What Are Nut Tree Pests: Learn About Bugs That Affect Nut Trees

What Are Nut Tree Pests: Learn About Bugs That Affect Nut Trees

When you plant a walnut or a pecan, you’re planting more than a tree. You’re planting a food factory that has the potential to shade your home, produce abundantly and outlive you. Nut trees are amazing plants, but along with their enormous size, they’re a large responsibility. They’re often the target of insect pests, so you should familiarize yourself with what common nut tree pests are problems. Bugs that affect nut trees are significantly easier to treat when the problem is caught early, before the infestation is serious, so a keen eye is a necessity.

What are Nut Tree Pests?

It may seem like nut trees are impervious to pest insects, but the truth is that they can succumb just like any other plant. Their relatively large size in comparison to many other plants only means that it takes a larger insect load before you notice significant nut tree pest symptoms. Regular inspection of your nut trees can keep them pest free, which is why we’ve compiled a list of the most common nut tree pests and how to treat pests on nut trees below:

Aphids. These soft-bodied insects are present on almost any sort of plant you can think of, and can be particularly devastating on producers like nut trees. Their sticky honeydew gives sooty mildew plenty of opportunity to obscure and block photosynthesizing leaves, reducing overall vigor and the aphids’ own feeding can cause blooms and buds to emerge disfigured, making it increasingly difficult for biological processes to continue normally.

Getting rid of aphids in nut trees requires a two-pronged approach, since they’re almost always being farmed by ants. You can treat the tree with one of many insecticides, or simply spray the leaves with a hard blast of water from a garden hose daily, while also maintaining a sticky barrier at the bottom of the tree and baiting the ants to eliminate the colony.

Scale. Many species of scale attack nut trees, but unless your tree is suffering significantly, don’t panic if you see scale. First, verify that the new bump or fuzzy spot is actually a scale insect by gently separating the protective covering from the tree with a thin blade.

If a soft-bodied insect is inside, plan to spray your tree with a three percent concentrate of superior oil during the dormant season. Reducing pesticide applications can actually help reduce scale numbers by encouraging the number of beneficial insects that will readily feed on these insects.

Mites. Mites can cause various types of damage to nut trees. The most obvious is bronze-colored stippling to leaves and fine webbing, in the case of spider mites. If the situation is very localized, you can wait and watch to see if natural predators will take care of the situation, but if there’s widespread damage, you’ll need to intervene.

You can apply superior oil during the dormant season at a three percent concentration or at one percent during late spring or summer. Applications of abamectin can also be applied, but be sure to observe a 21 day pre-harvest interval.

Codling moth. Because these pest caterpillars crawl into nuts early in their lifecycle, it’s vital that you monitor for them throughout the season. They overwinter behind bark or in the soil in cocoons, then emerge as adults to lay eggs on nearby fruit and nut trees. Once a population of codling moths has started breeding on your nut trees, it’s very difficult to get rid of them.

Choosing late-leafing nut trees can help avoid them, but if your trees are already in place, removing any nuts that appear to have been tunneled into or have frass coming out of the end immediately can slow the spread. Bagging nuts four weeks after bloom provides excellent control, but is also highly intensive work. Utilizing sticky traps can help thin adult codling moth populations, as well as inform your timing of safer insecticides like Bacillus thuringiensis.

Other possible pests of nut trees include weevils, though these insects are rarely a problem unless found in significant numbers.


Leaf-feeding aphids are usually not damaging, but large populations can cause leaf changes and stunting of shoots. Aphids also produce large quantities of a sticky exudate known as honeydew, which often turns black with the growth of a sooty mold fungus. Some aphid species inject a toxin into plants, which further distorts growth.


Pest & Disease Control for Walnut Trees

Every tree has the future potential for disease and insect damage. Factors such as location and weather will play a part in which issues your tree encounters. If available, disease-resistant trees are the best option for easy care and for all trees, proper maintenance (such as watering, fertilizing, pruning, spraying, weeding, and fall cleanup) can help keep most insects and diseases at bay.

NOTE: This is part 6 in a series of 10 articles. For a complete background on how to grow walnut trees , we recommend starting from the beginning.

Dieback

Causes a slow death, branch by branch. The bark changes from normal greenish-brown to reddish-brown and finally gray in color.

  • Pruning out infected branches promptly. If the disease reaches the trunk, the whole tree needs to be removed.

  • Consult County Extension Agent

Walnut Bunch

  • Tufting brooms a profuse development of branches appears on stem, branches and tips. A cause and control is not known. Try pruning out the brooms. Be sure to sterilize shears between each cut.

  • Consult County Extension Agent

Scale

Tan to gray 1/16” hard, scaly shell covers developing young. This usually on bark of twigs and branches but may also be on the nuts. Sap feeding weakens the tree.

  • Bonide® Citrus, Fruit & Nut Orchard Spray
  • Rub off with burlap.

Aphids

They are the size of a pinhead and vary in color depending on the species. Cluster on stems and under leaves, sucking plant juices. Leaves then curl, thicken, yellow and die. Produce large amounts of a liquid waste called “honeydew”. Aphid sticky residue becomes a growth media for sooty mold.

