Biological control of conifer root aphids in Christmas trees

Final report for ONE14-220

Project Type: Partnership
Funds awarded in 2014: $14,995.00
Projected End Date: 04/15/2017
Grant Recipient: University of Vermont
Region: Northeast
State: Vermont
Project Leader:
Dr. Bruce L. Parker
University of Vermont
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Project Information


Root aphids have become a relatively new problem in Christmas trees. Infested seedlings are slow to grow, stunted, chlorotic, susceptible to root rot, delay maturity and impact revenues. In most plantations trees are cut intermittently throughout the field. Growers fill gaps with young seedlings, planted adjacent to stumps of harvested trees. Ants transport root aphids from harvested trees to seedling roots. Historically, growers have relied on insecticides for management. The first line of defense against this pest is early detection through scouting, which is difficult given that they live most of their life under ground. Once an infestation is found biological control should be considered. Two commercially available biological control agents, Stratiolaelaps scimitus (a  predatory mite), and Beauveria bassiana, an insect-killing biopesticide, has been shown to be effective against soil-dwelling pests. Our goal was to develop survey methods for root aphids and test the efficacy of S. scimitus and B. bassiana for management. Research was done cooperatively with Christmas tree growers in Bakersfield, Vermont. A survey method was adapted from methods reported in the literature and its efficacy was tested. This method was relatively rapid and reliable and was suitable for assessing treatment efficacy of the biocontrol agents and could be used by growers as well. Trials comparing the two biocontrol agents with the standard chemical pesticide, Imidacloprid, were conducted in Christmas tree plantations in Bakersfield, VT. Significantly fewer conifer root aphids were observed on trees treated with predatory mites, the insect-killing fungus or Imidicloprid than on the untreated (control) trees. When averaged among plots, fewer aphids were found on trees treated with Imidacloprid than on those with the mite or fungal treatments, but differences were not significant due in part to the large tree-to-tree variation in aphid populations among plots.  Our results showed that the biological control treatments tested provided a suppressive effect on the root aphid populations similar to that obtained for the traditional chemical treatment. Therefore, growers may want to consider replacing their insecticide applications with one of these more environmentally-friendly biological control options. This will also minimize the potential secondary pest outbreak by spider mites. Presentations were given on this research at two grower educational events, reaching over 125 producers. Handouts on root aphids were prepared and distributed, alerting growers to the problems, and describing integrated pest management options they could follow to minimize the impact of this pest.


In 2009, it was estimated that over 30.8 million live Christmas trees were purchased in the US, with a national value of $1.1 billion. The 2009 Census of Horticultural Specialties reported there were around 2,700 Christmas tree operations in the US, containing 158 million trees. Almost 34% are located in the Northeast, containing over 19% of the trees. VT ranks 15th nationally, and 5th among Northeastern states in the number of operations. This industry employs over 100,000 workers.  In the Northeast, Christmas tree production is a supplemental crop whose income plays a critical role in preserving the working landscape and small family farms. According to the 2007 Census of Agriculture (the most recent data available), revenue generated from Christmas trees in Vermont increased 7% since 2002, and even more in some other Northeastern states, demonstrating the income-generating potential of this commodity.

Christmas tree plantations are coniferous monocultures, and several insect pests regularly recur, including spider mites, balsam twig aphid, balsam gall midge and adelgids, for which chemical pesticides are commonly applied. Perhaps as a result of the disruption of the naturally-occurring biological control agents, root aphids have emerged as a persistent regional problem in Christmas trees. Because they feed on tree roots in the soil, they often go undetected until populations are high and tree health is impacted. Infested seedlings are stunted, chlorotic and particularly susceptible to root rot. Trees that are able to survive a root aphid infestation grow slowly, delaying tree maturity and impacting grower revenues. The root aphid problem is likely exacerbated by the production practices used. Growers commonly interplant seedlings beside stumps of harvested trees. Root aphids often appear in this 2nd planting cycle. In general it takes around 7 years to produce a marketable tree, depending on size. Production costs are greatly increased if, because of root aphids, the time to maturity is increased by 2-3 years.

