Broadnecked Root Borer Prionus laticollis (L.)<br/> Tilehorned Prionus Prionus imbricornis (Drury)

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Order	Coleoptera
Suborder	Polyphaga
Infraorder	Cucujiformia
Superfamily	Chrysomeloidea
Family	Cerambycidae
Subfamily	Prioninae
Tribe	Prionini

Scientific Name

Prionus

Common Name

prionid beetles

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 broadnecked root borer, Prionus laticollis (L.), and the tilehorned prionus, P. imbricornis (Drury), are pests of pecan and hickory as well as many other trees from southern Canada throughout the Eastern United States and extending west to the Plains States (Payne and others 1976, Linsley 1962). Injury occurs from the larvae feeding on and destroying the roots. The larvae feed first on the root bark, but they soon enter the wood, completely hollowing large roots and often severing then. The larvae move from root to root through the soil, feeding on the surfaces of smaller roots as they go, causing many injuries and wounds. Open-grown mature trees and those weakened by disease, drought, machanical damage, or soil conditions are most susceptible to attack, but young, vigorous trees can also be attacked and are occasionally cut off at the groundline. Severe root damage results in reduced growth, a poor nut crop, and eventual death of the tree. In southwest Georgia, pecan growers estimated that 3 to 10 percent of the trees showed signs of attack (Payne and others 1970).

Description

The adults are robust, broad, somewhat flattened, and blackish brown to reddish brown (fig. 12A)(Linsley 1962). They have antennae about half as long as the body, and there are broad spines on each side of the prothorax. Adults of P. laticollis measure up to 45 mm long and adults of P. imbricornis measure up to 37 mm long. The eggs are irregularly punctate and glossy yellow, but later darken and lose their glossiness (Farrar and Kerr 1968). One end of the egg is slightly larger than the other. Eggs average 3.5 mm long and 1.4 mm wide. The large rootboring larvae are fleshy, elongate-cylindrical, and creamy white to yellowish; they have three pairs of small legs and small heads armed with strong mandibles adapted for boring in wood (fig. 12B) (Payne and others 1970). Mature larvae attain lengths of 9 cm or more and weigh up to 15 g. Pupation takes place inside earthen cells, and pupae are white initally, but their eyes, mandibles, and appendages darken as adult transformation approaches.

Evidence of Infestation

Since injury to the roots occurs below ground level, correct diagnosis is often difficult (fig. 12C) (Payne and others 1970). Only by excavating the tree and examining the roots can one confirm the infestation . The above-ground syndrome is a gradual decline, characteristic of any tree under severe, prolonged stress (fig. 11) . Symptoms often resemble a nutrient deficiency—leaves may be sparse, reduced in size, and have a light-green to yellowish tinge. Nut yields from infested trees may be three to seven times less than from healthy trees. As an infestation progresses over a period of several years, 70 to 90 percent of the root system may be destroyed, resulting in a limb-by-limb death of the tree (Sparks and others 1974).

Biology

Adults emerge from the soil in late spring and early summer (Baker 1972, Benham and Farrar 1976). The beetles are normally nocturnal or crepuscular in habit and during the day remain hidden beneath debris or loose bark at the base of the tree. The females are short-lived—about 1 week—but deposit 300 to 500 eggs either singly or in groups in the soil near the base of host trees. When the eggs hatch in 2 to 3 weeks, the young larvae dig down to the roots and begin feeding on the bark. They move from root to root through the soil feeding on the surfaces of smaller roots as they go and causing many injuries and wounds. They enter the wood of larger roots and hollow, girdle, or sever them. In the summer, the larvae feed on roots in the upper 15 to 45 cm of the soil, but in winter they are often found at depths down to 80 cm. The feeding period lasts 3 to 5 years. In early spring, mature larvae come to within 6 to 12 cm of the soil surface and prepare large, oval earthen cells in which they pupate and transform to the adult stage.

Control

Root borers usually attack trees weakened by other factors such as disease, drought, mechanical damage, or soil condition (Sparks and others 1974). Therefore, cultural practices should be followed to keep trees thrifty and vigorous. Since these root borers are also pests of other tree species, it is advisable to establish new plantings some distance away from orchards and tracts of woodland that are already infested. Although little is known about natural enemies, they undoubtedly play a role in regulating root borer populations. A tachnid parasite (near Dexilla) has been reared from P. laticollis (Benham and Farrar 1976), and several bacteria and fungi have been isolated from dead larvae and pupae of P. laticolis. It may be necessary to use insecticides occasionally if root borer populations threaten high-value trees (Payne and others 1976).