  • Bonide® Citrus, Fruit & Nut Orchard Spray

Caterpillars

Many kinds can damage walnuts.

  • Bonide® Citrus, Fruit & Nut Orchard Spray
  • Bonide® Thuricide Bacillus Thuringiensis (BT)

Codling Moth

The adult is moth gray with brown patches on the wings. The worms are about 1” long. The nuts have holes from side to core.

  • Consult County Extension Agent

Anthracnose

Causes irregular purplish or reddish-brown spots on leaves and these spots may merge to form irregular shaped blotches. Although significant defoliation may occur after cool, wet spring weather, this disease is usually not serious to the health of the tree.

  • Bonide® Fung-onil™ Fungicide

Bark Beetle

Young twigs wilt because of boring done by a dark brown beetle, 1/5” long. The larvae bore into sapwood, which can girdle branches. Remove and burn severely infected trees, peel bark from stump.

  • Consult County Extension Agent

Cigar Case Bearer

Leaves are mined, turn brown and fall. Mining is done by larvae that is 1/5” long with a black head. The adult is a moth with brown wings, fringed hairs along edge. The larvae over-winter on twigs and branches in cigar shaped cases 1/8” long.

  • Consult County Extension Agent

Walnut Blight

Black, dead spots on young nuts, green shoots and leaves. Many nuts fall early but some will reach full size with husk, shell and kernel black and ruined.

  • Bonide® Citrus, Fruit & Nut Orchard Spray

Gall Aphids

Hollow green galls turn black in July. On leaves, stems and small twigs. Insides are lined with living young. Galls are ‘pea sized’ to ½“.

  • Consult County Extension Agent

Mites

Pinpoint in size with many different colors. Found on underside of leaves. Sap feeding causes bronzing of leaves. Severe infestations have some silken webbing.

  • Bonide® Citrus, Fruit & Nut Orchard Spray

Omnivorous Leafroller

Adult is bell shaped, blackish gray snout-like mouthparts, forewings dark rusty brown with tan tips. Over winters in larval stage in mummified berries, in weeds and other trash. Moths emerge in spring and lay egg masses on leaves. Eggs hatch in 5 days and larvae tie two young leaves together to form nest in which they feed. Does not roll leaves. Later nests can be found in flower clusters and in bunches. Damage is not only from feeding on leaves, flowers and berries, but feeding sites allows rot organisms to enter fruit.

  • Bonide® Thuricide® Baccillus Thuringiensis (BT)

Fall Webworm

Damage the leaves by both feeding and web building. Webworms over-winter within cocoons located in protected places, such as crevices in bark or under debris and fences. Adult moths emerge in summer. They have a wingspan of about 1 1/4” and vary from pure satiny white to white thickly spotted with small dark brown dots. Females lay white masses of 400-500 eggs on the undersides of the leaves. The caterpillars hatch in 10 days and all from the same egg mass live together as a colony. They spin webs that enclose the leaves, usually at the end of a branch, to feed upon them. After they have defoliated a branch, they extend their nest to include additional foliage. ?When caterpillars are mature, they leave the nest to seek a place to spin gray cocoons. The mature caterpillars are about 1 1/4” long with a broad dark brown stripe along the back, and yellowish sides thickly peppered with small blackish dots. Each segment is crossed by a row of tubercles with long light brown hairs.

  • Bonide® Captain Jack’s™ Deadbug Brew
  • Bonide® Thuricide® Bacillus Thuringiensis (BT)

Husk Flies

Over winter as pupae in the ground and midsummer adult fly emerges, nearly as large as the common housefly. The bodies of the fly are yellowish-brown and have a dark color pattern on the wings. They feed on the husks of nuts. Female lays its eggs in the husk and the larvae feed on the green husk of nuts.

  • Bonide® Captain Jack’s™ Deadbug Brew

Fruit Tree Leafroller

Moth is brown with irregular yellowish to brown bands across the wings and is about ½ inch long. Larva is about 7/8 inch long, head and back may be amber to light brown or black with rest of its body light green. They feed on opening buds and new leaves and will roll up a leave, webbing them together, for protection.

  • Bonide® Captain Jack’s™ Deadbug Brew


Scale

Some insects pose serious threats to fruit trees by killing young branch and leaf growth. Armored or hard scales include San Jose scale (Quadraspidiotus perniciosus) feeding on stone fruits such as apricots and plums and California red scale (Aonidiella aurantii) feeding on apples, peaches and olives. Soft scales, such as calico scale (Eulecanium cerasorum) and Kuno scale (Eulecanium kunoense) feed on stone fruits, including cherries, while brown soft scale (Coccus hesperidum) and black scale (Saissetia oleae) feed on citrus, causing more damage due to their mobility. Scale infestations require timely pruning and targeted insecticides.


Identifying Insect Pest Damage Through Harvest Sampling

With almond and walnut harvest underway, it is a good time to review harvest nut sampling strategy and protocols for the best estimation of field loss by common pests. It is a general understanding that the grade sheet from your processers only represents about half of what is going on in the field. Also, the whole sum percent damage from the grade sheet does not identify which pest is causing the most economic loss. Insect population in orchards builds over time therefore, knowing the history of damage helps to address potential risks and strategies to next year’s pest management program.

