Leafminers usually belong to two insect orders, either they are Diptera (flies) or Lepidoptera (butterflies/moth) [There are some exceptions] If you are sure that your leafminer species belongs to the Lepidoptera then B.T. is in fact an acceptable spray for organic control, however, it may not get into sufficient contact with the leafminer since it is hidden away inside the leaf. Early detection and early applications of B.T. are crucial. Some growers reported that sprinkling plants with wood ashes as a prevention has helped.

If your leafminer belongs to the order of Diptera you will probably be more successful with a pyrethrum spray, however, again early detection and applications are important.