Winter is spent as full-grown larvae in cocoons in small earthen 
cells 1 or 2 inches below the soil surface. Pupation occurs in the 
spring and the adults begin to appear about mid-May. Eggs are 
deposited singly in slits cut in the central areas of young leaves, 
usually near the tips of branches. The larvae feed on the tissues 
between the leaf surfaces. At first, they feed singly, forming 
small kidney-shaped mines near the egg. Then, as they increase 
in size, the mines of different larvae coalesce and form large, 
hollowed-out brown areas in the leaf. These areas (fig. 186) 
wrinkle and turn brown. Full-grown larvae chew their way out 
of the leaf and drop to and enter the ground where they form 
earthen cells in which to pupate. There are three or four genera- 
tions per year in the southern portions of the infested region 
(271). 
Outbreaks occur frequently in the Northeastern States and re- 
sult in the browning of birch stands over wide areas. Some tree 
killing may occur, but the greatest damage is the weakening of 
affected trees, which leads to attack by other insects. 
The elm leaf miner, Fenusa ulmi Sundevall, an introduced 
species, occurs in southeastern Canada and the Northeastern 
States west to the Lake States. Its preferred hosts are English, 
Seotch, and Camperdown elms. American elm is also attacked 
occasionally. Full-grown larvae are about 6 mm. long, flattened, 
and whitish with a greenish cast. The head is brown and the legs 
are encircled with brown. 
Winter is spent as full-grown larvae in brown papery cocoons 
in the topsoil. Pupation occurs in the spring and the adults appear 
in May, usually during the first half of the month. Eggs are laid 
in the upper surfaces of leaves. The larvae mine the tissue be- 
tween the leaf surfaces, causing large blotch or blister-like mines. 
Several attacks may occur on a single leaf. When this happens, 
the various mines may coalesce and the entire leaf be hollowed 
out (fig. 187). These leaves soon wither and fall. Where only a 
small portion of a leaf is mined, the surfaces dry out and crack, 
COURTESY CONN. AGR. EXPT. STA. 
FIGURE 186.—Mines of the 
birch leaf miner, Fenusa 
pusilla, in leaves of gray 
birch. 
455 
