Biological control of conifer root aphids in Christmas trees

Biological control of conifer root aphids in Christmas trees

Summary

The goal of this project is to determine reliable methods for surveying root aphids in Christmas trees and test two biological control agents for root aphid management. Accurate pest identification is critical for management. Root aphids from infested trees were sent to several specialists (from USDA, Univ. of Montreal and Univ. of Vermont), and it was confirmed they are Prociphilus americanus. Several species of ants associated with root aphids were collected and sent for identification and were Tetramorium bicarinatum, Brachymyrmex depilis, Camponotus novaeboracensis, Formica incerta, and Lasius neoniger. All are known to tend aphids and coccids. A method for estimating root aphid populations was validated allowing for a rapid assessment of treatment effects. Three field trials have been initiated at the Bakersfield site. One trial involved release of predatory mites, Stratiolaelaps scimitus. In September 2014 one was evaluated by digging up the trees and counting root aphids with the clear grid technique. These data indicated that trees treated with predatory mites had significantly fewer root aphids (114 aphids) than the control trees (174). Populations will be sampled again in 2015 to see if the impact of the predatory mite increases over time. The second study used the same design with two treatments applied in June 2014: predatory mites, and an insect-killing fungus (Beauveria bassiana), and an untreated control. Treatment effectiveness will be assessed in June 2015. These treatments were also applied to different trees in October 2014, for evaluation in Oct. 2015. A presentation was given to 90 members of the VT/NH Christmas Tree Growers Association. Information on root aphid id, biology, and damage to host trees was presented. Biological control was discussed and our research introduced. The audience was receptive and asked many questions. Arrangements were made to give a presentation at their January meeting.

Objectives/Performance Targets

Our project objectives are: 1) Conduct studies to develop grower-friendly survey methods for root aphids; 2) Test the efficacy of predatory mites and fungal drenches of BotaniGard/Mycotrol, a registered insect-killing fungus, against root aphids. The primary target audience of this research is Christmas tree growers, an underserved agricultural sector, but our results will also apply to other root aphid species that cause damage and crop loss in the production of vegetables and ornamentals.

Accomplishments/Milestones

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

Identification of the target pest and associated arthropods in the target agricultural system is critical for evaluation of treatment efficacy. Aphid and ant specimens were sent to the USDA for identification, but a definitive species id was not possible. Research Entomologist Dr. Gary L. Miller, USDA’s Systematic Entomology Laboratory (SEL) in Beltsville, MD, identified the aphid specimens as belonging to the genus Prociphilus, stating, “Group needs revision; adults not identifiable to species.” Samples of the winged adults were sent to Dr. Colin Favret, Universite de Montreal, Dept. de Sciences Biologiques, Centre sur la Biodiverite, Montreal, Quebec, Canada, an aphid specialist, who definitively identified them as Prociphilus americanus (Walker 1852). Little is known about this group of aphids, including their life cycle or alternate hosts. This information is needed to further substantiate the species.

Several species of ants are known to “tend” aphids, moving them from place to place, protecting them from natural enemies and feeding on the “honeydew” excreted by the aphids. USDA research entomologist Dr. Matthew Buffington identified the following ant specimens associated with conifer root aphid: Tetramorium bicarinatum Nylander, genus Formica, stating, “Group needs revision; adults not identifiable to species.” T. bicarinatum is an exotic species known to be an agricultural and natural environment pest but has also been used as a biological control agent against crop pests (Dr. Olivier Blight, Institut Mediterraneen d’Ecologie et de Paleoecologie, Universite Paul Cezanne, Europole, Mediterraneen de l’Arbois, Aix-en-Provence, France, pers. comm.). Samples of additional ant specimens collected from Abies tree roots infested with conifer root aphids were identified by Univ. of Vermont entomologist Dr. Ross Bell as: Brachymyrmex depilis (Emery), the little hairless ant; Camponotus novaeboracensis (Fitch), the New York carpenter ant; Formica incerta (Emery), the uncertain ant; and Lasius neoniger (Emery), the Labor Day ant. All tend aphids and coccids and B. depilis is known to specifically tend root aphids.

Twelve trees in a randomly-selected plot were excavated and brought to the laboratory to estimate total numbers of root aphids per tree. Using a modified method published by Straw et al. (2000), transparent 5cm x 5cm grids, each with 25 equal sized squares, were used to count aphids and 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 saw an aphid or waxy material was 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. After completing the grid counts, the root system of each tree was inspected under a microscope to determine the total number of aphids. This took over 8 hours per tree, demonstrating the need for a 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 for determining aphid counts using the grid technique for any subsequent sampling.

Yellow sticky cards were placed in the plantation in mid-May to monitor root aphid dispersal activity. Four cards were randomly placed within each of six test plots for a total of 24, placed within 5 cm from the base fir seedlings. Of each four cards, two are placed horizontally approximately 5 cm above the ground, and two vertically oriented east/west with the bottom of the card at about 5 cm off of the ground. The cards were checked or collected weekly until early December. The cards will be inspected in the coming months to determine the number of conifer root aphids by species.

2. Test the efficacy of predatory mites and fungal drenches of BotaniGard/Mycotrol, a registered insect-killing fungus, against root aphids.  

A 20,000-tree Christmas tree plantation in Bakersfield, VT was found to have extensive root aphid problems, and was determined to be a suitable site for the trials. A block of trees (approx. 1 acre) within the plantation that had been planted with 225 4-yr old Canaan fir trees in 2013 was identified for test purposes. Most of these trees were heavily infested with conifer root aphids and served as the location for our field trials. 

Three field trials have been initiated at the Bakersfield site. One trial was initiated in Oct. 2013, which involved release of around 18,000 predatory mites, Stratiolaelaps scimitus, applying a total of 500 mites/tree to each of 36 Canaan fir trees in three randomly-selected plots (12 trees/plot). In late September 2014 12 trees each were carefully dug up and root aphids counted using the clear grid technique. This was done in the field after which the trees were replanted and watered. After consultation with a UVM Statistician (A. Howard), a T-test ANOVA was done on these data and determined that the trees treated with predatory mites had significantly fewer root aphids on their roots than the control trees. The average number of root aphids on the mite-treated trees was 114 compared to 174 aphids per tree on the control trees (t= -2.587, df=430, p=0.03). Populations will be sampled again in 2015.  

The second study used the same design in a section of the plantation adjacent to the first. The following treatments were applied in June 2014: 500 predatory mites/tree, an insect-killing fungus (Beauveria bassiana) in the commercial formulation Mycotrol (at the recommended concentration for root aphids (2 liters/378 liters water) at 0.5 liters/tree as a root drench, and an untreated control. Treatment effectiveness will be assessed in mid-summer 2015.

 For the third study, the same treatments used for the second study were repeated, making the treatments in October 2014. These trees will be dug and assessed for root aphid populations in Oct. 2015.

• Figures for Biocontrol of conifer root aphids report

Impacts and Contributions/Outcomes

Identification of the species of conifer root aphid infesting Christmas tree plantations in Vermont and their associated ant species was confirmed by specialists. 

A rapid system for estimating conifer root aphid populations using a grid to count the number of aphids or aphid wool was developed and calibrated. 

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, a chemical insecticide, to control root aphids and are hopeful that our work will lead to the development of viable environmentally-friendly alternative methods of control. A presentation on conifer root aphids was given to 75 members of the NH/VT Christmas Tree Association at their annual summer meeting in Greensboro, VT. 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. Arrangements are underway to give additional presentations including results of our experiments at their upcoming winter or summer meetings.

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