Chronic Water Deficiencies, Biotics, Insects, Mites, Borers + More

A weekly blog present by a Kephas landscaping & Property maintenance services, a Calgary based year round landscaping, tree services, snow removal company :

"All plants can suffer from water deficit, which results in slow growth or even plant death, although some species, known as xerophytes, are more adapted to dry areas. These adjustments allow for better water retention and/or water uptake. Adjustments include thicker, leathery leaves, often with a waxy cuticle or hairy coating; more extensive root systems; or photosynthetic adaptations. 

Chronic water deficit refers to a prolonged water shortage lasting for days or months. This leads to a decrease in photosynthesis, slowing the tree’s growth and energy storage. Reduced growth can often be identified by measuring the distance between the terminal bud scale scars of the individual plant or by long-term observation of the species.

Prolonged periods of below-normal rainfall can lead to low soil moisture and drought conditions. When moisture decreases below the permanent wilting point, soil water is bound so tightly to the soil particles that the plant roots can no longer absorb water, causing the plant permanent damage.

If water deficit continues, tissues may dehydrate to the point of necrosis. In addition to reduced growth rate, leaf size is often reduced, leaf color may be less intense, and early fall coloration and abscission are often evident. Prolonged water stress can leave the plant highly susceptible to secondary infection by pests and pathogens. 

Just as water deficit can be a serious threat to the health of trees, so, too, can flooding and excessive irrigation. When soil becomes saturated, it drives most of the oxygen out of the pore spaces of the soil. This stops aerobic respiration in the roots, causing them to switch to anaerobic fermentation. Anaerobic fermentation uses a great deal of storage reserves to release a small amount of energy, thereby starving the tree. The byproducts of anaerobic fermentation can be toxic. Flooded conditions promote the growth of bacteria and fungi that can be harmful to trees and kill beneficial bacteria and fungi that are present in the soil.

When trees are flooded and oxygen is driven out of the soil, the lack of oxygen in the soil suffocates roots, changes chemical composition of essential elements, can create mineral toxicities, and can lead to fermentation in root cells. The longer the excess water remains, the more tree health deteriorates. 

Girldling Roots

Girdling roots are roots that grow around or across the stem or other roots. Symptoms of girdling may occur in young nursery plants or in mature trees. Trees with severe defects show symptoms at planting time and usually do not survive to maturity. Symptoms of girdling roots may be difficult to detect because they often resemble disorders associated with other stressors.

As the trunk and roots increase in diameter, these roots may begin to choke (or girdle) the tree. Phloem tissues in the trunk become compressed, which may cause swelling directly above the girdling root. The compressed stem weakens the tree and leaves it more prone to failure. Affected trees show symptoms similar to water deficiency, and early fall color and abscission are very common. Eventually the tree may also show significant dieback.

Girdling roots often develop from container-grown plants or poor planting techniques. Research has shown that trees planted too deeply are more prone to develop girdling roots than those with the trunk flare exposed, although some species are more likely to develop girdling roots than others. Often, girdling roots are not detected until symptoms begin to appear in the aboveground portions of the tree, when the girdling roots are too large to be removed without severely damaging the structural support and uptake capabilities of the root system.

Often the only way to deal with girdling roots is to prevent them from occurring. Arborists should carefully inspect nursery stock for girdling or circling roots. It is very important not to plant the tree too deeply and to prevent soil or mulch from collecting around the base of the trunk to minimize the development of adventitious roots. 

Girdling roots may cause minimal damage or can become a serious problem and can lead to the decline and even death of trees. Additionally, when girdling root symptoms become obvious, it may be too late to remedy. Because removing large, woody, girdling roots can compromise a tree’s support and uptake capabilities, the best remedy against girdling roots is to prevent them.

If girdling roots develop, it may be possible to remove the portion in contact with the stem. Roots that have become grafted to the trunk or roots that are too large (larger than 1/3 of the trunk diameter) should be left in place because severing them will cut off a large portion of the tree’s root system. Moreover, removal of girdling roots should be avoided during the growing season. If the girdling roots have compromised the structural stability of the tree, the tree may have to be removed.

Insects

Insects have complex life cycles; one stage may cause problems, while the next does not. Knowledge of the pests’ life cycles is important in identification and treatment. Some insects are predators or parasites of harmful insects. An arborist must be able to identify both harmful and beneficial insects and know the extent of damage the plant can tolerate before any control is necessary. Reference books and diagnostic labs should be used when needed.

Most insect damage to trees is the result of either feeding or egg-laying activity. For example, periodical cicadas can destroy small twigs by ovipositing (laying eggs) in long lines under the bark. 

Because cicadas may be present in such large numbers, this wounding can be a significant problem during years when large cicada broods emerge.

Insect feeding damage is characterized by the type of mouthparts the insect has. Insects, such as caterpillars, webworms, beetles, and weevils, chew on plant parts. Chewing insects eat plant tissue, such as leaves, flowers, buds, and twigs. Some insects, such as gypsy moths, eastern tent caterpillars, and cankerworms, eat the entire leaf. Others may feed between the leaf surfaces or only along the leaf margin.

Insects with chewing mouthparts include beetles, caterpillars, grasshoppers, and weevils, among others. Chewing insects physically remove portions of leaves and/or flowers directly or even consume entire plant parts.

