Pacific Flatheaded Borer

Pacific flatheaded borer

Chrysobothris mali Horn

* Hosts -- At least 70 species belonging to 40 genera in 21 plant families have been reported as hosts, fruit and nut trees seem particularly susceptible

* Range -- Widely distributed throughout western North America west of the Rocky Mountains from California to British Columbia. Its economic effect are particularly felt in California.

* Adult -- Dark bronze to reddish copper beetle with distinct copper spots on the elytra; 6 to 11 mm long.

* Egg -- About 1 mm in diameter, subcircular or disk-like, and yellowish white. Form of egg varies with the crevice or depression in which they are laid.

* Larva -- Varies from yellowish white to yellow and 15 to 18 mm long when mature. Thoracic segments--not head, as name implies--greatly enlarged and flattened. Abdomen bent back, making larva look like hook when exposed in feeding burrow.

* Pupa -- Translucent white when first formed, dark bronze near adult emergence, and 6 to 11 mm long.

* Biology -- Adults emerge from April through August but mostly in June and July. Soon after emergence, mating and egg-laying begins. Eggs are deposited singly but may be laid close enough together to form clusters. The eggs are usually deposited in bark crevices or depressions. During eclosion, larva bore through the bottoms of their egg shells directly into the bark. Most larvae reach maturity by September or October, construct pupal chambers in the heartwood, and then molt into the last larval instar. Borers overwinter as prepupae. Pupation occurs mid-March into June, with most larva pupating between mid-April and mid-May. Adult emerge in the pupal cells and chew their way to the outside. Usually, there is one generation per year, but the life cycle may be longer at higher elevations and in its northern range.

* Injury and Damage -- The first evidence of feeding is usually dark wet spots on the bark, which may later crack slightly and expose borings. Some tree species, especially those in the genus Prunus, show a slight flow of gum from the affected area. Little (sometimes no) frass is ejected to the outside, but as they burrow, larvae pack it behind them within mines. The area around and over the wound often becomes roughened. Feeding burrows are winding, shallow mines in the inner bark and outer wood of the tree. Mines, oval in cross section, sometimes spiral and girdle branches or the trunks of small trees. Mines end in the outer wood in pupal cells that open to the surface through oval exit holes. Usually, bark over the tunnel cracks and peels, but thick bark may not crack. Either a ridge or a depression