BORING INSECTS

Some boring insects attack buds or other soft plant parts; others bore into the wood itself. Bud borers are perhaps the most serious insect pests of black walnut because they can kill terminal buds and cause multiple forking in a young tree, destroying its value for veneer or high quality timber. Insects that bore directly into wood usually attack only weakened, dying, or dead trees.


AMBROSIA BEETLES

Description
    At least two species of ambrosia beetles attack black walnut trees but the most serious is Xylosandrus germanus (Blandf.). This beetle occurs throughout most of the northeastern and north-central walnut growing regions.

    The adult female beetle is dark brown to black and about 1/8 inch long. The immature stages are rarely seen because they occur in tunnels made inside the wood by the adult female. External entrance holes to these tunnels are about 1/32 inch in diameter, and are sometimes referred to as pinholes.

Injury
    Young walnut trees up to 8 years old are most often attacked.

    A Xylosandrus germanus female may introduce a Fusarium fungus into the tree as she excavates her tunnel into wood. This fungus causes a cankered area in the wood, usually causing top dieback and resprouting from the base of the tree. Cankering, however, is not always apparent. In some plantations, dieback in 1 year due to ambrosia beetle/Fusarium canker attack has been reported on 30 to 40 percent of the trees.

    Ambrosia beetle attack is usually not detected until there is profuse sprouting from the base of the trees or until the trees are dead. Close examination is necessary to locate the tiny pinholes in the lower stem area or in small, lowhanging branches.

Control
    Cut and remove dead or Fusarium-cankerinfected tree tops and branches and burn, if possible.


FLAT-HEADED APPLE TREE BORER,
CHRYSOBOTHRIS FEMORATA (OLIV.)

Description
    The flat-headed apple tree borer is a common insect in the eastern United States, attacking primarily apple trees but also black walnut.

    The adult beetle is flat with a dark-green bronze back and a metallic brassy color on its abdomen. The body is bullet-shaped, ranging in length from 1/3 to 2/3 inch. The wing covers are usually marked with two, wavy, depressed, lightcolored bands.

    Full-grown larvae have flat, broad heads and yellowish white bodies about 1 inch long.

flat-headed apple tree borer Injury
    Adults feed on the foliage of trees but the larvae feed in the phloem and outer sapwood area. They make large tunnels, sometimes several inches long. The flat-headed apple tree borer often attacks newly planted trees or trees weakened or stressed by drought, defoliation, or disease, thus hastening the tree's decline.

Control
    Maintain tree vigor by pruning dead or diseased branches and fertilizing the soil if nutrients are limiting. Wrap trunks with high-grade wrapping paper or burlap when trees are planted or pruned to prevent female beetles from laying eggs.


WALNUT CURCULIO,
CONOTRACHELUS RETENTUS (SAY)

Description
walnut curculio     The walnut curculio is commonly found throughout eastern United States wherever walnut trees are grown. The adult curculio is about 1/5 inch long and is reddish-brown with two small, white spots on its wing covers. It has a long snout with which it feeds. The larvae are small, legless, and a dirty white in color.

Injury
    The adult female curculio lays eggs in young nuts in May, June, and July. The larvae bore into the developing nuts and cause great losses during the so-called "June drop" of walnuts. Walnut curculio larvae also cause the meager filling of walnuts that remain on the tree. A small exit hole in the side of a fallen nut is evidence that walnut curculio larvae have been present.

    Walnut curculio has caused losses of 60 percent or more of the nut crop.

Control
    Consult your local service forester or county extension agent for recommended chemical control. If you have only a few trees, immediately pick up and discard any immature nuts that fall during the growing season.


WALNUT HUSK MAGGOT,
RHAGOLETIS SUAVIS (LOEW) AND
WALNUT HUSK FLY,
RHAGOLETIS COMPLETA CRESSON

Description
walnut husk fly     The walnut husk fly and the walnut husk maggot both occur commonly throughout central United States.

    The walnut husk fly and the husk maggot look alike. They are light brown in color and have two transparent wings with dark cross bars. They are smaller than the house fly. The larvae of both species are legless and pale yellow in color. Full grown larvae are up to 1/2 inch long.

Injury
    The walnut husk fly and the walnut husk maggot breed and lay eggs in the husks of nearly mature walnut fruits in early autumn. The larvae burrow into and feed on the husk, producing black, slimy husks that stain and stick to the shell. The maggots can sometimes be seen crawling in the husks.

    Husk maggots and husk flies do not penetrate into the nut, so the taste and color of the nutmeat are not affected. However, the slimy nature of the husks reduces their value to commercial nutmeat producers because the husk is difficult to remove. The infested husks also make the nuts unattractive and undesirable to the private walnut grower.

Control
    Pick up and remove infested walnuts from the plantation as soon as possible after they fall from the trees. Contact your county extension agent for recommended controls.


WALNUT SHOOT MOTH,
ACROBASIS DEMOTELLA GROTE

Description
    The walnut shoot moth is closely related to the pecan leaf casebearer, also an insect pest on black walnut. They are nearly identical in appearance but habits differ and serve to differentiate the two species.

    Larvae of the walnut shoot moth are drab, olive-green with a black head capsule. They reach a length up to 3/4 inch. Adult moths are gray-brown with a small white patch on each front wing.

    Newly hatched larvae overwinter in protective cases near the base of the terminal bud. In early spring each larva leaves its overwintering case and bores into the still unexpanded bud. Evidence of attack is indicated by a small pile of excrement and webbing deposited near the entrance hole.

Injury
    Many terminal and lateral buds are killed, causing multiple forks and crooks in the main stem. If the attacked bud is not killed immediately, the stem of the expanding leaf is usually hollowed out and subject to breakage during high winds.

Control
    Contact your county extension agent for recommended controls.


Back to the Introduction