A wide range of insects, diseases, and other pests can seriously affect our trees. This is a very broad topic …. but here are some of the common and challenging issues.
Insects
Borers – The Nemesis of Most Trees
Boring insects can be a challenge to control because they spend most of their time under the protective bark layer and in the impervious heartwood. Often times, they are not detected until a tree is in decline. Small holes in the bark are the only sign of their presence. When the insects are active, small bits of sawdust are visible at the hole openings. In the case of a serious infections, the sawdust may accumulate at the base of the tree.
4 Steps to Controlling Borers:
Inspect trees monthly from April to August.
Reduce stress:
- Watering during times of drought. Consider fertilizing if feasible.
- Soil compaction and digging within the root zone also cause stress.
- Weed eater damage and sunscald wound the bark, inviting borer damage.
- Always prune trees when the sap is down in winter.
Apply Imidichloprid annually as a prevention, sometime between March and September.
Consult with an arborist for alternative and additional control techniques.
Caterpillars – Leaf-Chewers with a Bad Rap
Hundreds of varieties of butterflies and moths emerge from the worms that eat leaves of many trees. Though it appears they cause a lot of damage, these insects turn into lovely, much loved pollinators.
Many types of trees and shrubs support this class of insects. Missouri oak trees alone support over 700 different kinds of caterpillars. Contrasting, sometimes a worm has a specific host such as the Spice Bush Swallowtail, feeding only on Lindera (Spice Bush).
Generally the damage done by caterpillars is minimal. The massive source of leaves in a forest and on even just one healthy tree seldom cause permanent damage to our prized yard trees.
Bagworms, however, are notorious for the damage they do to the evergreen trees Juniper, Dwarf Alberta Spruce, and Arborvitae. Using control measures for bagworms may be necessary as these critters can do a lot of irreparable damage in a short time. Spinosad is an effective and safe way to control bagworms and other caterpillars.
Beetles –
Japanese Beetles– A non-native pest that has a voracious appetite, devouring leaves as the masses move in over a 6-12 week period in early summer. The shiny multi-colored insect is only about 1/2″ in size. Though sprays can be applied, traps placed a couple hundred feet away from the tree(s) have been proven to be an effective method of reducing Japanese beetle damage.
Bark Beetles– A category of insects that tunnel just under the bark and are usually small, only 1/16 to 1/4 “. Overall, they are attracted to stressed or dying trees, though some species may attack healthy trees.
Dutch Elm Disease and Oak Wilt are two examples of bark beetles seen in Missouri, as well as Shothole Borer (fruit trees), pine and spruce beetles, and ambrosia beetles.
Control bark beetles by removing dead and dying branches on healthy trees to eliminate a possible source of infection. Burn or remove prunings from the area as the insect can complete it’s life cycle in the refuse.
To prevent infection, keep trees healthy by watering during times of drought and fertilize if the growth rate is slow.
Smaller Insects & Assorted Pests –
A plethora of assorted insects fall in the “small-size” category, each with it’s own threat to our trees. Some serious and some not-so-much. Here is a quick list:
- Aphids – Not too serious but can be messy, leaving behind a sticky substance on surfaces below the infestation. Accompanied with sooty mold, often times the first sign of their presence.
- Scale – Notorious for being difficult to control because of their hard outer shell, scale feeds on plant fluids and often goes undetected for a long time.
- Lace Bugs – small, often clear-winged insects affect the beauty of tree leaves but, in the long run, don’t affect the health of the tree. If control is desired, research the type of lace wing (per the type of tree) and follow the guidelines for prevention or early control.
- Leafhoppers – These hopping insects also suck plant sap from the leaves of trees, grasses, and shrubs. The damage is usually minimal, but the sticky honeydew left behind can be unsightly with a dark mold lingering through the summer.
- Cicadas – This annual and sometimes periodic visitors do cause damage to small twigs, laying their eggs on the underside so that when a wind comes along, the twig will snap off and fall to the ground where the larvae then move to the ground for the winter. The only real threat is to smaller trees, as large well-established trees can resist the on-slaught of tip pruning.
- Galls – Usually caused by a mite and needs to be controlled by spraying early, as leaves start to emerge from their buds.
For most of these insects, Pyrethrin and Neem Oil are the preferred treatment, as both products are non-selective and have a low toxicity to mammals.
Diseases
Anthracnose –
This disease spreads early, just as leaf buds are emerging, especially if conditions are moist from spring rains. Sycamore, Ash, and Maple are commonly affected, but anthracnose can show up on other types of trees, such as dogwood. The foliage develops dark brown irregular patchs days to weeks after infection. Moisture is key in the spread of this disease, so dry weather as the leaf buds emerge usually results in little or no infection.
Fireblight –
A disease common to members of the apple, pear, and rose family, fireblight spreads during the blooming season, especially as petals drop leaving behind a small entry point for the disease to infect over a few hours or days. Symptoms show up weeks later as branch tips turn dark brown or black and start to curl, plus the foliage turns dark brown. Pruning branches further back than what is visibily damaged is one approach, but prevention is the best cure. Spray with Streptomyacin during the bloom time and as petals fall off. Pear is one of the most commonly affected.
Rust –
Also a common disease in the apple and rose family, it is commonly seen in apple, pear, blackberry, rose, hawthorn. These diseases must have a host plant for the disease to complete it’s life cycle, for example Cedar-Apple Rust much have both plants present (though not necessarily in close proximity). In this case, when the orange galls are present on the cedar (native Juniper trees), the spores are spreading to the apple tree and will show signs of infection about 3 weeks later with yellow/orange spots on the underside of the apple leaves. When this shows up, there is no way to eliminate the spots, and heavily infected leaves will turn yellow and drop to the ground waiting to reinfect cedars another year.
Verticillium Wilt –
The most common wilt disease we see in Missouri is Verticillium Wilt, affecting mainly elm, maple, magnolia, and redbud, though other types can also be affected. Sudden wilting, yellowing of leaves, and stunting of growth are common symptoms.
The mycelia of this fungus can clog vascular pathways blocking the water flow to all leaves and branches. Sanitation and planting resistant varieties are the best approaches to avoid this disease.
Oak Wilt Fungus
A common problem among stressed red oaks in Missouri, Oak Wilt disease is spread from tree to tree by sap-feeding beetles and root grafts. The most common symptoms are browning and wilting of leaves in the upper crown by early summer.
White oaks can live with the disease for many years, but red oaks have less survivability once symptoms show up. Fungal injections by an arborist can protect prize oaks for awhile.
Remove diseased trees before spring. The wood can be cut up into firewood but should be covered with tarps or 4-mil plastic, tucked in securely, so that beetles don’t continue to harvest the sap for several months. Avoid pruning or damaging oaks from mid-March until June when they are most susceptible to the disease.
Powdery Mildew – an unsightly but not threatening disease. It is brought on during certain weather conditions, warm days and cool nights. The best solution for avoiding powdery mildew with landscape trees is to plant resistant varieties.
