In many ways, a pest update is a rundown of what’s new, what’s changed and what hasn’t changed. While this is true, there are several other factors that are important to consider as well. Some pests, like aphids, cedar apple rust and eastern tent caterpillars, have become year-in and year-out pests, while others have seemingly gone through periods of abundance followed by periods of nonoccurrence. For example, bagworms were one of the major pests of the ’70s and ’80s; then in the mid ’90s, for reasons unknown (lots of speculation, but nothing definitive), they were hard to find. The pendulum seems to have swung back with outbreaks in recent years occurring on a regular basis.
Numerous small cankers are associated with thousand cankers disease.
PHOTOS BY LORI STEPANEK, NEBRASKA FOREST SERVICE, UNLESS OTHERWISE NOTED.
Regionality of pests
Due to the presence or absence of specific host plants across the U.S., certain pests are considered a scourge of one region of the country, yet arborists in other areas may have hardly heard of them. This is certainly the case with trees of Mediterranean origin such as jacaranda and palm, but even temperate zone species such as oaks or ash trees can be regionally distributed and affected by damaging pests only in certain areas.
The pests that are most currently thought of as “new” are typified by two culprits: the mountain pine beetle and thousand cankers disease.
• Mountain pine beetle: The mountain pine beetle has killed millions of acres of pine trees in the western U.S. and portions of Canada. Scotch pine is highly susceptible to the beetle, whereas Ponderosa and Austrian pines are less susceptible. Mountain pine beetles infest pine trees from July through October leaving small, marble-sized masses of resin called pitch tubes on the trunk. Adults (beetles) are black, about .25-inch long. Females lay their eggs in the tunnels between the bark and wood, and larvae hatch from the eggs and tunnel in the tree until July of the following year. These larvae can kill the tree if sufficient numbers are present.
To reduce the risk of the beetles spreading to nearby pines, heavily infested and dead trees should be removed and either chipped or burned before the end of May. High-value and recently infested trees with only a few pitch tubes on the trunk can be treated with insecticide trunk sprays containing bifenthrin, carbaryl or permethrin. Sprays should be applied in the late spring but before mid-June.
Thousand cankers disease symptoms.
• Thousand cankers disease: Caused by the fungus Geosmithia morbida, thousand cankers disease has been on the rise in many western U.S. states. The fungus is spread by the walnut twig beetle, a tiny black beetle that feeds under the bark. The beetle and the fungus are readily transported in infested walnut wood. As a result, quarantines are being set up to limit movement of the wood from state to state in an effort to contain infestations locally.
Thousand cankers disease symptoms include yellowing foliage followed by brown wilted foliage, branch dieback and tree death. Black walnut is highly susceptible, while English walnut, butternut and other walnut species show varying degrees of susceptibility; pecan and hickories are resistant. Tree death occurs two to three years after initial symptoms appear, and trees may be infected for many years without showing visible symptoms, eventually dying as a result of the collective effects of many individual cankers, hence the name of the disease. Unfortunately by this time, there are no effective chemical controls.
What’s changed – new, but not-so-new
Pests that have been an issue for the last decade or so include the emerald ash borer (EAB) and pine wilt disease. Again, the regionality of the host is a consideration with these pests. Throughout much of the Midwest, arborists and tree workers have heard the horror stories of death and destruction of various ash species caused by EAB over the past few years or so, yet no confirmed cases have been reported in Iowa or west of the Missouri River as of the spring of 2011. Yet, in the lake states of Michigan, Ohio, Indiana and Illinois, EAB has become a problem on the order of the magnitude of Dutch elm disease.
Emerald ash borer adult.
PHOTO BY BARRY LYONS, CANADA FOREST SERVICE.
• Emerald ash borer: The emerald ash borer (EAB) is an exotic, .5-inch long metallic-green beetle from Asia that has already destroyed millions of ash trees in the upper Midwest since its introduction in 2002. Nearly all ash species are susceptible to EAB. An early sign of EAB infestation is the appearance of weak and dying stems and branches in the crown of the tree. Closer inspections will reveal 1/8-inch D-shaped holes on the trunk where adult borers have exited and zigzag tunnels packed with frass (insect excrement and sawdust) under the bark. Later symptoms may include watersprouts and suckers around the trunk, split or loose bark, and increased woodpecker activity. While most borer species are only attracted to weak or dying trees, EAB will attack young and old, healthy and stressed trees. Emerald ash borers generally have one generation per year, but may require two years to complete their life cycle in cooler regions.
In landscape settings, insecticidal control of EAB on specimen trees is possible using bark and canopy sprays containing bifenthrin, carbaryl, cyfluthrin, dinotefuran or permethrin; soil injections/drenches of imidacloprid; or trunk injections containing acephate, dicrotophos, emamectin benzoate or imidacloprid.
