Christmas Tree Notes
|
|
Christmas Tree Notes |
|
Prepared by:
Jill Sidebottom, Ph.D.
Area Extension Forestry Specialist, Mountain Conifer IPM
Extension Forestry, College of Natural Resources
Original Christmas Tree Note written January 1998. Update August 2002
The hemlock rust mite, Nalepella tsugifoliae, is a frequent springtime problem on hemlocks grown in the foothills of western North Carolina. Mite numbers can increase quickly from February through May depending on the weather. Most growers in the foothills use an insecticide at least once every spring to prevent damage from this pest in hemlocks. By early summer when temperatures average in the high 80's rust mite numbers and damage decline.
White pine and Fraser fir Christmas tree growers also have occasional problems with this pest at higher elevations (>3000 ft). Because damage doesn't occur yearly on these species, many fir and pine growers do not scout regularly for this pest. They only notice damage when it is too late to achieve control. Some growers never realize mites are the problem, since they are so small and hard to see, and will attribute the problem nutrient deficiency or winter burn.
Rust mites are eriophyid mites and are biologically different from the more familiar spruce spider mite. Eriophyid mites have four legs at maturity rather than eight such as spider mites and predatory mites. They are very small, requiring magnification such as a hand lens (7X to 15X) or dissecting microscope (30X) to see them clearly. Rust mites can be cream-colored, tan or orange. They are similar in color and size to pollen, but unlike pollen their motion can be detected if watched carefully.Rust mites suck juices from conifer needles. When many mites are present, their damage gives the needle a dusty, rust-colored appearance. To distinguish rust mite damage from a nutrient deficiency, examine needles with a hand lens. Rust mite feeding causes tiny marks or scratches running parallel to the midrib. If damage is severe, affected needles will drop off the tree or shed prematurely in the fall.
Rust mites produce many generations over a short period. The weather affects their reproduction and survival. Rust mites prefer moderate, spring-like temperatures and dry conditions. These weather patterns allow them to reproduce quickly. Mite numbers can explode from just a few to more than 50 on a single needle in as little as two weeks. The worst outbreaks of rust mites in white pine and Fraser fir Christmas trees have been when a warm fall is followed by a long, dry spring.
Figure 1. Drawing of an eriophyid mite. To see mites well, you need a hand lens or a microscope. Drawing courtesy of Dr. James Baker, NCSU
Rust mites can quickly cause serious damage in Christmas trees. Damage can be prevented by frequent scouting in the spring and prompt treatment if mite numbers have reached the treatment threshold (see below). Unfortunately, scouting is difficult because mites are so small. Many growers aren't confident in their ability to see such tiny pests. Also, in early spring, growers are busy planting, fertilizing, and applying herbicides; scouting is often neglected.
To know whether a pesticide is needed, look for the rust mites themselves, not just the damage. The goal of scouting is to treat when mites are present at great enough numbers that damage is likely but before the damage has occurred. Rust mite numbers can decline as quickly as they build if the weather warms or predators have been active. To establish the need for using a pesticide and to accurately time the pesticide treatment before serious damage has occurred, each block of Christmas trees requires scouting.
There have been some observations that rust mites problems increase after the use of Asana and other materials used for control of the balsam woolly adelgid. Even treatment with Asana the summer before has been followed by rust mites in the spring. Just because Asana or another woolly adelgid treatment is used doesn't guarantee that rust mites will be a problem. Also, fields that have never been treated with an insecticide have had serious damage from rust mites. Still, enough observations have been made to recommend scouting for rust mites the spring after balsam wooly adelgid applications.
It is not necessary to scout white pine and Fraser fir Christmas trees every spring for rust mites. Earlier mite activity on hemlocks grown in the foothills can help predict rust mite problems at higher elevations. Keep in touch with your county extension office in the spring to decide if rust mites are becoming a problem in your area. Rust mites can also be detected when scouting trees for other pests such as spruce spider mites. However, if rust mite problems are found when scouting for another pest, go back and rescout for rust mites alone.
The Sampling Procedure. To scout for rust mites, examine 10 to 20 trees per acre. Choose trees that had damage the previous year. If there is no previous damage, choose trees at random.
When
scouting in white pine, pull needle clusters on the upper third of the plant.
Mites are most often found on the southeast side of the tree. Look for mites between
the needles near the base.
When scouting Fraser fir and hemlocks, take shoots of the most current growth. Examine some from the upper portions of the plant, and some from the lower. Scan both the upper and lower surfaces of the needles with a hand lens, looking for mites. If the buds have recently opened, take both the new growth and previous year's growth, to learn if the mites have moved onto the new growth.
Treatment Threshold. A few rust mites on a shoot will not cause damage. Only when rust mite numbers begin to build will pesticide treatment be necessary. The following is a treatment threshold that can help assess the need for treatment. It can be modified depending on each individual grower's treatment costs and markets. The treatment threshold can apply to hemlock, Fraser fir and white pines.
Both the percentage of trees with rust mites and the total number of mites on the heaviest infested needle are considered in the treatment threshold. This is different from the treatment threshold for spruce spider mites, which only takes into account the percent incidence. Therefore, for a pesticide to be necessary, both of the following criteria need to be met. A counter and a scouting form will aid in keeping track of these numbers.
What should I do if mites are present but numbers are below treatment threshold? Rust mite numbers can increase rapidly given favorable weather. Weekly scouting may be necessary during critical periods in the spring. This is accomplished through hot spot scouting - choosing areas of the field with the greatest numbers of mites to assess quickly if mite numbers are increasing or declining. Keep accurate records of each scouting assessment to compare results from different scouting trips.
What should I do if the threshold has been reached but it is almost summer? If the treatment threshold is not reached until late May or early June, it may be possible to delay a pesticide treatment and let the warmer summer temperatures "control" the mites. Scouting weekly may again be necessary to decide if the numbers are declining. Recheck these fields in the fall when temperatures start to cool as rust mites may again become active.
Many miticides are commercially available for the treatment of mites. However, some miticides, especially the newer materials, are effective only against the tetranychid or spider mites. Rust mites are eriophyid mites and are biologically different from spider mites. Be sure that a given miticide specifically controls rust mites before using it. Commonly used materials that are effective against rust mites include horticultural oil, Dimethoate, Sevin, and Joust (morestan). Elemental sulfur can also control rust mites at the rate of 4 lbs/acre. Sanmite also gives good control of mites. Other chemicals such as Di-Syston 15G have not given consistent or long-lasting control of this pest. Rust mites may quickly rebuild after these treatments.
Rust mites are typically not difficult to control. An air-blast mist blower or light coverage with a high-pressure sprayer and hand-held gun can provide sufficient coverage. Be sure to scout one to two weeks after trees are treated to learn if control was successful.
If weather favorable to the mites continues, rust mite numbers may build back up, even after a successful pesticide treatment. Always rescout trees in a few days to make sure any pesticide application was successful. Check trees again in four to six weeks to make certain rust mites are not becoming a problem again.
Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention of commercial products or services in this publication do not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact an agent of the North Carolina Cooperative Extension Service in your county.
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.