Organic Pest Control in Fraser Fir Christmas Trees
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The organic production of Fraser fir Christmas trees is not easy. Fraser fir is a slow-growing tree that only produces one flush of growth each year. Grasses and weeds can choke out and kill young Fraser fir seedlings. It is harder to get soil nutrition for trees through organic fertilizers. Pesticides such as horticultural oil are harder to apply and more costly. Therefore, when it comes to pest control, it is important to start with a good site, scout regularly, and control problems before they become widespread.
Since pest control will not be as easy with organic pesticides, it is important follow good cultural practices that promote tree growth and reduce problems with pests. These include the following:
Site Selection and Spruce Spider Mite
Spider mites (SSM) are not easy to control organically. Therefore, it’s important to grow trees in a habitat that favors predatory mites and not spider mites. Predatory mites prefer cooler and more humid conditions than the SSM. Frasers grown at elevations lower than 3,000 almost always have problems with SSM. So do trees grown on a south or south-west exposure. A Christmas tree field surrounded by woods almost never has a problem with spider mites. Avoid windy ridges. Dust from gravel roads also create SSM problems. The dust that settles on the needles will kill the predatory mites by damaging their exoskeleton as they run in search of prey, causing them to dry out and die. The more sedentary SSM are not affected. Providing a habitat that does not favor mites will go a long way in reducing the need to treat for them.
Avoid Close Proximity to balsam woolly adelgid
The same thing is true for the balsam woolly adelgid (BWA). Though it can be controlled organically, treatments will need to be made every year. To reduce pressure from BWA, avoid placing an organic Christmas tree field near natural stands of Fraser fir, near true firs that are growing in yards or in Christmas tree fields that are abandoned or not properly taken care of.
Promote Flowering Weeds
Biodiversity of flowering plants is key to attracting predators. Promote flowering weeds such as yarrow and clovers in ground covers around the trees. You may need to establish clover ground covers the year before setting trees. Allow field borders to grow up in grass and flowers. Even briars and brush help bring in natural predators. Bush hog these field borders every 2 to 3 years to keep vegetation in a state of early succession.
Culling Problem Trees
Fraser fir is a seed run crop. There is tremendous genetic diversity. Some trees are more prone genetically to pests such as SSM, BWA, elongate hemlock scale (EHS), and rosette bud mite (RBM). Cutting down and removing these trees will keep these pests from spreading to other trees.
One method of pest control is to grow trees as quickly as possible. A good organic grower must be an expert shearer to make the best use of tree growth. Shear trees early to increase budset. Prune out cross-over branches that will become horns. Do not extend leader length to keep trees from becoming too thin and narrow. For more information on shearing see Shaping Fraser Fir Christmas Trees.
Organic pesticides do not give as good or as quick a kill as synthetic materials. Therefore it is important to scout to learn when pests first come into a field and treat them promptly when needed. Another issue with organic insecticides is the potential for soaps and oils to burn or discolor foliage. These materials, if misapplied, will cause yellowing and browning of the needles, which is often followed by needle shed. In most cases, the bud is not affected.
Most organic pesticides require complete coverage. A high pressure sprayer gives the most thorough coverage. You also need good agitation in the spray tank when horticultural oils are being applied so that the oil and water do not separate. For the smaller acerage grower who either cannot afford a high pressure sprayer or cannot find a reliable contract applicator, a backpack mistblower is a lower cost alternative.
Horticultural Oil — The Workhorse of Organic Pest Control
A good organic pesticide which will control or at least suppress almost all Fraser fir pests is horticultural oil. Oil controls pests by smothering them. Therefore, you have to get complete coverage of the tree so that pests on all surfaces are coated with the oil solution. For pest control, a 2% solution of oil (that is 2 gallons in 100 gallons) is required. At this concentration, however, the tree may be damaged. Needles may turn brown and drop off the tree.
Types of Oils to Use
To get the best pest control without foliage burn, use a highly refined horticultural oil. These are often called summer oils. Do not use a dormant oil as these will cause foliage burn. Look for oils with at least 92% unsulfonated residues on the label. It will list this value under the active ingredients.
There are also an encapsulated oils on the market. The encapsulation process results in an oil that does mix with water. This is more expensive than regular oil, but you will not need good agitation in the spray tank.
What Oils Will Control
A 2% solution of oil applied in mid-March will do a good job of controlling the following pests:
- BTA eggs
- SSM eggs
- Rust mites (HRM) and eggs
- BWA nymphs, crawlers and adults
Control of the following pests is not as good. A 2% solution of oil will only suppress the following:
- BWA eggs. However, there shouldn’t be any BWA eggs present in mid-March.
- BTA nymphs or adults. Therefore be sure to treat before the twig aphid eggs start to hatch.
Timing of Oil Applications
The best time to treat for pests of Fraser fir using oil is in mid-March. Going ahead and applying oil preventatively each year regardless of what pests are present or how soon trees will be harvested is a good strategy. A second application may be necessary to control SSM or if Cinara aphids are a problem in the fall. A summer application of oil may be necessary to control EHS. Good control of EHS may take repeated applications of oil made two to three weeks apart.
Other Organic Pesticides
The following are other organic pesticides that have been successful at reducing pest numbers.
- Pyrethrims. These naturally occurring insecticides are extracted from flowers. Like synthetic pyrethroids, the man-made insecticides patterned for them, they are very broad spectrum and will kill natural insect predators.
- Insecticidal soap. This is particularly effective against Cinara aphids. There may be issues with yellowing foliage.
- Neem oil. This oil acts like an insect hormone mimic and interferes with the insect’s ability to feed and reproduce. Neem oil has been used successfully to control BTA.
- Cinnamite. This cinnamon extract has been used to control BTA successfully.
- Elemental sulfur. Sulfur has been successfully used to control HRM.
Special Considerations for Cinara Aphids
Most conventional Christmas tree growers treat all go-to-market trees with a bifenthrin product such as Sniper in the fall. A fall treatment for Cinara aphids can also be made in organic Christmas trees with either horticultural oil or insecticidal soap. However, it is expensive to treat all the harvested trees and most will not have aphids. Soaps and oils may cause discoloration of the foliage. If individual trees with Cinara aphids are identified at harvest, then those trees can be treated either with insecticidal soap or even washed out of the tree using water.
Special Considerations for Rosette Bud Mites
Rosette bud mites are the only pest that cannot be controlled with organic insecticides. Control of RBM in organic trees will have to rely on cultural practices such as selective harvesting and not interplanting. For more information on RBM control, see CTN-018a: Rosette Bud Mite on Fraser Fir.