How to Get Rid of Caterpillars on Arizona Cypress Trees – A Comprehensive Guide

Leyland cypress (x Cupressocyparis leylandii) has grown in popularity in the Southeast over the last few decades, especially in its use as a fast-growing screening plant. With the increased planting that has resulted from its popularity, various diseases and pest problems have become more evident.

As with any plant, the first step to a healthy Leyland cypress is to provide it with the cultural conditions that it needs. Ultimately, a vigorously growing plant is always better able to survive disease and insect problems than is a stressed one. In general, Leyland cypress requires full sun and prefers moderately fertile soil that is moist and well-drained. For further details on providing a Leyland cypress with the conditions under which it grows best, see HGIC 1013, Leyland Cypress.

Seiridium Canker/Dieback: In the Southeast, this disease is caused most often by the fungus Seiridium unicorne. Generally speaking, cankers are dark, oval or elongated lesions that are usually dry, may be sunken with a raised edge, and are surrounded by living tissue. More specifically, Seiridium cankers form on stems, branches, and in branch axils. They are often thinly elongated, shallow, and the surrounding bark shows a dark brown to purplish discoloration. A single Seiridium canker does not enlarge to girdle a branch. Instead, multiple cankers form around a branch, reducing water flow significantly. In addition, the cankers are often accompanied by an extensive flow of resin from cracks in the bark. However, resin flow by itself is not a defining characteristic of this disease as it often occurs in otherwise healthy Leyland cypresses. A shallow cut at the site of the canker typically reveals tissue beneath the bark that is reddish to brown in color and sticky. Branches affected by Seiridium canker experience dieback. They are often yellow to grayish tan to reddish-brown in color and usually appear randomly distributed on the tree. When a cankered branch has only recently shown a change in color, needles will fall off easily when a hand is run along the branch. Generally, the discolored branches are the symptom noticed first. If the fungus gets into the main trunk, it can kill the entire tree.

Environmental conditions that stress Leyland cypress (especially drought, but also spring freeze damage) favor the development of infection. The fungus survives winter in infected tissue. Spore-producing structures of the fungus appear on the surface of the canker as small, black dots that are barely visible without a magnifying lens. Spores can be spread (within the tree and to new trees) via rain, overhead irrigation, and pruning tools, typically entering the trees through wounds and cracks in the bark. From the point of infection until the development of cankers and other symptoms often takes years.

Prevention & Treatment: When planting as a screen, provide enough space (a minimum of 12 to 15 feet) between trees for good air circulation and to minimize stress as they mature and enlarge. Avoid over-fertilization. Place mulch under trees to at least the drip line (and preferably further) to reduce water evaporation and competition for water, as well as potential damage to trees from lawnmowers and string trimmers.

Drought-stricken trees are significantly more susceptible to infection. However, field trials have shown that trees inoculated with Seiridium unicorne that developed cankers were able to heal within 1-2 years once a regular irrigation schedule was implemented. As such, make sure Leyland cypresses are irrigated during drought or semi-drought conditions. Apply water at the base of trees to keep branches dry and reduce disease spread. If overhead irrigation is necessary, it should be applied very early in the morning.

Prune out and destroy diseased branches as soon as possible. Make pruning cuts 3 to 4 inches below diseased tissue. Sterilize pruning tools between each cut by dipping in a 10 percent bleach solution (1 part chlorine bleach to 9 parts water)*. Severely affected trees should be removed and destroyed. *Note: Be sure to clean and oil pruning tools after this procedure to prevent rust formation.

Fungicides are not considered an effective or practical means for controlling canker diseases in general or Seiridium canker in particular.

Phytophthora Root Rot: Leyland cypress is susceptible to the root-rotting fungus Phytophthora. Root rot is primarily a problem in soils with very poor drainage. Young plants are most often affected. Mature, established trees are seldom affected. Phytophthora species that cause root rot live in the soil and enter a healthy tree through wounds or the succulent parts of small roots. As the fungus destroys the roots, symptoms of distress become apparent above ground. Foliage becomes stunted, sparse, changes color (yellow, purple, tan), and dies. Cankers may be visible at or below the soil line.

