The elm leafminer, F. u/mi Sundevall, an introduced species, occurs in south- 
eastern Canada and the Northeastern United States west to the Lake States. Its 
preferred hosts are English, Scotch, 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 prepupae 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 
between the leaf surfaces, causing large blotch or blisterlike 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. 191). These leaves soon wither and fall. Where 
only a small portion of a leaf is mined, the surfaces dry out and crack, leaving holes 
in the leaf. The larvae usually become mature in late June. Then they vacate their 
mines and drop to and enter the ground to spin their cocoons. There is one 
generation per year (/097). This species appears to be most injurious to small trees 
in nurseries and ornamental plantings. 
: A < ‘ 
ce, é 
Courtesy Conn. Agric. Exp. Stn. 
Figure 191.—Mines of the elm leafminer, Fenusa 
ulmi, in elm leaves. 
The European alder leafminer, F. dohrnii (Tischbein), an introduced species, 
occurs in southern Canada and the Northern United States. Its hosts are listed as 
alders, especially the introduced European alders. Winter is spent in the prepupal 
stage in cocoons in the ground. Pupation occurs in the spring, and the adults appear 
and lay eggs during late May and early June. Larvae feed in the tissues of the leaf 
for about 3 weeks, forming blotch mines (/097). Up to 12 larvae may feed on a 
single leaf. Full-grown larvae drop to the ground to pupate, and a second generation 
of adults appears from late July to early September. They also lay eggs and give rise 
to a second generation of larvae. These become full grown by late fall and then 
enter the ground for the winter. 
400 