Most Christmas tree growers rely extensively on chemical pesticides to control arthropod pests, most commonly using the systemic insecticide, Imidacloprid. This has several negative aspects. Most importantly, it is toxic to honey and bumble bees, and has been implicated in colony collapse disorder. In addition, because it is a systemic, it tends to persist in the environment for months or years, encouraging resistance in pest populations. Aphids are particularly apt to develop resistance due to their parthenogenic reproduction. Secondary pest outbreaks can occur following a pesticide application. For example, spider mites often become a problem after applying Imidacloprid; probably because they are resistant to this pesticide, whereas their natural predators are not.

Christmas tree growers came to us several years ago complaining of recurring root aphid problems and asking for solutions. Aphids, which feed with piercing/sucking mouthparts, create an ideal entry point for root rot diseases that are often present in the stump of the harvested tree. This represents a classic chronic insect/disease complex, which together represents a serious threat to Christmas tree growers. Because root aphids are below ground, their infestations go largely unnoticed by growers, though their impact directly and indirectly may be significant. Little is known about the biology of this insect. Growers have said they would prefer not to use chemical pesticides to control this pest, recognizing they are not good for the environment or human health (theirs and their customers). However, to date, pesticides are the only option available. Until research is conducted to demonstrate the potential efficacy of biological control, growers will continue to rely on chemical approaches, despite its drawbacks.

Historically growers have relied solely on chemical insecticides for management. Research is needed to determine the species infesting the roots and assess biological control solutions. Two commercially available biological control agents, Stratiolaelaps scimitus , a predatory mite, and Beauveria bassiana, an insect-killing biopesticide, have been shown to be effective against soil-dwelling pests. Our goal was to develop survey methods for root aphids and test the efficacy of these two biocontrols in comparison with a standard chemical pesticide treatment or no treatment at all.

Project Objectives:

Obj. 1. Conduct studies to develop grower-friendly survey methods for conifer root aphids.

Obj. 2. Test the efficacy of predatory mites and fungal drenches of BotaniGard, a registered insect-killing fungus against conifer root aphid.


Click linked name(s) to expand/collapse or show everyone's info
  • Larry Krygier
  • Brian Spencer


Materials and methods:

The research was conducted within a commercial Christmas tree farm, Larry’s Tree Farm, in Bakersfield, VT. One block within the plantation was designated for the research based on a survey of the site. This block was relatively level to slightly sloping, and in full sun with sandy soil. It contained 2-3 year old seedlings that showed evidence and symptoms of conifer root aphid (CRA) based on a survey by Entomology Research Lab personnel. These trees had yellow needles, stunted growth, while others appeared healthy. Sampling in the fall and early spring confirmed presence of moderate to heavy infestation of CRA.

Root Aphid Plant Sampling and Monitoring with Sticky-cards

Root sampling methodology. Twelve trees in one randomly-selected plot were excavated and returned to the laboratory to determine a reliable procedure for estimating total numbers of root aphids per tree. Using a modified method published by Straw et al. (Forest Ecology and Management, Vol 134, Issues 1-3, September 2000, pages 97-109), transparent 5cm x 5cm grids each with 25 squares were used to count either aphids or the waxy wool left behind on each seedling. The fir seedling was laid out as flat as possible with most of the soil removed and three of the grids placed randomly on top of the root system. The number of squares where the observer looking straight down through the grid, could see an aphid or the waxy material were counted. The procedure was repeated on the opposite side after flipping the seedling. Using these data, a mean number of squares counted per tree were generated. Immediately after doing the grid counts, the root system of each tree was carefully dissected under a microscope and the total number of aphids counted. This task took over 8 hours per tree, demonstrating the need for developing a more rapid system of assessing aphid populations. The total number of aphids was correlated with the grid counts using polynomial regression and an equation generated. This equation can be used to determine aphid counts using the grid technique for rapid and reliable root sampling.