Prevention & Treatment: Remove and destroy infected plants, including the entire root system. There is no practical chemical control for home gardeners. Improve soil drainage by adding organic material to heavy clay soils and avoid overwatering. If replanting, do not plant a Leyland cypress or other susceptible species where Phytophthora is known to be present.

Botryosphaeria Dieback/Canker: This disease is caused by the fungus Botryosphaeria dothidea. As with Seiridium canker, symptoms include dieback with branches turning yellow to brown in color – typically the first symptom noticed. However, unlike Seiridium canker, needles generally do not fall off when running a hand down a recently affected branch. Examination of dead stems often reveals more deeply sunken (often V-shaped) cankers than found with Seiridium canker. In addition, these cankers often girdle a stem, killing the stem beyond the canker quickly. There may be cracks on the surface of the canker, and surrounding bark may be darker than that seen on a healthy branch. Botryosphaeria canker may exhibit little or no oozing resin. Cutting into an affected branch reveals dark brown discoloration with a definite margin between diseased and healthy tissue.

The fungal pathogen (disease-causing agent) overwinters in the bark and diseased tissue. Spore-producing structures of the fungus appear on the surface of the canker as small, raised black dots that are best seen with a magnifying lens. Spores are spread by water from rain and overhead irrigation, as well as pruning tools.

Prevention & Treatment: As with Seiridium canker/dieback, environmental and cultural stresses predispose Leyland cypress to Botryosphaeria canker/dieback. Follow Prevention & Treatment recommendations provided above for Seiridium canker.

Passalora Needle Blight: Although often referred to as Cercospora or Cercosporidium needle blight, this disease is caused by the fungus Passalora sequoiae (previously known as Cercosporidium sequoiae, Asperisporium sequoiae, and Cercospora sequoiae).

Typically, this disease only affects plant growth that is at least one year old. Symptoms usually appear during the summer months. They include browning of needles and eventual needle drop. These symptoms start on lower branches near the trunk and then spread outward toward branch tips. Over time, the disease moves up the tree. The portion of a tree displaying symptoms may increase from one year to the next until only the tips of upper branches are still green, or the tree dies completely. The disease will sometimes be more prevalent on one side of the tree than the other, especially when irrigation spray is a contributing factor. When irrigation spray is not an issue, it is more likely to be seen on the north and west sides of trees where the morning sun is not able to dry branches as quickly.

In cases of severe disease, all needles (except the current year’s growth) turn brown, resulting in green needles being present only on branch tips. Spores develop during late spring to summer. They are spread primarily by rain, overhead irrigation, and wind, but also by tools.

Passalora needle blight symptoms somewhat resemble symptoms seen in a Leyland cypress that is responding to severe environmental stress, such as drought, in which its lower interior needles turn yellow and drop.

Prevention & Treatment: At planting time, space trees properly to allow for adequate airflow. To minimize the spread of spores, avoid overhead irrigation or restrict it to the early morning hours. Preferably, use drip irrigation and make sure that enough water is applied during drought situations. Drip irrigation hoses should be no more than 75 feet long as pressure drops significantly after that point. Prune out diseased limbs, disinfecting pruning tools between cuts using 10 percent bleach solution (1 part chlorine bleach to 9 parts water). Note: Be sure to clean and oil pruning tools after this procedure to prevent rust formation.

Fungicides such as thiophanate-methyl and myclobutanil (see Table 1 for examples of products) are recommended for use against needle blight. However, to be effective, these sprays need to begin in late spring and continue through summer until the cooler, less humid months of fall. In addition, when applying these materials, it is essential that needles, including those on the inside near the trunk, are thoroughly sprayed to run-off. Once a tree is tall, adequate coverage by a homeowner is generally not feasible.

