Question
i have a large lilac tree it has been invaded by an orange grub with a black head,it is about 1.5ins long and has a fairly hard skin(well it crunched when i stood on one!)
it has tunneled through the center of large branches and the main body of the tree. do you have any idea what it is?

Answer
ash/lilac borer

If you have had problems with the canes or stems of lilac and privet suddenly wilting, or ash trees that show borer holes in the trunk and larger branches, the ash/lilac borer may be to blame. This insect causes the base of infested lilac stems to swell and the bark to separate from the wood. A fine sawdust-like material is present around holes in the canes. Ash and mountain ash also are affected. The borer attacks the trunk, which may cause bark to swell and crack if there are repeated infestations.

   

Ash/lilac borers overwinter as larvae in infested trees and shrubs. Moths generally begin to emerge in mid to late April. Emergence peaks in May, dwindles by mid to late June and ends by the first week of July. The moth has clear wings and resembles a wasp. There is one generation per year.



Public and commercially managed properties often use pheromone traps to determine the presence of adults. Spray treatments are started seven to 10 days after the capture of the first moths. Sprays can also be timed using phenology, the practice of timing one event by another. The first spray for ash/lilac borer should be applied when the Vanhoutte spirea is in full to late bloom, probably by about May 1 this year. A second spray should be applied four weeks after the first. Thoroughly treat the trunk and larger limbs of ash or the lower portion of the stems of lilac or privet. Heavily infested ash should be cut and burned during the fall and winter.

   

Infested stems of lilac or privet should be removed as well. Permethrin (Hi-Yield Garden, Pet, and Livestock Insect Control) is labeled for control. Though there are a number of other homeowner products that contain permethrin, the product listed above is the only one I've found that specifically lists the Ash/lilac borer on the label. (WU) Check with your local nursery/garden type store for an insecticide for lilac borers.