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Northern Research Station
One Gifford Pinchot Drive
Madison, WI 53726
(608) 231-9318
(608) 231-9544 TTY/TDD

Hemlock Woolly Adelgid

Chemical Control

Research Issue

[photo:] Hemlock tree damaged by adelgids. Soil application of imidacloprid is effective in suppressing hemlock woolly adelgid (HWA) populations. A noteworthy characteristic of this application method is that it is slow-acting, requiring 3 months to observe any adelgid mortality and at least 2 years following application to see the full treatment effects.  However, the long duration of control (typically about 5 years following one soil treatment) and the extraordinary degree of control observed at the labeled application rate suggest that far less than labeled dosages of imidacloprid could be used to manage adelgid populations and to preserve tree health.  A new insecticide active ingredient, dinotefuran, has greater mobility in trees than imidacloprid and could be valuable where a more immediate reduction of adelgid populations is needed for preserving tree health.  Direct absorption of dinotefuran through the bark could permit rapid treatment of trees, minimize soil contact of the insecticide, and improve distribution uniformity over trunk injections of insecticides to be similar to that achieved with soil application.

Our Research

  • Is there continued mortality of HWA due to the presence of imidacloprid parent compound or its metabolites?
  • Do higher dosages achieve faster suppression of HWA populations?
  • If it takes 2 years to achieve the full treatment effect, how many additional years of control may there be before re-treatment becomes necessary?
  • What is the relationship between tree size and efficacy?
  • Are trunk sprays of dinotefuran as effective as soil injection?

Expected Outcomes

  1. A description of the metabolism of imidacloprid and the concentration found in tissues of the principal insecticidal compounds: imidacloprid and its olefin
  2. Guidelines on the minimum effective dosage of imidacloprid (liquid applications) that may be used to control HWA, relative to tree d.b.h.
  3. Guidelines on the number of years expected before the need to re-treat hemlock trees.
  4. A comparison of the efficacy for an imidacloprid tablet formulation versus the wettable powder
  5. A comparison of dinotefuran efficacy applied as a basal trunk sprays vs. soil application for suppression of HWA and elongate hemlock scale
  6. A determination of the effects of surfactants on efficacy of dinotefuran applied as a trunk spray

Research Results

The long-term benefits in suppression of HWA with imidacloprid are due to the continued presence of imidacloprid and its olefin metabolite in new growth.  Both chemicals reached maximum concentrations in hemlock tissue approximately 18 months following soil application, whereupon the imidacloprid concentrations decreased more rapidly than did its olefin.  The olefin could still be detected about 8 years following soil application.  The practical outcome is that one soil application of imidacloprid should not have to be re-treated for about 5 years. 

Because imidacloprid is so highly effective, it can be applied at lower than labeled dosages for most trees.  Adelgid populations responded to imidacloprid dosage with approximately a linear relationship between the percent population reduction (probability scale) vs. log of dosage; 50 percent reduction in populations could be achieved with 0.15 g per 2.5 cm of trunk diameter at breast-height (d.b.h.), or 10 percent of the usual labeled dosage.  However, effectiveness was found to vary with d.b.h.: the dosage predicted to give approximately 90 percent reductions in adelgid populations is given by the equation log(dosage) = 0.0153 · d.b.h. – 1.074, where the dosage is grams of imidacloprid per 2.5 cm of trunk d.b.h. and d.b.h. is measured in centimeters. d.b.h. tapes could be calibrated in dosage units using this logarithmic equation. For trees less than 82 cm d.b.h., these dosages are less than the labeled dosage of 1.5 g imidacloprid per 2.5 cm d.b.h.. