Utah Pests News Spring 2010

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Lilac-Ash Borer: A Common Pest of Ash Trees

The lilac-ash borer (Podosesia syringae), belongs to a group of insects known as the clear-winged moths (Sesiidae), which fly during the daytime and have partially transparent wings.  They are abundant in Utah, and most ash trees in the state have been attacked at one time or another.

Larvae feed primarily in the trunks and larger limbs of lilac, ash, and privet, and occasionally attack related plants in the family Oleaceae.  They initially feed beneath the bark and then move into the sapwood.  Larvae enlarge their galleries as they grow, frequently pushing frass (sawdust-like excrement) out of the entrance hole.  Completed galleries may be over 12 inches long and 1/3 inch wide.  Full grown larvae are about 1 inch long and white with a brown head (Fig. 1).  They overwinter in the heartwood.

In spring, larvae cut an emergence hole in the bark, leaving a thin flap of tissue over the hole.  Pupation occurs within the gallery, and when development is complete, the pupa wriggles through the protective flap, and emerges as an adult moth. Reddish-brown pupal cases are left protruding from the plant after moth emergence (Fig. 2).   Adults emerge around early April in southern Utah, and mid May in northern Utah.  They will continue to emerge for 6 weeks.   Adults resemble the common paper wasp in color, size, and shape (Fig. 3).

Soon after emergence, females emit a pheromone to attract males for mating.  Within an hour of mating, they begin to lay eggs in cracks, crevices, and bark wounds.  A single female can lay about 400 eggs.  Eggs hatch within 14 days, and larvae bore into the plant.  There is one generation each year in Utah.

Trees attacked by lilac-ash borer may have the following symptoms: 

  • frass pushed out of the burrows by larvae
  • circular exit holes
  • protruding empty pupal cases
  • oozing sap
  • tip dieback on smaller, or young shoots

For chemical control, sprays must be applied to the trunk and larger limbs just before egg-laying.  The insecticide will create a chemical barrier, killing larvae as they hatch and attempt to bore into the tree.  Once larvae are in the tree it is almost impossible to kill them.

To properly time preventive sprays, it is important to determine when moths begin to emerge.  Traps baited with clearwing moth pheromone lures will attract male moths from long distances, and do not need to be hung near an active infestation.  Trees should be protected soon after first trap catch.

Alternatively, lilac-ash borer emergence can be predicted by tracking cumulative growing degree-days (GDD), calculated from daily maximum and minimum temperatures.  Table 1 summarizes approximate monitoring and treatment dates for north, central, and southern Utah, using historic data.  Preventive trunk sprays begin at 507 GDD after January 1, and should continue, according to the reapplication interval stated on the insecticide label until 1369 GDD.  For more details, see the lilac-ash borer fact sheet

Insecticides used in managing lilac-ash borer kill the egg or newly hatched larva, so are considered preventive.  Options include permethrin (insecticide group 3A*), bifenthrin (group 3A), and endosulfan (restricted use; group 2A).  Imidacloprid (Merit, group 4A), a systemic insecticide often used for borer control in trees, is ineffective against lilac-ash borer and should not be used for this insect pest.

Non-chemical methods for managing lilac-ash borer include:

  • Reduce plant stress by properly watering, fertilizing, and planting.
  • Diversify the landscape by planting non-host trees and shrubs that are adapted to Utah’s climate.
  • Prune older branches near the tree base (fresh pruning wounds are prime egg-laying spots; avoid pruning prior to moth flight).
  • Remove severely infested trees from the property before adults emerge, or cover logs with thick plastic.
  • Paint trunks of newly planted trees white or apply white tree wrap in the winter/spring to reduce wounding from sunscald.

-Ryan Davis, Arthropod Diagnostician

* Each insecticide is classified in a group, and to prevent resistance, the user should rotate between groups.  

Potter, M.F., and Potter, D.A. University of Kentucky College of Agriculture.  Insect Borers of Trees and Shrubs.

Fig. 1.  Lilac-ash borer larva (Podosesia syringae) in a gallery.
Fig. 2.  Pupal skin of a lilac-ash borer extruding from a hole in an ash tree. Fig. 3.  Adult lilac-ash borer.


Table 1.  Management actions based on the lilac-ash borer phenology model (Potter and Timmons) for three regions in Utah.  Growing degree-days (GDD) and calendar dates are based on weather station data (climate.usu.edu/pest) from 2004 - 2007.  For more information on degree days, see the “Using Degree Days to Time Treatments for Insect Pests” fact sheet. 
*GDD accumulated since January 1, using a lower threshold of 50°F.  **St. George includes weather data only from 2004, so is not a true average.