Sticky card monitoring. Sticky-cards were placed at the base of 24 randomly selected 5-year old Fraser fir seedlings in block 3.  Half of the cards were placed horizontally at 5 cm above the surface of the ground and the other half vertically with the card bottom at 5 cm off of the ground.  Traps were set out in mid-May of 2014 and 2015 and monitored and changed regularly until early December when snow and frozen ground effectively ended the flight season.  The traps were field-checked or collected weekly. When checked in the field, all aphids were recorded based of coloration and size.  Since the dispersal stage of CRA is winged, we could do an initial sort based on those that were winged vs non-winged individuals.  When the sticky cards were collected, they were placed in small plastic bags and brought back to the lab for storage in a freezer for later analysis (identification) under a microscope. 

Biocontrol Trial Year 1

Experimental Plot Delineation. Twelve rectangular plots, each containing 12 balsam fir trees in 3 rows with 4 trees per row.   Plots were located in adjacent rows with neighboring plots at least 1.8 m (6 ft.) apart.

Pre-treatment Sampling. Ten trees from an adjacent area also infested with CRA, chosen at random, were dug up in the fall and brought back to the laboratory for analysis. Each tree was cut with pruning shears at ground level and the entire root system placed into a 1-gallon Ziploc bag. Volume of soil was around 500 ml per sample. The number of CRA on the roots was counted in the laboratory, first using a modified grid-count method from Salt 1996, and then hand counted to get an exact number per tree. The grid counts were then correlated with the 100% counts to get a regression equation to be used with future grid counts to get a “total aphids per tree” number.

Treatment. Treatments were carried out on 12 June, 2014. The 12 plots were treated as follows: Three plots, chosen at random, were treated with predatory mites, supplied by Applied Bio-nomics Ltd. of British Columbia, Canada. Each tree received 25 ml of product applied to the soil at the base of tree. Each tree received between 375 and 500 predatory mites.

Three plots were treated with the fungus, Beauveria bassiana, in the commercial formulation Mycotrol O. The recommended application rate for soil insects including weevils and aphids is 8 oz./1000 ft2 , which is equivalent to 10 ml Mycotrol O/3,240 ml water in the sprayer. Approximately 60 ml was applied to each tree following application procedures on the product label. Another three plots were treated with the chemical insecticide Imidicloprid. The product was applied as per label instructions for root aphids, and another three plots were used as controls and received no treatment.

Post-treatment sampling methodology. In the early summer of 2015 after approximately one year, the trees were lifted and all CRA counted. Using the grid-count procedure developed earlier, trees were dug and assessed in the field and then re-planted. Specifically, each tree was carefully lifted and excess soil carefully shaken off or worked loose by hand being careful not to dislodge any CRA. Trees were then laid out in a large tray which was lined with black plastic sheeting to act as a contrasting backdrop. Three 5 cm x 5cm clear plastic grids divided up into 25 1-cm2 blocks were randomly laid onto the exposed root surface (modified Salt (1996) method developed earlier). Each square where at least one aphid or white residue from an aphid was seen was counted and tallied and then the total number per grid recorded. The tree was then flipped over and the same process done on the other side for a total of six grid counts per tree. A mean of these counts was calculated and used to determine an estimate of total number of aphids per tree using the regression formula developed earlier from the pre-treatment tree sampling. Aphid counts from the Stratiolaelaps scimitus, Mycotrol O, and Imidicloprid treatments were compared among themselves and to the control using a nested random effects model ANOVA (Proc Mixed, SAS).

Biocontrol Trial Year 2

Site Description: Same site as in Year 1, but a different block of trees.

Experimental Plot Delineation: Nine rectangular or nearly rectangular plots each containing 12 Canaan balsam fir trees, in three rows wide with four trees per row. Plots were located in adjacent rows with neighboring plots at least 1.8 m (6 ft.) apart.

