Pin-Hole Borers Platypus compositus (Say) and Xyleborus spp.

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Order	Coleoptera
Suborder	Polyphaga
Infraorder	Cucujiformia
Superfamily	Curculionoidea
Family	Curculionidae
Subfamily	Platypodinae
Tribe	Platypodini
Genus	Myoplatypus

Scientific Name

Myoplatypus flavicornis

Common Name

ambrosia beetle

Solomon, J. D.; Payne, J. A. A guide to the insect borers, pruners, and girdlers of pecan and hickory. Gen. Tech. Rep. SO-64. New Orleans, LA: USDA Forest Service. Southern Forest Experiment Station; 1986. 31 p.

Importance

The pin-hole borers, Platypus compositus (Say), Xyleborus affinis Eichoff, X. ferrugineus (F.), and X. saxeseni (Ratzeburg), also called ambrosia beetles, are best known for their damage to green logs and unseasoned lumber. They inhabit the trunks and branches of pecan and hickory as well as many other deciduous trees (Baker 1972, Bright 1968). The Xyleborus spp. are widely distributed from Canada throughout the eastern half of the United States. Platypus compositus is found from Mexico throughout the Southern United States northward to West Virginia. Ambrosia beetles seldom attack healthy, vigorous trees; their attacks are largely limited to stressed trees weakened from drought, disease, old age, insect defoliation, wounding, and other factors that produce tree stress. Thus, when trees are successfully attacked by ambrosia beetles, one can be sure that the trees have been under stress of some sort. The beetles tunnel through the bark directly into the sapwood and sometimes even into the heartwood, especially P. compositus. Although ambrosia beetles are not tree killers, the physical wounds produced by large numbers of beetles provide ports of entry for disease agents that may cause tree death. The pinholes and associated stain quickly degrade the wood for lumber and other wood products.

Description

Adults of the Xyleborus spp. are small, brown, reddish-brown, or black elongate beetles with compact cylindrical bodies that vary from 1.5 to 3.0 mm in length (Bright 1968, Chamberlin 1939, Blackman 1922). The adults of P. compositus can be distinguished by having longer and more slender bodies and wide heads flattened in front (fig. 24). The first segment of the tarsus is as long as all the other tarsal segments combined and about 4.5 mm in length. The eggs are elongate oval and pearly white. The larvae of Xyleborus spp. are C-shaped, legless, white to cream colored, and reach 3 to 4 mm in length; larvae of P. compositus are straight to only slightly C-shaped, legless, white to cream colored, and reach 5 to 6 mm in length.

Evidence of Infestation

Infested trees are usually attacked by numerous beetles that bore many uniform, round (0.8- to 1.7-mm diameter) holes directly through the bark into the wood (Chamberlin 1939). As the beetles construct their galleries, they push the fibrous (P. compositus) or granular (Xyleborus spp.) boring dust to the outside through the entrance holes. The boring dust is usually in evidence on the bark just below the entrance holes or in loose piles at the base of the tree; during humid weather it may stick together as it is pushed out to form string-like masses. Sap frequently oozes from many of the entrance holes, staining the bark around and below the entrances. Active galleries are kept free of boring dust and are light colored; vacated galleries are stained black by fungi. The galleries extend directly into the sapwood and then branch several times—the branching pattern depending on the species. These galleries cause dark-stained pinholes that will show up as defects in sawn lumber.

Biology

In the deep South the beetles are active most of the year, but further north they hibernate in brood galleries in the host tree (Chamberlin 1939). In the spring the adults emerge and initiate new attacks on the same tree or on different trees. Both adults and larvae feed on moldlike fungi that they culture in the galleries. The insects carry the specific inoculum from one tree to another and grow the fungi in pure culture on the walls of the tunnels. Galleries 0.8 to 1.6 mm in diameter are bored horizontally into the sapwood, then branch several times for a distance of 10 to 40 cm. Galleries of P. compositus are much more extensive and extend deeper into sapwood and heartwood than those of Xyleborus spp. Females deposit eggs in loose clusters in the galleries and may lay up to 200 eggs each. The eggs hatch in 6 to 10 days, and the young larvae wander freely about the mines and feed on the ambrosia fungus. Several adults may occupy each gallery. Both adults and larvae help to excavate and extend the galleries. When full-grown, the larvae of P. compositus excavate cells along the main gallery in which they transform to pupae and adults; those of Xyleborus spp. pupate freely in the galleries without making cells. Development from egg to adult during the summer requires 5 to 8 weeks. There are several generations per year in the Gulf Coast States. Successive broods may continue to attack a tree as long as it remains suitable. Infested trees that die become unsuitable for brood development as soon as the moisture content drops below about 48 percent.

Control

Since ambrosia beetles rarely attack healthy vigorous trees, good cultural practices should be followed to promote and maintain tree vigor (Baker 1972, Payne and others 1979). Trees that are weakened or stressed apparently emit fermenting-like odors that attract ambrosia beetles. Because healthy trees do not emit these odors, they remain unattractive. Insecticides may be needed occasionally.