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
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.