Insects & Related Pests

Bagworms: Bagworms (Thyridopteryx ephemeraeformis) infest many shrubs and trees, but conifers (evergreens) are the preferred hosts. Damage to plants results from feeding by the caterpillars, which causes loss of needles. Mild infestations of this pest slow the growth of Leyland cypress. Heavy infestations can kill a plant.

The adult male bagworm is a dark-colored, hairy moth with a 1-inch wingspan and clear wings. The adult female does not have wings or legs, is yellow, and appears almost maggot-like. The larvae (immature forms, aka caterpillars) range in size from about 1/8 inch to 2 inches, depending on maturity. Each larva produces a carrot- or cone-shaped bag that it carries as it feeds. The bag is formed from silk that the larva produces. As it feeds, the larva adds bits of plant material to the bag for camouflage. The bag is about 2 inches long when complete. Home gardeners sometimes mistakenly identify it as a pine cone.

In South Carolina, bagworms survive the winter as eggs in a bag. The larvae hatch during May. Each one produces a strand of silk that allows it to be blown by the wind to a new location on the same plant or to a new plant. They soon begin to spin their cases. When mature, each larva pupates (transforms into an adult) within its bag. An adult male moth emerges from its bag in late summer (August/September). It locates an adult female in her bag. After mating, the female lays 500-1,000 eggs in her bag and dies.

Prevention & Control: Several parasites and predators feed on bagworms, generally keeping their numbers under control so that damage is not noticed. Removal of the egg-containing bags during winter and early spring is a very effective method for preventing problems before the next growing season. Once removed, the bags should be destroyed or placed in a deep container (5-gallon bucket), which allows beneficial parasites that may also be present in the bags to escape while retaining the bagworm larvae.

If an infestation is severe or the bags are out of reach, spray with the bacterial insecticide, B.t. This insecticide contains spores of the bacterium Bacillus thuringiensis, which, when eaten, kill the caterpillar. Young larvae are much more susceptible to B.t. than are older larvae. As such, apply this pesticide in the spring as soon as bagworms are seen (usually in May) and repeat two weeks later. Control is most effective when spraying is done in the late afternoon or early evening. This insecticide is very safe to use. Once the bags have reached ¾ inch long, the efficacy of B.t. sprays decreases rapidly.

Contact insecticides such as permethrin, cyfluthrin, cyhalothrin, cypermethrin, malathion, or acephate should be applied later in the season (May and June) when bagworms are larger and harder to manage with B.t. Note that contact insecticides will also reduce populations of beneficial insects (predators and parasitoids) that help control spruce spider mites, which can result in an outbreak of this occasional Leyland cypress pest. Soil application of neonicotinoid insecticides, such as imidacloprid or dinotefuran, give minimal (less than 10%) control from bagworm damage and should not be substituted for spray control. See Table 2 for examples of products. As with any pesticide, read and follow all label directions and precautions before using. Again, once a tree is tall, adequate coverage by a homeowner is often not feasible.

Spruce Spider Mites: Mites are not insects but are more closely related to spiders. Spruce spider mites (Oligonychus ununguis) are occasional pests of Leyland cypress. They are very small and not seen easily with the naked eye. They have piercing mouth parts that they use to suck plant sap. Their feeding results in speckling (formation of tiny yellow spots) on needles. Some needles may turn brown and drop off. With heavy infestations, fine webbing may be seen on the plant. Several seasons of heavy mite feeding may kill a Leyland cypress. Although most spider mites increase in number during hot, dry weather, spruce spider mites are cool-weather mites. Their population peaks during spring and fall but drops dramatically during the heat of summer when predators feed upon them.

Prevention & Control: Naturally occurring enemies of mites include various predator mites, lady beetles (ladybugs), and other insects. These predators will usually suppress mite populations. Since insecticide use kills beneficial predators as well as mites, insecticides should be avoided unless absolutely necessary. Overuse of insecticides can result in population explosions of mites by their natural predators. However, insecticidal soap and horticultural oil sprays are less harmful to beneficial insects. Mites can be removed with a strong spray of water when applied on a regular basis as needed.