Other insects feed by piercing and sucking. Aphids/adelgids, scales, leafhoppers, and true bugs feed by piercing plant cells and sucking out the contents. Symptoms of this type of feeding include chlorosis, stippling, drooping, distortion, and the growth of sooty mold fungus.

A few of these insects can also cause phytotoxic effects as a result of chemicals they secrete into the plant while feeding. Generally, while most sap- or leaf-feeding insects do not kill trees outright, they can be an additional stress factor and can affect the aesthetics of the tree. 

Insects with piercing and sucking mouthparts include aphids, whiteflies, and soft scales, among others. These insects insert their mouthparts into the vascular tissues of plants and extract plant fluids. This may result in plant stunting, wilting, and leaf distortion. Some of these insects may use their piercing-sucking mouthparts to penetrate the outer covering of another insect and feed upon its internal fluids.

Borers are chewing insect larvae that tunnel under the bark and often into the wood of trees. Because each kind has its own style and tunnel pattern, borers may be identified by their work even after they have left the scene. 

Trees infested with borers typically show a thinning crown and a decline in vigor. Conclusive signs are small emergence holes in the trunk or branches with frass (semi-digested wood). Borers eat the inner bark, phloem, cambium, or xylem, thereby destroying the tree’s ability to transport water and nutrients between the roots and shoots.

Some borers, like the Asian longhorned beetle, tunnel into the wood of the plant and cause structural damage.

Other insects are classified as defoliators. Defoliation can dramatically decrease tree resistance to trunk-invading organisms such as wood borers, cankers, and vascular wilt diseases. 

The reduction in photosynthetic area causes the tree to draw on energy reserves to maintain growth, weakening its defense. Time of year can also be a factor with defoliation and may dictate how severely the tree is impacted. 

Some insects, such as the Japanese beetle, eat only the interveinal tissue, creating a skeletonized leaf (photo on the left). Other insects, such as the elm leaf beetle, create what is called a "windowpane effect" because they eat the interveinal tissue but leave the epidermal layer (photo on the right). Leaf notching is a distinctive type of defoliation caused by black vine weevils and their relatives. These beetles remove tissue from the leaf edge and leave a characteristic notch.Some insects, such as the Japanese beetle, eat only the interveinal tissue, creating a skeletonized leaf (photo on the left). Other insects, such as the elm leaf beetle, create what is called a "windowpane effect" because they eat the interveinal tissue but leave the epidermal layer (photo on the right). Leaf notching is a distinctive type of defoliation caused by black vine weevils and their relatives. These beetles remove tissue from the leaf edge and leave a characteristic notch.

Leaf miners are immature larvae of beetles, flies, wasps, and other insects that feed between the upper and lower leaf surfaces, creating tunnels inside the leaves. 

This process, known as leaf mining, causes a distinctive discoloration in one of two patterns: serpentine mines or blotch mines. Serpentine leaf mines are snakelike, while blotch mines have an irregular shape.

Certain insects, such as aphids, pierce into the vascular system of the plant and feed on the sap. These insects remove elements from the sap and excrete the remaining compounds in a liquid form called honeydew. 

Honeydew can become a problem in large amounts by attracting wasps, ants, and other insects; coating surfaces below the tree in sticky sap, and providing a medium for the growth of sooty mold fungus.

Another group of sucking insects is known as scales. Scales can be a problem on leaves or twigs, and sometimes on larger branches. Scales on twigs can sometimes be confused with galls.

A quick way to distinguish between the two is to scrape off the bump and observe the bark below. The bark below a scale insect will remain intact.

Scale insects can be divided into two categories: soft shell and armored shell. Soft scales are rounded (helmet shaped) and feed off the tree’s vascular tissues, producing honeydew. Armored scales are flatter than soft scales and have a target-like appearance. They feed on plants by bursting cells, not by penetrating into the vascular system, and, therefore, do not produce honeydew.

Some insects, bacteria, fungi, and nematodes can produce hormones or chemical compounds that trigger a wounding response in the plant, causing it to form abnormal growths called galls.

There are hundreds of types of galls, each characteristic of the organism that stimulates its growth. These organisms use galls for food and protection.    

Some insects are vectors, or carriers, of plant diseases. This means that they transmit or spread the pathogen, or disease-causing organism, from tree to tree.

Dutch elm disease is an example of a fungal pathogen that is often spread by an insect vector, bark beetles. Fireblight bacteria can be spread by bees as they collect nectar from flowers. Aphids and leafhoppers can transmit viruses. 

Some insects are predators of other insects and are seen as a natural control of certain insect pests. Control of insect pests by natural enemies can be effective, long-lasting, and minimally disruptive to the ecosystem. The use of predatory insects may be valuable in controlling exotic pests that have been introduced without their natural enemies. Research is being done in this area, with mixed results. 

Other insects can parasitize or displace harmful insects. Most parasitic insects are types of small wasps or flies that lay their eggs in the host insect. The emerging larvae feed on the host, usually killing or sterilizing it. Populations of certain insect pests are naturally kept in check through their parasites.

Mites are close relatives of insects but are actually arachnids, as are spiders and ticks. The mites that cause significant damage to woody plants are very small, and a hand lens is usually required for identification.

There are many species of mites. Some species, such as the spider mite, discolor leaves by extracting leaf fluids, resulting in symptoms similar to other sucking insects. Other mites are gall producers. Not all mites are harmful to woody plants. In fact, some mites prey on other mites in the landscape." (ISA study guide material)

Tel: 403-971-4722

Email : kephaslandscaping@gmail.com