• Pine wilt disease: This pine disease is caused by the pinewood nematode which is moved from infested to non-infested pine trees by the pine sawyer beetle. The disease typically kills Scotch pines within a few months after the pine sawyer beetle introduces the nematode to the tree. The needles of infested trees turn from grayish green to tan and finally brown. While primarily a disease of Scotch pine, pine wilt can also occur in Austrian, jack and mugo pines with a similar pattern of symptoms.
Sanitation is the most important management practice to prevent or slow the spread of pine wilt disease. Scotch pines dying from the disease should be cut down then burned, buried or chipped before pine sawyer beetles can emerge from the trees. High-value pines can be protected with trunk injections of the insecticide/nematicide abamectin.
What hasn’t changed
Integrated pest management (IPM) is a set of considerations that can serve as a common-sense guiding force in landscape pest control. IPM takes a holistic approach, encouraging the arborist to consider all factors necessary for a tree to be healthy, including soil nutrition, pH, planting depth, soil compaction, spacing, hardiness, competition from turfgrass or other trees, inherent disease resistance and soil moisture. Even though IPM was introduced several decades ago, it remains an excellent vehicle to ensure that trees are provided with all the essential inputs necessary to thrive.
The green industry’s catch phrase “Right Plant, Right Place” (RPRP) looks like it’s going to be around for a while because it is straightforward yet multifaceted. Key RPRP components, all of which can have an impact on the frequency and severity of tree diseases and insect pest, include:
•Sun/shade requirements: Misplaced trees and shrubs are unlikely to prosper.
• Mature tree height and width: Trees and shrubs that are pruned excessively to fit into a restricted space are more subject to foliar diseases, cankers, borers and other common pests.
• Soil moisture and pH: Adequate or inadequate drainage can either increase susceptibility or help trees resist diseases such as Armillaria root rot.
• Disease resistance: Disease resistance means lower maintenance, which is highly desirable and frequently requested, especially by commercial clients.
• Flower/fruit/fragrance: Butterflies are always welcome, but bees can be a real nuisance.
• Native choices: Chances are good that native plants are going to survive and be less likely to succumb to various pest problems.
• Hardiness zones: Lack of cold and heat tolerance can be a major cause of pest problems when plants are not sited according to hardiness zones.
• Level of maintenance: High levels of maintenance can be justified in high-visibility areas, but other areas can be zoned for lower maintenance plantings.
Unfortunately, phrases such as RPRP tend to be overly simplistic and can lead arborists to overlook or disregard other important species/cultivar characteristics. For example, sun/shade requirements and eventual tree height are two of the most commonly referred to criteria in plant selection guidelines. Using only these criteria, however, the arborist may not make the best selection for the site, especially if the decision must be made in a hurry.
Scouting and monitoring
For the most part, the terms scouting and monitoring can be used interchangeably. Monitoring involves regular inspections to detect early signs of pests and their damage. The primary goal is to detect, identify and describe pest infestations. All trees should be monitored on a regular basis during the growing season. The scouting interval may vary from once a week to once every several months, depending on the value of the trees to the customer and nature of anticipated pest problems. Landscape monitoring can be delivered and invoiced for, replacing the number of pounds of insecticide or fungicide sprayed on a tree as your source of revenue.
Scouting and monitoring is important not only for your client’s property, but also for the early detection of potential pests in neighboring landscapes as well. If Zimmerman pine moth or Sphaeropsis tip blight are evident at the next door neighbor’s place, it would be wise to inform your client and suggest a management program to prevent damage to their trees. Your monitoring activities and observations could be conspicuously placed in the future problems section of an invoice.
Site assessment and analysis
The health of any landscape, regardless of function, can be improved through site assessment and site analysis. Though more helpful when conducted prior to installation of plant materials, considering all aspects of the site is an immensely advantageous yet underutilized process.
The difference between a site assessment and a site analysis is simple. A site assessment involves documenting the status of each plant in the landscape including their growing conditions. In short, it’s the raw information on which the analysis will be based. A site analysis is conducted after an assessment and involves using your observations to diagnose the site, make value judgments and recommendations.
Here’s how it works: Step one is to walk the site with a clipboard, sketching in the various hardscape (anything nonliving) pieces and plants. As each plant is encountered, notes such as “spots on leaves,” “gash in trunk” or “stunted current season’s growth” are written on the sketch. This may be done by section of the landscape or all at once; there are advantages and disadvantages to each approach. It’s usually easier to focus on specifics when smaller areas are reviewed, while a more cohesive and overlapping view is useful when larger areas are assessed.
Step two is to determine the cause and seriousness of each noted concern; to make a value judgment for each. For example, a tree might be struggling because the sprinkler system has been in disrepair recently and runs for an hour and a half every morning regardless of natural rainfall. Thus, “pale leaves and stunted growth” could translate into a recommendation for an audit of the sprinkler system. It’s also possible that weather conditions have led to the infection by common pathogens such as apple scab and anthracnose, and a treatment program to address the issue may be necessary.
John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.