To determine whether miticide use is needed, it helps to know how many mites are present. Hold a white sheet of paper under a branch and tap the branch with a pen. The mites that are knocked off will be seen crawling around on the paper. If dozens of mites are seen per tap, serious damage can result. Continue to check population numbers at 7- to 10-day intervals. Populations will be greatest during the spring and fall.

Pesticides labeled for homeowner use against spruce spider mites include insecticidal soap, horticultural oil, neem oil, tau-fluvalinate, cyhalothrin, and malathion. See Table 2 for examples of products that contain these active ingredients. As with any pesticide, read and follow all label directions and precautions before using. Again, once a tree is tall, adequate coverage by a homeowner is often not feasible.

Scales: Various scale insects, including juniper scale (Carulaspis juniperi), Maskell scale (Lepidosaphes pallida), and minute cypress scale (C. minima), feed on Leyland cypress trees.

Scales are unusual insects in appearance, and as a result, are sometimes misidentified by gardeners either as parts of the plant itself or as disease organisms rather than insects. Adult female scales are small and immobile, with no visible legs. They secrete a waxy coat that varies significantly in shape and color depending upon the species. Adult males tend to be very small and have wings that allow them to fly to locate females. Immature scale insects are called crawlers, and as the name indicates, they have legs and are mobile.

Adult females survive the winter on the plant. In spring, they lay eggs under their shells. The crawlers hatch and crawl around before settling to feed. They feed by piercing a leaf, stem, or branch with their mouthparts and sucking sap. Their feeding can weaken or kill branches.

Symptoms of a scale infestation of Leyland cypress are very similar to those of spruce spider mite infestation. Initially, the Leyland cypress appears off-color, and infested branches show little growth. The needles eventually turn yellow or brown. Branches may die back. If ignored, a scale infestation may kill the plant within two to three growing seasons.

Prevention & Control: Depending on the tree’s size and how extensive the infestation is, sometimes scale can be removed by scraping them off the plant. If the scale infestation is somewhat localized, removal of infested branches can reduce the population significantly. Various beneficial insects help keep scale insects under control. If insecticides become necessary, try to use those that are “beneficial insect friendly”. These include insecticidal soap and horticultural oils.

The presence of adults or crawlers determines which treatment will be most effective. The waxy coating on adult scales protects them from traditional insecticides, but their crawlers are susceptible. Use a horticultural oil spray in late winter or very early spring before new growth occurs to control adult females by suffocation. Insecticides recommended for use against crawlers include horticultural oil, insecticidal soap, permethrin, cyfluthrin, cyhalothrin, malathion, and acephate. See Table 2 for examples of products. Read and follow all label directions and precautions before using.

Table 1. Fungicides to Control Leyland Cypress Diseases.

Fungicides	Examples of Brand Names & Products
Myclobutanil	Spectracide Immunox Multi-purpose Fungicide Concentrate Ferti-lome F-Stop Lawn & Garden Fungicide Concentrate Monterey Fungi-Max
Thiophanate-methyl	Cleary’s 3336-WP Turf & Ornamental Fungicide Southern Ag Thiomyl Systemic Fungicide
Note: Chemical control of diseases on large trees is usually not feasible since adequate coverage of the foliage with a pesticide cannot be achieved.

Table 2. Insecticides to Control Leyland Cypress Insects & Related Pests.

