Flatheaded alder borer—Agrilus burkei
This flatheaded borer (family Buprestidae) attacks only alders, especially white alder (Alnus rhombifolia) that is drought stressed.
Plant damage as described below is generally the first clue of an infestation of flatheaded alder borer. The elongate adult is dark, shiny, metallic blue or green and 1/4 to 3/8 inch long. It is widest in the middle of the abdomen and tapers toward the rear. The whitish egg is oval to round and flattened.
Larvae and pupae occur hidden under bark. The slender larvae are pale yellow to whitish with brown mouthparts and swollen immediately behind the head. Mature larvae are 4/5 to 1 inch long. The oblong pupae are initially pale then darken as they age.
Adults emerge from infested alders in late March through May. They chew and feed on the outer edges of alder foliage, although this slight damage is usually not apparent. Each adult lives for about 2 weeks. After mating each adult female lays about 3 to 10 eggs in a mass on the lower trunk or main branches of alders, especially on stressed or declining trees. Eggs hatch in 1 to 2 weeks and the larvae bore into the bark. Larvae chew and feed to form winding tunnels in cambium. In chambers just beneath bark mature larvae (prepupae) overwinter then pupate in spring. There is 1 generation per year.
Larval chewing damages alder's vascular system. Noticeable from a distance is foliage yellowing and wilting and dieback of infested branches. Wet spots, dark staining, and gnarled, ridged or swollen growth appear on bark where larvae feed underneath. After larvae complete feeding, they pupate under bark. Each emerging adult leaves a round to D-shaped hole in bark about 1/8 inch in diameter. Infested limbs commonly die and entire trees can be killed by flatheaded alder borer.
Provide proper cultural care and protect trees from injury. Providing alders with adequate irrigation throughout their lives is especially important where summer drought prevails. White alders are native to sites near year-round surface water and can become highly drought stressed when planted in landscapes. Consider replacing problem-prone trees. If an alder is preferred, plant more borer-resistant species, such as black alder (e.g., Alnus glutinosa). See the Solutions section of the similar Pacific flatheaded borer for detailed preventive and management recommendations.
During late summer or fall, prune out and dispose of all branches showing bleeding, dieback, swelling, or other evidence of infestation by borer larvae. Avoid pruning white alder anytime between March and the end of May because egg laying adult beetles are attracted then to recent pruning wounds.
If insecticide will be applied to reduce future infestations, beginning late March inspect leaves for adult chewing feeding. Look for adult beetles during mid to late afternoon by examining foliage and beating or shaking branches over a light-colored surface onto which insects will drop. If adults are known to be present, foliage and wood can be thoroughly sprayed with a residual insecticide (e.g., carbaryl or a pyrethroid) during about mid-April.
Insecticide products available to homeowners are not effective and a professional applicator with access to effective products is needed to make the application. Consult Pest Notes: Hiring a Pest Control Company for guidance when selecting a company for hire. Do not substitute insecticide applications for providing proper growing conditions and good cultural care or sprayed trees are likely to die from the conditions that caused them to become unhealthy and attractive to borers.
For more information see Flatheaded Alder Borer in White Alder Landscape Trees (PDF) from the University of California and Guide to Insect Borers in North American Broadleaf Trees and Shrubs from the U.S. Department of Agriculture.
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).