Leafminers are pests which tunnel through the mesophyll of leaves creating distinctive and unsightly squiggly patterns. They are, collectively, mostly comprised of members of two insect orders: Lepidoptera (moths and butterflies) and Diptera (flies). The latter will be the focus of this page as caterpillar pests are covered in the previous page. There are about 98,000 members of the order Diptera, and many are pests in some way, shape or form, but only a handful are active as leafminers. In the United States they are not as common as pests like aphids and caterpillars, but do deserve attention. Two decades ago, and we don’t know why, they were a more serious problem than they are today, but are still a concern for growers and, thus, mentioned on this website.
How They Become a Problem
Leafminers can attack outdoor and indoor crops of many kinds, but the damage they cause is not usually life-threatening. Aesthetically speaking, though, these flies can destroy the value of a crop. Larger plants, shrubs and trees can be infested by leafminers every year without noticeable growth setbacks unless the infestation is serious-in which case photosynthesis can be hindered. And cosmetic issues are not always a concern unless the plants, shrubs or tree are to be sold outright. Growers’ concerns usually involve leafminer infestations on smaller specimens which will be marketed as plants for their appearance and not for their fruits, etc. Chrysanthemums are one example of such a crop. On mums and other such flowering plants, leafminers are a more serious concern.
A Serious Pest
The infestation process begins as the female flies lands on your plants and begin to feed. They do this by puncturing the leaf with their sucking mouth parts. This activity creates a tiny white spot which, by itself, is not of great concern to growers-though a lot of feeding activity can make the plant’s leaves unsightly. The real damage begins when the female fly lays an egg in the puncture wound. The egg hatches and the resulting larva or maggot begins to tunnel through the leaf, eating the cells of which the mesophyll is composed, creating a void, tunnel or mine as they go. The outer leaf surfaces remain intact, but with no cells between the upper and lower surfaces the feeding becomes very noticeable and ugly. As the maggot continues to feed, it tunnels randomly. Moreover, the maggot increases in size so the mines get larger and more noticeable as progress is made. Additionally, the maggot’s feces is forced out of the mine’s entrance creating yet another unsightly mess (and this is exacerbated when a maggot has been parasitized). The growth process continues until the maggot is ready, through complete metamorphosis, to pupate. At this point they usually drop to the ground. On the ground, control of these pests can obtained by employing one or more soil-pest controls. When the entire process is complete, the pupae give forth a new generation of adult flies ready to mate and lay eggs in leaves so that the cycle may continue.
For The Scout
You will probably not be bothered with large numbers of leafminers and, unless they are a known problem-pest in your area, we wouldn’t recommend special treatments or scouting measures be taken. Become visually familiar with them-which can be difficult as they are unremarkable in appearance and look like, well, little two to three millimeter flies with a yellow spot on each side of their thorax, in essence, stay alert and do nothing else. Utilizing yellow sticky traps during a regular scouting regimen should be the only step you’d take in an effort to scout for them, aside, that is, from looking for those telltale mines during your normal plant inspections. Which, by the way, if you do find mined leaves, remove and destroy them.
There are few biological pest control agents available to specifically address leafminer pests from the order Diptera. The parasitoids which are available have been known to do a very good job, though. If possible, use them preventively or, at least, at the very first sign of these flies. Other biocontrol agents, those used for soil pests, for example, can also help you with leafminers while being used primarily to thwart other pests like fungus gnats. Chemical controls can also be effective, but expect difficulty when trying to effectively treat the leaf-protected larvae and soil-protected pupae.