“Leaf miner” is a term used with several groups of insects including some flies, wasps, and moths. The group we are focusing on is the genus Liriomyza which are flies. There are over 400 species in the genus. All stages are pretty small, so their existence is usually not noticed until “mines” are noticed on foliage. The larvae are legless and start out clear. As they mature, they start to yellow and pupate the same color. The pupae are between 1.2 and 2.5 mm long and less than 1 mm wide.
Initial damage is caused by the adults through piercing the leaves for feeding and oviposition. This will resemble yellow stippling on the leaves. However, the most significant damage is caused by the larvae. They create tunnels in the mesophyll, leaving the epidermis intact. This will resemble light colored “squiggles” on the leaves. Many times, you can see frass (more pleasant word for poop) in the mines. This damage can interfere with growth of the leaves and the plant overall. Females can lay up to 400 eggs in their less than 3 week lifespan.
These are typically most active during the spring, but following generations can be present throughout the year.
This fly genus can be found in all temperate and tropical zones around the world.
Many species in this genus are polyphagous (they eat a lot of different stuff) and tomato plants are a known host, but surprisingly not a favorite.
Identification is the first step since there are so many insects that cause similar damage. Place plastic trays under infected foliage. As the larvae mature, they will evacuate the leaves and pupate on the trays. This can be a good method for checking thresholds as well. Once identified, there are several methods that can be used to control them before you reach for the pesticide. Cultural controls such as removing broadleaf weeds and tilling or mulching can decrease miner activity. Since the larvae pupate in the soil, it is difficult for them to emerge if they are buried too deep. Parasitic wasps such as Diglyphus begini have been shown to be effective, even in field studies. On a side note, I did run across a paper that mentioned staking tomatoes seems to decrease parasitoid activity. That may be something you should keep in mind. As always, there are several chemical routes you can take, but check to make sure you are using the least toxic but effective application available.
While these little flies are a plague on crops and ornamental plants, they are actually pretty cool. They are one of the few groups of flies that can make noise! The males have a stridulating organ (think of a washboard instrument) that is used in mating.