In the San Joaquin Valley, it is important to distinguish between L. trifolii and L. sativae, which can occur in the same areas, because L. trifolii is much more resistant to most chemical controls. Using a strong hand lens, observe the compound eye of the adult. The eye of L. trifolii is nearly completely surrounded by yellow with only a small portion of black touching the rear of the eye. The top of the thorax of L. sativa is shiny black whereas on L. trifolii it is more dull dark gray colored. The larvae of leafminers are tiny bright yellow maggots about 2 mm long when they emerge from the leaves to pupate.
Pupae look like tiny brown grains of rice.
There can be many generations a year. Females insert the white, oval eggs into the leaf tissue by puncturing the leaf at the tip and along the edges. They may cause leaves to dry, resulting in sunburning of fruit and reduction in yield and fruit quality. In severe infestations, the leafmining may reduce yields or cause plant death. The most serious infestations usually appear late in the season.