Parasitic Wasps Are Making A Meal of North Carolina’s Christmas Trees

Whether it’s bought at a retail lot or farm, a live Christmas tree is an integral part of the holiday season for millions of Americans. In recent years, however, some growers in major tree-producing states like North Carolina and Oregon have struggled to remain profitable amid a looming shortage. 

Experts with the National Christmas Tree Association and NC State Cooperative Extension say the shortage is partly a result of the 2008 recession when decreased demand led growers to plant fewer seeds. Seeds are harvested from the cones of mature Christmas trees – and because it can take several years for trees to reach maturity, the shortage is likely to continue beyond 2019.   

In North Carolina – which is the second-largest producer and exporter of Christmas trees – growers face changing weather patterns, diseases, and a number of other challenges that could potentially prolong the shortage. That includes parasitic wasps that eat the seeds of Christmas trees before they ever get the chance to grow.

Originally described in the 1930s, Megastigmus specularis is one of several thousand species of chalcid wasps. It mostly infests Fraser fir trees in the Appalachian Mountains, according to Dr. Lilian Matallana, a research associate at NC State’s Department of Forestry and Environmental Resources. 

The tiny wasp – about 3 mm long – overwinters in fir seed on the ground, with adults emerging in spring. Females then fly to developing cones and use an elongated ovipositor (like a modified stinger) to drill into the seeds and deposit their eggs. Once the egg hatches, the larvae feed on the seed until they’ve matured. 

Matallana said this cycle – which leaves seeds inviable for commercial planting – is repeated annually except some larvae delay emergence for an additional year or more, presumably to preserve the species in case of crop failures.

Feasting on Fraser Firs

Until recently, there was little reason to be concerned about Megastigmus specularis and similar species in North Carolina, according to Matallana. But, because of increased interest in growing Fraser fir for Christmas trees, information about the effect of these insects on the state’s seed supply is needed. 

North Carolina produces more than 6 million Christmas trees each year, about 15 percent of the nation’s natural Christmas trees – second only to Oregon. Nearly all of the state’s trees – 96 percent – are Fraser firs, mostly grown in 14 mountain counties. 

The state’s Christmas tree industry is mainly supplied with seeds from natural stands and genetically-improved material from clonal seed orchards, according to Matallana. Trees derived from clonal seed orchards have more desirable characteristics than trees that originate from seeds collected in natural stands. These attributes provide a considerable increase in income for Christmas tree plantation owners across the state. 

Previous work by Matallana and other NC State researchers suggested that there may be genetic differences in Megastigmus infestation rates among trees in clonal seed orchards, and that these differences could affect the cost of planting stock for Christmas tree growers due to downstream impacts on the viability of seeds from the same clone during long-term storage. Additionally, the team found that pesticide treatment may impact infestation rates. 

With funding from the North Carolina Christmas Tree Association, Matallana is now working with Dr. Ross Whetten, a professor in Department of Forestry and Environmental Resources and member of NC State’s Molecular Tree Breeding Lab, to determine clone-specific infestation rates before and after pesticide treatment and to identify candidate clones with reduced susceptibility to Megastigmus specularis and other chalcids.

“The proposed research will contribute basic and practical knowledge to improve seed quality and leverage chalcid development to control future infestations in other fir orchards in North Carolina,” Matallana said. 

Protecting the Seed Supply

Although chalcid infestations aren’t currently considered a major threat to North Carolina’s Christmas tree industry, they could eventually become a costly issue for growers, according to Matallana.   

Growers often have difficulty identifying chalcid infestations, because infested seeds not only appear normal on the outside but also weigh about as much as healthy seeds. As a result, infested seeds are often deposited with healthy seeds during the cleaning process. 

“It’s really a big problem for growers, because they might be buying and planting infested seeds that look completely healthy,” Matallana said. 

The most accurate methods of determining infestation rates include obtaining x-ray images of Fraser fir seeds or dissecting them under a microscope. Matallana and her team are using both methods.

Since June, Matallana and her undergraduate research assistant, Nick Moore, have collected – and dissected – more than 10,000 Fraser fir seeds from a clonal seed orchard in Ashe County in order to identify larvae and document infestation rates. Their preliminary lab observations show infestation rates ranging from 3 to 30%.

Dr. Jill Sidebottom, an Extension Specialist with the Department of Forestry and Environmental Resources, recently wrote that most Christmas tree pests can cause “considerable damage if left untreated.” That’s why growers are encouraged to use a combination of land management practices and insecticides to control or reduce pests. 

Unfortunately, though, most insecticides used by Christmas tree growers only coat the outer layers of Fraser fir seeds, leaving the inside vulnerable to chalcids, according to Matallana. 

Growing Pest-Free Christmas Trees

As part of their research, Matallana and Whetten are working determine the efficiency of a seed cleaning method in reducing chalcid infestations and the effects of x-ray imaging in chalcid development and seed germination rates. The team recently sent several batches of seeds to the U.S. Forest Service’s National Seed Laboratory in Georgia for x-ray imaging and genotyping. They plan to publish their results later this year.

Matallana said she hopes the project’s findings will not only help boost growers’ profits but also improve the seed supply – both important factors as rapid changes in the world’s climate, globalization and current trade practices influence pest populations. Research shows that these changes directly impact the development, survival, reproductive cycle and dissemination of pest species while also altering plant host susceptibility. 

She concluded that the project – and other research efforts across campus – wouldn’t be possible without the assistance of undergraduates like Moore. 

Moore, who is a junior majoring in environmental science and minoring in applied ecology, was recently awarded an NC State Undergraduate Research Grant and receives financial support from the Provost’s Professional Experience Program. He said his experience in Matallana’s lab has not only exposed him to the basics of research but also helped prepare him for future projects.   

“I feel like before I was on the project, I didn’t know how research actually happened,” Moore said. “But now I know how it all works. You have to be engaged at every moment and ready to shift gears when it’s needed. Being a research undergraduate is almost essential if you want the confidence and knowledge to do your research later in life.”