to the ground and the prepupa remains in its cocoon throughout the winter. There is 
one generation per year (977). 
Heavy infestations of the birch leafmining sawfly occurred in Maine during the 
1920’s and 1930’s, and severe defoliation of birch occurred in many areas. Very 
little tree mortality occurred, but there was a considerable loss in annual growth. 
During this period several species of parasites were imported against the sawfly 
(327), two of which, Chrysocharis laricinellae (Ratzeburg) and Phanomeris phy- 
llotomae Muesebeck, became established. 
The birch leafminer, Fenusa pusilla (Lepeletier), an introduced species first 
recorded from North America in Connecticut in 1923, now occurs from New- 
foundland to Maryland, west to Ontario, Minnesota, and Iowa; also in Washington 
and Oregon (/097). Full-grown larvae are somewhat flattened, yellowish white in 
appearance, and about 6 mm long. Black spots occur on the venter of the thorax and 
the first abdominal segment. 
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. As the 
individual mines increase in size, they coalesce and form large, hollowed-out 
brown areas in the leaf. These areas (fig. 190) wrinkle and turn brown. Full-grown 
larvae chew their way out of the leaf and drop to and enter the ground where they 
form small earthen cells, 2.5 to 5 cm below the soil surface, in which they 
overwinter as prepupae in cocoons. There are three or four generations per year in 
the southern part of the range of this leafminer (450). 
Outbreaks occur frequently in the Northeastern States and result in the browning 
of birch stands over wide areas. Some trees may be killed, but the greatest damage 
is the weakening of affected trees, which leads to attack by other insects. 
Courtesy Conn. Agric. Exp. Stn. 
Figure 190.—Mines of the birch leafminer, Fenusa 
pusilla, in leaves of gray birch. 
399 
