Pacific Flatheaded Borer
Solomon, J.D. 1995. Guide to Insect Borers in North American Broadleaf Trees and Shrubs. Agriculture Handbook 706. Washington, DC. United States Department of Agriculture, Forest Service. 735 p.
Alder, birch, ash, ceanothus, oak, boxelder, mahogany, maple, poplar, sycamore, willow, apple, pear, beech, elm, cotoneaster, peach, plum, avocado, loquat, cherry, currant, fig, apricot. At least 70 species belonging to 40 genera in 21 plant families have been reported as hosts (Burke and Boving 1929). Sycamore, mahogany, ceanothus, and fruit and nut trees seem particularly susceptible (Davis and others1968, Essig 1929)
Widely distributed throughout western North America west of the Rocky Mountains from California to British Columbia and Manitoba (Burke and Boving 1929, Fisher 1942). Its economic effect are particularly felt in California.
Dark bronze to reddish copper beetle with distinct copper spots on the elytra; 6 to 11 mm long.
About 1 mm in diameter, subcircular or disk-like, and yellowish white (Brown and Eads 1965a). Form of egg varies with the crevice or depression in which they are laid (Burke and Boving 1929).
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.
Translucent white when first formed, dark bronze near adult emergence, and 6 to 11 mm long (Brown and Eads 1965a).
Adults emerge from April through August but mostly in June and July. Soon after emergence, mating and egg-laying begin (Brown and Eads 1965a). Eggs are deposited singly but may be laid close enough together to form clusters. The eggs are usually deposited in bark crevices or depressions (Burke and Boving 1929). 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 (Brown and Eads 1965a).
Injury and Damage
The first evidence of feeding is usually dark wet spots on the bark, which may later crack slightly and expose borings (Burke and Boving 1929). 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 may occur in the bark over a tunnel. After larvae mine extensively, the bark often loosens and drops away, leaving ugly cankerlike spots. Larval feeding may occur in any part of the bark from the roots to the tree top; however, feeding typically occurs in the main trunk, especially in smaller trees. The extent of damage to the tree is related to the location in the tree of feeding sites and often to whether or not the borers encircle and girdle the branch or trunk. This borer is considered one of the most damaging pests of newly planted deciduous trees. It is an important pest in nurseries, where trees may be killed or so severly injured that their value is disminished. Also, shade, fruit, and nut trees may be killed or disfigured. In native forests, branch mortality is common, but the insect rarely kills an entire tree (Burke and Boving 1929).
Five species of larval parasites have been found--two braconids, one chalcid, one ichneumonid, and one tetrastichid (Burke and Boving 1929). The chalcid Trigonura californica Rohwer is the most important parasite, being found in 29 of 151 infestations studied. A chalcid egg parasite has also been found, but little is known of its effectiveness. A mite--Pediculoides ventricosus Newport--the most important predator, was reported to destroy a high proportion of the developing brood in heavily infested plants in California. Field observations suggest that birds remove immature stages of the borer from infested trees. Control can be enhanced by cultural practices that encourage vigorous, healthy plants. Young transplanted trees can be protected by mechanical covers over the trunks. Mechanically removing larvae is an effective control for valuable trees. Sanitation practices include removing weakened, injured, dying, and dead trees from the area. Chemical controls are sometimes needed to protect valuable plantings (Davis and others 1968).