1. Harvest Sampling in Almonds

Taking a minimum of 500-1000 sample nuts from an average-sized orchard, anytime between shaking and sweeping, is recommended. Infestation can vary among different sides of the tree, and between edges and interiors of the orchard. This is especially true for navel orangeworm damage. Use paper bags to collect samples from multiple spots (>10 sampling spots, if possible) within the orchard. Store the sample bags in a cold room or freezer until you have time to do crack out. Look for damage signs associated with insect species described in the following paragraphs. Major insect pests for the damage evaluation are navel orangeworm (NOW), peach twig borer (PTB), oriental fruit moth (OFM), ants, leaffooted bugs (LFB), and brown marmorated stink bug (BMSB). BMSB is an invasive stink bug species, which is established and causing damage in almond orchards in the northern San Joaquin Valley.

1.1. Worm Damage (NOW, PTB, OFM). NOW feed in the kernel (nutmeat) and create deep feeding tunnels. Feeding by NOW results in a significant amount of white frass, and webbings on the kernel (Fig. 1a). Since NOW and PTB often infest the same nut, NOW feeding damage often masks the PTB damage. Feeding damage signs by PTB and OFM on the nutmeat are similar (i.e., the presence of the shallow tunnels and surface grooves on the kernels, and no webbings) (Fig 1b & 1c), except OFM leaves a small amount of reddish frass on the hull, which is absent in PTB damaged nuts.

Fig. 1. Almond kernels (nutmeat) damaged by: a) navel orangeworm, b) peach twig borer, c) Oriental fruit moth

1.2. Ant Damage. The percentage of almond damage by ants at harvest depends on the duration the nuts are on the ground after shaking. The longer almonds are left on the ground gives more time for ants to feed on them, results in more damage. Also, more damage is likely in orchards with drip or sprinkler irrigation compared to orchards with flood irrigation. Cover or vegetation in the orchards also favors ant activity. Nuts with tight shells or with narrower ( 1.3. Leaffooted Bug and Brown Marmorated Stink Bug Damage. Most nuts infested by the leaffooted bug or BMSB, early in the season (mid- March to mid-May), abort and drop. A small percentage of those infested nuts do not drop but end up becoming shriveled and gummy kernels at harvest (Fig 2b). Both LFB and BMSB feeding after the shell hardening can result in sunken dark spots on kernels (Fig. 2c), although the degree of damage tends to be higher with BMSB feeding than LFB. Late-season feeding (July-August) by BMSB, can cause dark stained kernels (Fig 2d). Varieties with soft shells such as Fritz, Sonora, Aldrich, Livingston, Monterey, and Peerless are more susceptible to bug damage and for a longer period during the season.

Fig. 2. Almond kernels damaged by: a) ants, b-d) leaffooted bug and brown marmorated stink bug

2. Harvest Sampling in Walnuts

It is recommended to take a minimum of 1000 nuts at the harvest and evaluate for the damage caused by navel orangeworm , codling moth, ants, husk fly, and sunburn. It is important to have representative samples (>10 samples with a minimum of 100 nuts/sample) from the orchard for better estimation of the infestation. The damage signs associated with these specific insect pests and sunburn are described as follows:

2.1. Worm Damage (NOW, CM). Navel Orangeworm damage can be identified by the presence of a large amount of frass and webbings (Fig 3a). NOW larvae are present in groups and can bore deeply into the kernel. Heavy infestation may give a nutshell an oily appearance. In contrast to NOW, a single codling moth larva infest the nut, and has a lot cleaner damaged area inside the nut. Frass is evident, but only at the entry point on the husk very little webbing present (Fig. 3b). If larva is present, look for crescent-shaped marking just behind the head to confirm navel orangeworm.

Fig. 3. Walnut damaged by: a) navel orangeworm, b) codling moth

2.2. Ant Damage. Similar to almonds, nut damage by ants increase as the duration of the harvested nuts on the ground increase. Ants enter the nuts from the soft tissues (i.e., stem end) and/ or through a codling moth injury. Ant damage on nuts is identified by the presence of deep chewing channels with clean kernels (i.e., no frass, no webbings, no deep boring) (Fig 4a).

Fig. 4. Walnut damaged by: a) ants b) walnut husk fly c) sunburn Fig. 5. (Left) Brown Apical Necrosis is shown on the left, not to be confused with Walnut Blight, shown on the right, and caused by the bacterial pathogen, Xanthomonas arboricola pv. juglandis (Figure provided by Themis Michailides).

2.3. Husk Fly Damage. Walnut husk fly larvae (technical term: maggots) feed in groups by boring into the husk. Early season damage results in shriveling and darkening of the kernels, with the increased potential for mold growth. Late- season infestation causes little kernel damage (Fig. 4b), although it may stain the shell and make the husk removal process difficult.

Fig. 6. (Right) Moldy, off color nuts which lead to economic loss due to downgrading (Figure provided by Themis Michailides).


Watch the video: How to Control Pests u0026 Diseases on Citrus Trees