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University
of Rhode Island GreenShare Factsheets
Arborvitae
Leafminers Argyresthia
spp.
There are four
species of arborvitae leafminers in the Northeast, although Argyresthia
thuiella is the most common. Mined arborvitae (Thuja
spp.) leaves first exhibit a translucent or straw color which later
turns to brown. Mines start in the scale-like leaves near the end
of a branchlet, extending rapidly into other branchlets. Injury
begins in the summer and reaches a climax in the fall. The death
of the mined branchlets gives the tree a sickly appearance. The
greatest injury probably occurs to hedge rows and ornamental plantings.
The adults
are white to light gray moths with a wingspan of 10 mm (1/8 inch)
and brown markings on the forewings. The larvae are 3 mm (1/8 inch)
long caterpillars with a light green (sometimes with a reddish or
yellow tinge) body and a shiny black head. The adults
are active from late May to early June. Many tiny glittering moths
may call attention to an infestation. After mating, the females
deposit eggs on the inner edges of the arborvitae leaves. The young
larvae enter the leaves and mine, or excavate, between the leaf
surfaces. The larvae overwinter in the mines and resume feeding
briefly the following spring. Pupation occurs in late May and the
adults emerge soon after. Under forest
conditions, leafminers are usually kept in check by natural control
factors including tiny hymenopterous (wasp) parasites. Cultural:
Some control may also be obtained by cutting off infested, discolored
tips in the fall and destroying them. Chemical:
It may be necessary to spray ornamentals in order to control an
infestation. Check for currently registered insecticides. One application
should be made in early June, with a second application in July
when adults are active or 7 to 10 days after adult activity is observed.
Follow manufacturer's directions carefully when using any pesticide.
Adapted
from the Cornell Cooperative Extension , 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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