Pre-treatment Sampling methodology: Same procedures as in year 1 of this study.

Treatment: Treatments were carried out on 16 October 2014. The 9 plots were treated as follows: Three plots, chosen at random, were treated with predatory mites. Each tree received 25 ml of product applied to the soil at the base of tree. Each tree received between 375 and 500 predatory mites. Three additional plots were treated with an insect-killing fungus, B. bassiana ( Mycotrol O). The application rate for soil insects including weevils and aphids is 8 oz./1000 ft2 which was 10 ml Mycotrol O/3,240 ml water in the sprayer. Approximately 60 ml was applied to each tree following application procedures on the product label. Three plots, chosen at random, were controls and received no treatment.

Post-treatment sampling methodology: In the fall of 2015 after approximately one year, the trees were lifted and all CRA counted. Using the grid-count procedure developed earlier, trees were dug and assessed in the field and then re-planted. We used the same procedures as for the post-treatment sampling and counting as in year 1. Aphid counts from the Stratiolaelaps scimitus and Mycotrol O treatments were compared among themselves and to the control using ANOVA.

Research results and discussion:

Developing a sampling methodology

Root Sampling. Based on polynomial regression analysis, the correlation between the total number of aphids observed and the number of grid squares with evidence of CRA was determined from which an equation generated to calculate the number of aphids per seedling. This equation y = -3.6997x2 + 121.05x + 2.1174 with an R2 = 0.9533 can be used to determine aphid counts using the grid technique for rapid and reliable root sampling. This was used for subsequent data analyses for the biocontrol trials.

Sticky-cards. All sticky-cards were viewed under the dissecting microscope.  A total of 744 cards were viewed, some with as many as 1000 aphids present.  However, the vast majority of the aphids were not CRA.  An easy identifier for CRA is the lack of cornicles on the abdomen, a trait present on all of the other species we found on the cards.  All cards that had an initial CRA identification were double checked by a second person. We found that there is a fall flight but did not see any evidence of a spring or midsummer flight as has been described in the literature.  We did comprehensive searches of nearby ash trees and could find none.  We searched the trunk and the crown and found no evidence of any life stage of CRA.  A fall flight of winged CRA adults leaving the seedlings was observed. This occurred from mid-October until early December.  It was impossible to determine where these individuals were going but surmise they may just be dispersing within the conifer plantation.  Also, only a portion of the population dispersed, many stayed on the roots and may not even become winged.  We did not capture enough individuals to determine if card orientation made a difference in catch rate.

Biological Control Efficacy

Year 1 trial. All three management treatments had significantly fewer root aphids than did the control. However, none of the comparisons between the treatments were significant, i.e., differences in the number of aphids on trees treated with Mycotrol O, Imidacloprid or predatory mites were not significant.

Year 2 trial. One year after treatment, trees treated with mites had an average of 35.3 root aphids, compared to 78.3 and 66.5 for the Mycotrol and controls, respectively. Though fewer aphids were found on the trees treated with predatory mites, than the other treatments, differences were not significant, primarily because of the high variability from tree to tree. While significant differences between the management treatments for both trials were not observed consistently, the data suggests a treatment effect from the predatory mites occurred. These results should encourage growers who prefer to avoid use of Imidacloprid to consider releasing S. scimitus mites on a regular basis to maintain low aphid numbers.

Research conclusions:

Our research trials demonstrated the potential of the predatory mite, S. scimitus, as a biological control tool for use in Christmas tree plantations. In both field trials, the number of root aphids on the tree seedlings where these predatory mites were released, was less than on untreated trees. Annual releases of these predators could enable this beneficial to build to sufficient levels over time to significantly reduce populations. The Imidacloprid treatment provided similar results as the predatory mites, but growers who recognize the negative environmental impacts of this compound and the potential for secondary pest outbreaks by spider mite may prefer this management strategy. The biological control option may provide adequate control without the negative side effects.

