Holly Leafminer (Phytomyza ilicicola)
Adult flies are about 1/8 inch long and black. The larvae are 1/8 inch long yellow maggots that tunnel through leaves, creating serpentine mine. Eggs are usually deposited in the midrib or leaf margin and early mining occurs there. There is one generation a year. Larvae overwinter in mines. Hard, late frosts extend adult egg-laying activity and increase the pest population.
Summer to fall mining occurs in the midrib. The obvious, linear, yellowish-green mine in the leaf surface occurs the following spring. Several mines per leaf cause premature leaf drop. Adult females of this imported fly puncture tender new holly leaves to feed on plant juices.
In heavy infestation, use systemics for larvae in March of late summer. Contact insecticides may be used for adults in early May, but this is the least desirable technique because beneficial parasites may be killed.