EXTENT OF LEAF-MINING HABIT 37 



tobacco split-worm (Pkthorimaea operculella) that is now in 

 the leaves and then in potato tubers in storage, or like the 

 lilac leaf-roller that mines the leaf while small, and then 

 rolls it up from the margin and lives and feeds thereafter in 

 the roll; or like the ribbed cocoon-maker of the apple 

 (Bucculatrix pomifoliella) that mines during its first instar 

 only, and thereafter feeds openly upon the leaf. Or, they 

 are general feeders like the beet leaf-miner, Pegomyia 

 hyoscyami, now here, now there, accepting whichever food 

 plant offers, only confining themselves to a group of related 

 plants. 



Leaf -miners infest many of our cultivated crops, but few 

 of them are of economic importance. The apple has at 

 least ten different North American species that mine its 

 leaves (see Chapter XVI, p. 313) ; two or three of them are of 

 some importance, occasionally, in ill-kept orchards. Newly 

 imported leaf-miners that are running ahead of their para- 

 sites are apt to be most injurious — until their parasites 

 catch up with them. The Fenusa miner on birch, the 

 Schizocerus miner on European alder, and the Gracilaria 

 miner on lilac are examples of recent immigrants into the 

 Eastern United States, that now (1925) are causing un- 

 sightly foliage wherever they occur. The so-called "wheat 

 leaf -miner/ ' Alphelosetia praematurella, a native species, is 

 only a scatterling in the leaves of wheat, its preferred food 

 plant being wild rye (Elymus) . 



The most important economic species are probably those 

 affecting vegetable crops, such as the beet leaf -miner, Pego- 

 myia hyoscyami, the tobacco split-worm Phthorimaea oper- 

 culella, and other species affecting cabbage seedlings, celery 

 and peppers. The chrysanthemum miner, Phytomyza chry- 

 santhemi is a real pest in the greenhouses of the commercial 

 floriculturist. The coffee leaf -miner, Leucoplera coffeella, 

 is a serious hindrance to the growing of that crop in Porto 

 Rico, since its great blotch mines reduce the working 



