By James A. Bethke, for Let's Talk Plants! November 2021.
My first job in the University of California Riverside (UCR) Department of Entomology was to assist in the rearing of an invasive pest from the southeast U.S., the serpentine leafminer, Liriomyza trifolii. I was thrilled to be doing real entomology and progressing toward a degree in the field. I was fascinated by the tiny fly and eventually completed a master’s degree with an emphasis on the biology and behavior of Liriomyza. When I joined UCR and began taking entomology classes, I thought there was so much known about the insects, how could I possibly do any original research. Much to my surprise, the majority of what was known was about pest insects of which there is relatively few compared to the diversity of insects in the world. Further, there was very little known about the serpentine leafminer, so everything I studied was new to the scientific literature.
There are many small flies in the insect family Agromyzidae (Figure 1) and many of them can be found in southern California attacking bedding plants and vegetables in homes and gardens. The adults are small (less than 2mm or 0.08in long) and rarely noticed. They lay their eggs under the epidermal layer of leaf tissue. When the egg hatches, they are a tiny maggot that begins to feed on the leaf tissues right below the epidermal layer, and they form a serpentine mine as they move and feed (Figure 2). Leafminers have a relatively short life cycle of about 20 days, so numerous generations occur in warm climates like California. Leafminers are common in my yard on many plants, and the damage they cause is usually heavy late in the season when the population size is large. However, because the populations are larger at that time, parasitism and predation dominate (Figure 3), and the population crashes as the days get shorter and the temperatures drop.
Leafminer damage can be severe and unsightly in ornamental plantings but are tolerable in vegetable plants. For example, on my lantana (Figure 2) the damage is unsightly, but the plants are perennial, and they are going to senesce through the winter and recover very nicely in the spring. There is no need to control for the leafminer in this case. Similarly, melon, squash or tomato leaves that are heavily mined have little effect on the fruit they produced, except in severe cases. In contrast, leafy vegetables (spinach, leafy lettuce, celery, etc.) that are heavily mined and destined for the dinner table are not very appetizing and will likely need some form of pest control.
Beneficial insects (parasites, predators, etc.) against leafminers exist naturally and in great numbers, so there is no need for their purchase and release. If pesticides are required in your situation because of the severity of damage, then the organic spinosyn products are your best bet. They are very effective against leafminers.