are very small, non-stinging, larva-loving insects that kill leafminers by laying their eggs in them. Unfortunately, this method can get expensive and it can be difficult to contain introduced thrips, so this is often recommended in more enclosed spaces such as greenhouses.

Last-ditch Effort

A dusting of kaolin clay is an option for those who need immediate help with their high-value crops and are having an outbreak of pests. Also, sticky traps can be used to catch leafminers but may also catch other beneficial insects. To see what the 2020 Canadian Organic Standards permits in regards to biocontrol agents, check out 311, Table 4.2 in COG’s Guide to the Canadian Standards