Biological control of conifer root aphids in Christmas trees

Biological control of conifer root aphids in Christmas trees

Summary

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. Growers rely on insecticides for management. Stratiolaelaps scimitus (a commercially available predatory mite), 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 an insect-killing fungus for management. Research is being done cooperatively with Christmas tree growers in Bakersfield, Vermont. A survey method has been developed and efficacy testing completed. Data are being entered and statistical analyses will be done.

Objectives/Performance Targets

Developing a survey method.  From the Bakersfield, Vt  plantation 12 fir seedlings 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. (2000), a transparent plastic sheet with a 5cm x 5cm grid  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.  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 surveying.

 We tested, predatory mites, an insect-killing fungus (Beauveria bassiana) in the commercial formulation Mycotrol® O, and the commercial insecticide Imidacloprid (AmTide Imidacloprid 2F) for management of root aphids  Twelve plots of 12 trees each were treated in early June.  Three random plots with S. scimitus (500 mites per tree), three plots with Mycotrol® O (2 liters Mycotrol® O/378 liters water) at 0.5 liters/tree as a root drench, and three plots were sprayed with Imidicloprid at the label rate for root aphids.  Three plots were left untreated as controls.

The trial described above was repeated in October.  After approximately one year, all of the trees in each trial were carefully lifted and root aphids counted using the clear grid technique we developed. In the first trial all three treatments had significantly fewer root aphids than did the control (i.e. 32.3 aphids/tree in the mite treatment vs. 93.7 aphids/tree in the control treatment) . However, none of the comparisons between the treatments were significant, i.e., we can’t say that the mites worked better than Mycotrol O or Imidacloprid or visa-versa.

  There were no statistically significant differences between the treatments and control in the second  trial although the means showed a trend especially comparing S. scimitus predatory mites (35.3 aphids/tree) to the control (66.5 aphids/tree).  The, high variability between individual trees is to blame for the non-significant results. 

Accomplishments/Milestones

The project progressed as expected. There were two major changes made. (1) In order to accurately evaluate the efficacy of predatory mites to manage conifer root aphids it was necessary to extend the length of the field trials to one year. This was done. (2) To assess the life cycle and behavior of conifer root aphids it was necessary to continue weekly yellow trapping in the plantation until approximately 5December. This was done thus delaying the counting of the aphids on the sticky cards etc. Both of these changes required that we obtain a no-cost extension to the research. This was approved.

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 are currently making arrangements to give another presentation with some results of our experiments at either their upcoming winter or summer meetings. (Update: 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 environ-mentally-friendly alternative methods of control. Though it is premature at this time, when available, 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.

Impacts and Contributions/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. Although no statistically significant results were obtained from a practical standpoint the trees with the predatory mites/and or the fungus had fewer aphids on their roots than the control trees. Root aphids continue to plague Christmas Tree producers and the information obtained will help them make decisions on management.

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