Rhododendron
Borer
Pest: Rhododendron Borer (Synanthedon rhododendri (Beutenmuller))
Order: Lepidoptera
Family: Sesiidae
Host Plants:
Rhododendrons, mountain laurel (Kalmia ), and deciduous
azaleas (Johnson & Lyon)
Description:
Early symptoms of attack, as described by Johnson and Lyon,
appear to be drought stress, with foliage losing its color and dying.
Entry holes are often near branch crotches or close to the ground
on the main stem. Being a clear-winged moth borer, this pest expels
sawdust and frass (excrement) from its tunnel during the early stages
of attack. This material will accumulate near the entrance holes
and on the ground. Infestations can be severe, especially in large
plantings and much branch and plant death may occur.
Life Cycle:
Larvae over-winter within the host and pupation occurs in the
spring right at the exit hole. Moths are active from late May through
much of June, in Massachusetts. GDD for adult flight period is 192-707.
Eggs are laid near wounds, roughened bark, branch crotches and pruning
sites. Larvae chew their way into the cambial area.
Management Strategies:
Pheromone traps are available to monitor the time of adult emergence.
Entomopathogenic nematodes may be of limited use given that the
larvae of this clear-winged borer do not entirely keep their tunnels
clear of sawdust and frass. Applied sprays of these must be directed
to the base of the major shoots and trunks to maximize effectiveness.
Oftentimes, heavy infestations are not realized until severe symptoms
are evident.
Images:
| 
An adult rhododendron borer clear-winged moth. (R. Childs)
The bark of this rhododendron stem has been cut away to reveal
a rhododendron borer larva within, its tunnel and frass. (R.
Childs)
Close up of a rhododendron borer larva within the host plant
stem. (R. Childs)
A rhododendron that was previously attacked by the rhododendron
borer. A major shoot was killed and then pruned away. Note
that the stub left from pruning is hollow. The sawdust on
the ground, is from the activity of the borer and not from
the pruning (R. Childs).
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Wood
attackers index:
Asian Longhorned Beetle
Bronze Birch Beetle
Dogwood Borer
Rhododendron Borer
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