Participation Summary
3 Farmers participating in research

Education & Outreach Activities and Participation Summary

3 Curricula, factsheets or educational tools
1 On-farm demonstrations
2 Webinars / talks / presentations

Participation Summary:

125 Farmers participated
3 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

Outreach Activities. A presentation regarding conifer root aphids was given to 75 members of the NH/VT Christmas Tree Association at their annual summer meeting in Greensboro, VT on June 28, 2014. All 75 participants were in attendance at our presentation. Information on root aphid identification, biology, and damage to host trees was presented. Biological control was discussed in regards to the root aphid and our research was introduced. Most of the audience was receptive and many asked questions. Exit surveys were not given out because we were invited speakers and were not present at the end of the conference.

We gave a follow-up presentation including results of the first trials at the NH/VT Christmas Tree Association winter meeting on January 27, 2015, with approximately 50 members in attendance.

Extensive interaction with several individual Christmas tree growers in Vermont has taken place during our search for plantations with root aphid infestations. These discussions have increased awareness among growers to the potential damage caused by root aphids and how to look for them. For example, the growers with a plantation adjacent to the test site in Bakersfield had no idea they had high root aphid populations infesting their trees until UVM researchers inspected their site. The owners of both farms are concerned about the negative consequences of using imidacloprid to control root aphids and are hopeful that our work will lead to the development of viable environmentally-friendly alternative methods of control.  Research results will be disseminated to New England growers. We will be able to provide suggestions for how and when to sample for root aphids to assess population levels and impact, as well as suggestions for biological control options for this underground pest.

3 farmers participating in research

An interim fact sheet was developed and shared called "Biological Control of Conifer Root Aphids in Christmas Trees: An Update" available here: BiologicalControlofConiferRootAphidDraft

Learning Outcomes

125 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

IPM, biological control, negative impact of chemical pesticides (esp. imidicloprid), importance of inspecting seedlings before planting, scouting for aphids in plantations

Project Outcomes

1 Grant received that built upon this project
$8,903.00 Dollar amount of grant received that built upon this project
2 New working collaborations
Project outcomes:


An accurate survey method was developed to determine numbers of conifer root aphids on the roots of Christmas trees. It involves the use of a clear plastic sheet with a 5 x 5 cm grid and counting numbers of white mass or aphids in the grids. Numbers are then converted to low, medium or high population levels. Two trials were conducted using, predatory mites and an entomopathogenic fungus for management of root aphids on Christmas trees. Our results demonstrated that based on statistical analysis of the data, the two biological control treatments (predatory mites and the insect killing fungus) provided suppression of aphids that was not statistically different from the chemical insecticide treatment, but all treatments were more effective than the untreated controls. Root aphids continue to plague Christmas Tree producers and the information obtained will help them make decisions on management. We will distribute our findings that have been compiled into a 2-page fact sheet to Christmas tree growers in the region. This information will also be added to our Entomology Research Laboratory website.

Assessment of Project Approach and Areas of Further Study:

Conifer root aphids are particularly challenging to study because most of their life is spent below the soil surface. In addition, their distribution and population levels in Christmas tree plantations are extremely variable. This makes assessing the efficacy of a treatment very difficult, in terms of showing impact. The effect of the corn field ants that move the aphids from one tree to another further complicates the determination of a treatment effect. Despite all of these difficulties however, we were able to demonstrate the potential of soil-dwelling predatory mites as a means of biological control. That was the primary question we sought to answer with this research. In the future, as more Christmas tree growers decide to shift to non-chemical production strategies, the use of these biocontrol agents is likely to expand. Currently, most Christmas tree growers are hesitant to shift away from their reliance on chemical pesticides. This will likely change when they observe resistance to imidacloprid, or if this compound is banned by state or federal agencies. These growers would benefit from further outreach to expose them to the benefits of IPM.

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Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.