Leaf
Miners > Birch Leafminer
Pest: Birch
Leafminer (Fenusa pusilla (Lepeletier))
Order: Hymenoptera
Suborder: Symphyta
Family: Tenthredinidae
Host Plants:
Gray birch (Betula populifolia ) and the native paper birch
(B. papyrifera ) are very susceptible ; this pest is much
less common to black, yellow, European and river birches (Johnson
and Lyon).
Description:
The tiny larvae of this leaf mining
sawfly can incur complete defoliation on the host plant. Repeated
infestations, in consecutive years, can weaken the host to the point
where it is invaded by secondary pests such as the bronze
birch borer.
Life Cycle:
The adults appear in May, in Massachusetts, and their appearance
is closely timed to the emergence of new foliage on the host plant.
Adult females require tender, newly expanding foliage within which
to lay their eggs. The eggs quickly hatch and mining begins, which
initially appears as small yellow spots
on the foliage.
Larval feeding occurs for about 4 weeks, whereupon, they chew a
hole through the bottom of the leaf and drop to the soil. One leaf
may support a dozen or more larvae and be completely browned from
their feeding. One moderate-sized landscape tree may support tens
of thousands of larvae in the first generation. Most of the larvae
will stay in the soil until the following spring but a limited number
(less than 30%) will pupate and emerge about a month later as the
second generation. There is even a third generation in southern
New England but it is smaller still.
Management Strategies:
The first generation is by far the most important; the second
and third rarely, if ever, warrant controls. The placement of yellow
sticky cards in the host tree prior to
adult emergence is an extremely effective method for monitoring
the adult emergence time. Otherwise, visually inspecting the foliage
for the active, tiny, black adults
will be required. Managing the adults is important in breaking the
cycle of infestation. Once the larvae are in the foliage, management
becomes much more difficult. When the larvae have just become active
within the foliage, certain foliar systemic pesticides may be effective.
Otherwise, a whole-tree systemic insecticide may be necessary. Removal
of adventitious growth, such as root suckers
and water sprouts
can reduce the number of ovipositional sites for the second and
third generations, in particular. This growth is more succulent
and may support larger than normal populations of this pest. The
GDD for the first generation of adults is 100 - 200.
The second generation appears from 530-700 GDD.
Images:
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Note the tiny (blackfly-sized) adults of the birch leaefminer
on the host foliage. Visual inspections can be performed by
gently shaking the foliage and observing their numbers. (R.
Childs)
This birch leaf is showing about a dozen
tiny yellow these. These represent individual
leaf miners at each of those sites. These
mines will expand and then coalesce into one giant blotch
mine. (R. Childs)
A yellow-sticky card used to monitor for the activity of adult
birch leafminers. Insects that require new, tender foliage
for egg-laying, often perceive this color as a super attractant.
(R. Childs)
A birch leaf that has been completely
mined by birch leafminer larvae. All of
the foliage on a susceptible tree may
appear like this by late May in New England.
(R. Childs)
Adventitious (sucker) growth emerging
from the base of a birch tree. This fast
growing and highly vegetative growth can
harbor birch leafminer populations.
Therefore, it should be removed. (R. Childs)
Birch leafminer larvae feeding within a birch leaf. (R. Childs)
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Leaf
miners index:
Arborvitae Leafminer
Birch Leafminer
Boxwood Leafminer 
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