Leafminer species causes California mum growers new problems.
Among leaf-miners damaging vegetables and ornamental plants in California Liriomyza trifolii (Burgess), L. sativae Blanch. and L. huidobrensis (Blanch.), which are illustrated in this paper, have often been confused. L. trifolii on chrysanthemum presents the greatest problems, because insecticides available for leaf-miner control are not as effective against this species as against the other 2, and L. trifolii appears to have developed resistance to chlorinated hydrocarbons and synthetic pyrethroids. Biological observations in the greenhouse in California in March 1981 showed that the life-cycle from egg to adult lasted up to 20 days, and the adults lived for up to 30 days, during which the females laid about 250 eggs each at a daily rate of 17 eggs. The females punctured the leaves both for feeding and for oviposition, and males fed at the same punctures. In the field, the egg stage lasted 2.5-4.5 days, the larval stage (including 3 instars within the mine) 5-7 days, and the pupal stage (in the soil) 10-12 days. Methods of monitoring the adult, larval or pupal populations in the greenhouse are outlined. In field control tests in 1977-79 at several localities, the only insecticide that gave satisfactory control was microencapsulated parathion-methyl (Penncap M), especially at 1 lb toxicant/100 US gal water; at the time of testing, this compound had not been registered for application on chrysanthemums, but a special local registration for use on greenhouse chrysanthemums in California was granted in view of the seriousness of the problem presented by this leaf-miner.