Plant Health Problems
The disease problems of walnut are similar to those of other Juglans. See Butternut for a detailed discussion of the common diseases of this host.
The large gray aphid, Longistigma caryae, is found on Japanese walnut. The three hickory aphids, Monellia caryae, M. caryella, and M. costalis infest the black walnut. Spraying with malathion or insecticidal soap, which are among the compounds registered for control of this pest in Connecticut, to contact the insects will destroy them. Consult the labels for dosage rates, safety precautions, and preharvest intervals.
Butternut curculio, Conotrachelus juglandis.
This insect breeds in the nuts and new shoots of walnut, and often severely injures Persian and Japanese varieties. The adult is a snout beetle or weevil, about 1/4" long, brownish gray in color, with a broad whitish band across the wing covers just beyond the center. The weevils overwinter in protected places, appear on the trees the latter half of May, and cut holes into shoots and leaf stems. Females lay eggs in both the fruit and stems under flaps in crescent-shaped punctures like the plum curculio. The young Iarvae tunnel in the new shoots, at the base of the leaf stems, and some go into the fruit. They mature in 4 to 6 weeks, then enter the ground where they pupate about an inch beneath the surface and from 16 to 20 days later the adults emerge. The weevils do some feeding, then early in September seek overwintering sites. There is one annual generation. Applications of carbaryl to control other walnut pests may also control butternut curculio adults. Consult the label for dosage rates, safety precautions, and preharvest intervals.
Oystershell scale, Lepidosaphes ulmi.
The oystershell scale often infests walnut. A dormant application of horticultural oil will control overwintering scales. Summer application of ultrafine oil cannot be used, because it causes injury to walnut foliage. Spraying with malathion, or insecticidal soap, which are among the compounds registered for control of this pest in
Walnut bud moth, Acrobasis caryae.
The caterpillars of this moth feed on the tender terminal leaves and shoots, webbing them together. They injure and distort the new growth or stop it entirely. The mature larva is about 5/8" long, and dirty olive-green with a shiny black head. The adult moth has a wingspread of about 3/4" and is gray with lighter areas at the base of the forewings. Sprays of carbaryl, malathion, or Bacillus thuringiensis var. kurstaki , which are among the compounds registered for use against this pest in Connecticut, should control the caterpillars. Consult the labels for dosage rates, safety precautions, and preharvest intervals.
Walnut caterpillar, Datana integerrima.
This caterpillar is usually noticed in August when clusters of black caterpillars with whitish hairs are found stripping the branches of walnut and hickory. On the trunks and larger branches, gray hairy patches can be seen where the groups of caterpillars have cast their skins. The mature caterpillars are about 2" long, black, and covered with whitish hairs. The adult moth has a wingspread of about 2", is light reddish brown, and darker reddish lines cross the forewing. The thorax has a bright mahogany-red spot. The moths emerge in July and lay clusters of eggs on the undersides of the leaves. The caterpillars become mature in September, pupate in the ground, and remain there until the moths emerge the following season. There is one generation each season. On small trees, the caterpillars may be gathered and crushed. Carbaryl, malathion, or Bacillus thuringiensis var. kurstaki , which are among the compounds registered for control of this pest in Connecticut, can be applied as sprays if caterpillars are numerous. Consult the labels for dosage rates, safety precautions, and preharvest intervals.
Walnut scale, Quadraspidiotus juglansregiae.
This circular light gray scale about 1/8" in diameter often infests Persian and Japanese walnuts. Its life history is similar to that of the San Jose scale. Among the compounds registered for control of this pest in Connecticut are horticultural oil, dormant use only, and malathion. Spraying with malathion late in June and at intervals of 4 weeks until the end of September will be helpful in controlling crawlers. Consult the labels for dosage rates, safety precautions, and preharvest intervals.