FAQ: Pesticides Used in Christmas Trees
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Collapse ▲What are the pesticides used to grow Christmas trees?
Pesticides used on Christmas trees grown in Western North Carolina are the same pesticides used to grow non-organic vegetables and fruits. Many trees that are shipped out of state are sprayed around two or sometimes three times a year. Farmers target invasive species such as elongate hemlock scale, cryptomeria scale, and balsam woolly adelgid. Unfortunately, it’s very difficult to grow Fraser fir without the use of pesticides due to these invasive pests. Fraser fir is especially susceptible to the Balsam woolly adelgid. This pest is a main cause for mortality in native Fraser fir stands.
WNC Christmas trees are usually sprayed with bifenthrin or another pyrethroid a month before harvest (in late September) to assure no insect pests are in the trees before they are harvested. Pyrethroids are the synthetic version of the naturally occurring pyrethrins found in chrysanthemum flowers. They are used in a variety of products, including indoor and outdoor household insecticides, pet sprays, human lice treatments, and mosquito repellents.
The EPA has determined it is safe for many vegetables and fruits (such as cilantro or oranges) to be harvested one day after bifenthrin is applied. Therefore, by the time you buy your tree November, you’ve probably already eaten something that has been sprayed with bifenthrin with a shorter pre-harvest interval. The safety in terms of pesticide exposure of a live cut Christmas tree in your home can be compared to the pesticide exposure of a non-organic cabbage in your home.
How do growers know if they need to apply a pesticide? What else do they do to control pests besides pesticides?
Growers are encouraged to use Integrated Pest Management (IPM) to control pests. Integrated pest management is a system of pest control methods that uses appropriate cultural practices and pesticide selection to reduce pest problems. Scouting is key to IPM.
Most pests of Christmas trees can cause considerable damage if left untreated. Scouting fields on a regular basis to estimate pest numbers is required to learn if a treatment is necessary. Growers use a handlens and tree symptoms to find pests as well as natural predators in the field. Scouting occurs on a regular schedule with multiple trips to the field. In 2018, 83% of growers reported scouting before they treated for pests and 21% of growers reported delaying treatments based on the presence of predatory insects.
Other IPM practices that growers use to combat pests include site selection, early harvesting of damaged trees, proper shearing and fertility practices, encouraging natural predators by keeping ground covers around trees and the carefully selection of pesticides, and not interplanting young trees among older trees (pests get on the smaller trees sooner than they would have).
Why do they have to use herbicides? Why can’t they just mow?
Growers prefer using herbicides to mowing or using a weedeater for several reasons. From the grower’s point of view, it’s quicker and cheaper to apply a herbicide than to mow. Mowing and weed eating can damage the bottom limbs on trees and even kill trees if the trunk is girdled. Mowing also encourages grasses to grow, which can choke out trees or grow up into the tree making weak bottoms. And mowing is also less environmentally friendly because of the gasoline and oil required to run them and the number of times a year a farmer is required to mow.
Many growers use a technique called “weed suppression.” Originally developed at North Carolina State University by Dr. Walt Skroch and others, weed suppression uses lower than labeled rates of herbicides to stunt rather than kill weeds.
For instance, Roundup is labeled at 32 to 160 ounces formulation per acre. But Christmas tree growers in western North Carolina use rates of 4 to 8 ounces per acre to stunt the weeds so that they don’t grow too tall. This treatment lasts 4 to 6 weeks before another application is required. Growers may treat two or three times through the growing season with Roundup at low rates, then come back in the fall and treat problem weeds such as briars and poison ivy at the full rate to eliminate these weeds.
This type of ground cover management results in a shift of common ground covers away from grasses which are competitive to the tree to small woodland perennials that are far less competitive. That’s because these plants are more tolerant to Roundup and aren’t killed by the lower rates. Clover is a common ground cover that fills in the places between trees with this type of management practice, as are dandelions, nimblewill, wood sorrel, and violets. Clover and bird’s foot trefoil are also being sowed by growers in cleared land or other sites where appropriate ground covers are not present; these are then managed further through weed suppression with Roundup.
Prepared by Jill Sidebottom, PhD and Jamie Bookwalter, PhD
Area Extension Forestry Specialist, Mountain Conifer IPM
NC State College of Natural Resources
N.C. Cooperative Extension