Insecticides	Examples of Brand Names & Products
Acephate	Bonide Systemic Insect Control Concentrate
Bacillus thuringiensis (B.t.)	Bonide Thuricide Bt Concentrate Garden Safe Bt Worm & Caterpillar Killer Concentrate Monterey Bt Concentrate Natural Guard Caterpillar Killer Spray with Bt Concentrate Safer Brand Caterpillar Killer Concentrate Southern Ag Thuricide Bt Caterpillar Control Concentrate
Cypermethrin	Garden Tech Sevin Bug Killer Concentrate Gordon’s Bug-No-More Lawn & Garden Insect Control Conc.
Cyfluthrin	Bayer BioAdvanced 24 Hour Lawn Insect Killer RTS1 Bayer BioAdvanced Complete Insect Killer for Soil & Turf I RTS1 Bayer BioAdvanced Insect Killer for Lawns RTS1
Horticultural Oil	Bonide All Seasons Spray Oil Concentrate Ferti-lome Horticultural Oil Spray Concentrate Monterey Horticultural Oil Concentrate Safer Brand Horticultural & Dormant Spray Oil Concentrate Southern Ag Parafine Horticultural Oil Summit Year Round Spray Oil Concentrate
Insecticidal Soap	Natural Guard Insecticidal Soap Concentrate Safer Insect Killing Soap Concentrate
Cyhalothrin	Martin’s Cyonara Lawn & Garden Concentrate; & RTS1 Spectracide Triazicide Insect Killer for Lawns & Landscapes Concentrate; & RTS1
Malathion	Bonide Malathion 50% Insect Control Gordon’s Malathion 50% Spray Concentrate Hi-Yield 55% Malathion Insect Spray Concentrate Martin’s Malathion 57% Concentrate Ortho MAX Malathion Insect Spray Concentrate Southern Ag Malathion 50% EC Spectracide Malathion Insect Spray Concentrate Tiger Brand 50% Malathion Concentrate
Neem Oil	Bonide Neem Oil Concentrate Bonide Rose Rx 3-in-1 Concentrate Concern Garden Defense Multi-Purpose Spray Concentrate Ferti-lome Rose, Flower & Vegetable Spray Concentrate Garden Safe Fungicide 3 Concentrate; & RTS1 Garden Safe Neem Oil Extract Concentrate Monterey 70% Neem Oil Fungicide/Insecticide/Miticide Concentrate; & RTS1 Natural Guard Neem Concentrate Safer Brand Neem Oil Concentrate Southern Ag Triple Action Neem Oil Concentrate
Permethrin	Bonide Eight Insect Control Vegetable Fruit & Flower Conc. Bonide Eight Yard & Garden Ready to Spray (RTS1) Bonide Total Pest Control – Outdoor Concentrate Hi-Yield Indoor/Outdoor Broad Use Insecticide Concentrate Southern Ag Permetrol Lawn & Garden Insecticide Conc. Tiger Brand Super 10 Concentrate
Tau-Fluvalinate (miticide)	Bayer Advanced 3-in-1 Insect, Disease & Mite Control Concentrate; & RTS1
Note: Chemical control of diseases and insects on large trees is usually not feasible since adequate coverage of the foliage with a pesticide cannot be achieved. 1RTS = Ready to spray (a hose-end spray bottle).

Caution: Pollinating insects, such as honey bees and bumblebees, can be adversely affected by the use of pesticides. Avoid the use of spray pesticides (both insecticides and fungicides), as well as soil-applied, systemic insecticides unless absolutely necessary. If spraying is required, always spray late in the evening to reduce the direct impact on pollinating insects. Always try less toxic alternative sprays first for the control of insect pests and diseases. For example, sprays with insecticidal soap, horticultural oil, neem oil extract, spinosad, Bacillus thuringiensis (B.t.), or botanical oils can help control many small insect pests and mites that affect garden and landscape plants. Neem oil extract or botanical oil sprays may also reduce plant damage by repelling many insect pests. Practice cultural techniques to prevent or reduce the incidence of plant diseases, including pre-plant soil improvement, proper plant spacing, crop rotation, applying mulch, applying lime and fertilizer based on soil test results, and avoiding overhead irrigation and frequent watering of established plants. Additionally, there are less toxic spray fungicides that contain sulfur or copper soap and biological control sprays for plant diseases that contain Bacillus subtilis. However, it is very important to always read and follow the label directions on each product. For more information, contact the Clemson Extension Home & Garden Information Center.

Pesticides are updated annually. Last updates were done on 2/21 by Joey Williamson. Originally published 01/01

If this document didn’t answer your questions, please contact HGIC at [email protected] or 1-888-656-9988.

Joey Williamson, PhD, HGIC Horticulture Extension Agent, Clemson University

This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.

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