Acrobasis Shoot Borers Acrobasis spp.

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Superorder	Holometabola
Order	Lepidoptera
Superfamily	Pyraloidea
Family	Pyralidae
Subfamily	Phycitinae
Tribe	Phycitini
Genus	Acrobasis

Scientific Name

Acrobasis juglandis

Common Name

pecan leaf casebearer

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 acrobasis shoot borers are pests of pecan and hickory as well as black walnut and butternut from Ontario and Quebec in southeastern Canada southward to Florida and west to Texas in the United States (Baker 1972, Neunzig 1972). Some of the more important species include Acrobasi nuxvorella Neunzig, A. juglandis (LeBaron), A. caryivorella Ragonot, and A. demotella Grote. The acrobasis shoot borers are perhaps best known for their damage to foliage and nuts, although they often act as shoot borers during spring when growth begins (Payne and others 1979, Neunzig 1972). Boring and tunneling by the shoot borers cause many new tender shoots to become stunted, distorted, or die. Seedlings in nurseries can suffer serious damage. Mortality to the terminals of young trees intended for timber production is perhaps the most damaging type of injury (Kearby 1978). The destruction of terminals causes reduced growth and dichotomous branching, which results in forks, crooks, and abnormal branching. Repeated terminal injury during early growth can adveresly affect the tree from that is so important when the goal is production of saw logs for lumber, veneer, and other products.

Description

The adults are grayish to brownish moths with reddish, black, or white markings on the forewings and wingspan of 12 to 20 mm (fig. 6A) (Payne and others 1979, Gill 1925, Neunzig 1972). The eggs are oval to elliptical, convex above and flattened below, with a rectangular micropattern on the outer surface of the chorion. When first deposited, the eggs are greenish white, but they gradually assume a whitish and then reddish tinge as development progresses. Eggs vary from 0.5 to 0.8 mm in length and 0.25 to 0.36 mm in width. Newly hatched larvae are pale reddish brown and about 0.8 mm long. Mature larvae (fig. 6B) are cylindrical, taper slightly toward each end, and 10 to 19 mm in length. Head and mouthparts are dark yellowish brown; the prothoracic shield is pale brown; and the body is olive green to jade green, usually darker dorsally than ventrally. The newly formed pupa is olive green but gradually changes to light brown.

Evidence of Infestation

Shoot damage occurs almost entirely during the spring (Martinat and Wilson 1978, Neunzig 1972, Payne and others 1979). The earliest evidence of injury may be holes in the swelling and unfolding buds. The majority of attacks to the shoot occurs after the buds have opened, but before there is much elongating or unfolding of the leaflets. Entrance holes are usually made into the basal part of the shoot just above the terminal leaf scar of the previous season's growth. However, some larvae make entrance holes some distance out on the shoot, usually where the inner base of a petiole joins the shoot. Small amounts of frass are extruded from the gallery entrance and silked together to make a short loose tube, often forming an extension of the tunnel in all except A. demotella, whose larvae do not form a frass tube. The frass tube often becomes prominent as loosening bud scales and additional frass are silked together. Tunnels excavated in the tender shoots by the larvae range from 6 to 45 mm long. The injury usually causes the terminal parts of the tender shoot to wilt, turn yellow and then brown, and did. Tunneled shoots sometimes become enlarged, swollen, or gall-like. Injured shoots that survive often become stunted and deformed and lost apical dominance to a lateral shoot. The different species of Acrobasis can be partially separated by habits in attacking buds and shoots, site of entrance into the shoot, amount of tunneling, and characteristics of the frass tube and/or silking together of adjacent leaves, petioles, and shoots (Neunzig 1972).

Biology

The shoot borers overwinter as partially grown larvae in tightly woven cocoons called hibernacula, typically found where a bud joins the stem (Neunzig 1972, Martinat and Wallner 1980, Payne and others 1979). The larvae emerge from the hibernacula in early spring and tunnel into the swelling buds and tender new shoots. When mature, a few larvae pupate within the galleries, but most vacate the tunnels and pupate under barkplates or drop to the ground and burrow into the litter or soil and pupate. The pupal stage lasts 11 to 18 days. Moths emerge and deposit eggs singly or in small groups, usually on nuts near the base of calyx lobes, on or near buds, and on the underside of leaflets. The eggs hatch in about 7 to 10 days. There may be one to four generations per year depending on species and north-south location. In the fall, third instar larvae construct and overwinter hibernatula on or adjacent to a bud. Only during the spring do the larvae attack the shoots; later broods feed on the nuts and foliage (Payne and others 1979).

Control

Natural enemies help in keeping the acrobasis shoot borers in check. Parasites are probably the most imprtant group of natural controls; many species of parasities have been reported (Neunzig 1972, Martinat and Wallner 1980, Gill 1925). When new plantings are established, they should be located away from existing areas containing Carya and Juglans spp. Corrective pruning during early growth may be the best alternative to control, especially when plantings are intended for timber purposes (McKeague and Simmons 1978). Pruning should be done in May or early June, soon after the current season's shoot damage ceases. The pruning technique should retain the strongest newly developing shoot as the new terminal. Corrective pruning aims to reestablish apical dominance by one shoot in order to correct forks that may have resulted from terminal bud injury, thus improving tree form. Insecticides may be necessary when infestations are heavy (Payne and others 1979).