ARBORVITAE LEAF MINER (Argyresthia thuielld).
Leaf miners are chewing insects which tunnel between upper and lower
leaf surfaces. Their mines are irregular blotches, or conspicuous winding
or serpentine markings, or, as with the birch leaf miner, a blighting of
the major portion of a leaf. Miners are fly maggots, beetle grubs, sawfly
larvae, and occasionally caterpillars. Until recently they could be con-
trolled at only one brief period-when the adult emerged to lay eggs-
but with some of the new insecticides it is possible to kill young larvae
inside a leaf.
Larvae of this moth and
two other species mine the foliage tips, eating out the inside of the
leaves and turning them conspicuously white or tan. They are small green
caterpillars with a reddish tinge, black heads. They pupate in May, and
small gray moths lay eggs in May and June.
Control. Two applications of DDT during the month of adult flight
give excellent control but markedly increase injury from spruce mites.
Parathion is effective but dangerous to use; perhaps it can be replaced by
the somewhat safer malathon. Nicotine sulfate gives some control.
AZALEA LEAF MINER (Gracilaria azaleelld).
BIRCH LEAF MINER (Fenusa pusilla).
This is a greenhouse pest in
the North, sometimes a garden problem in the South. Small yellow
caterpillars feed between leaf surfaces causing blisters; later they come out
of the blisters, roll the leaves, feed inside that protection, and then make
a cocoon there. Parathion dust has been satisfactory in greenhouses;
lead arsenate can be used on garden shrubs before leaves are rolled.
This sawfly causes the brown
blighted birches that are such a blot on the landscape through the
Middle Atlantic States. Larvae winter in small cells in the ground,
pupating in spring. Small black sawflies emerge about the time new
leaves develop and lay eggs in leaf tissues over a 3-week period. As many
as 15 or 20 larvae develop in a single leaf, and their mines run together
to form a large brown blotch, often covering more than half the leaf,
in which you can see small black specks of excrement. The larvae drop
to the ground to pupate, and second-brood adults appear in late June.
Control. The old nicotine sulfate and soap recommendation was not
very effective, and DDT does not do so good a job here as for other
leaf miners. Chlordane and lindane are both efficient. Start spraying when
you see small gray kidney-shaped areas in leaves when held up to the
light (about mid-May). Spray for the second brood in late June, and
repeat 2 weeks later.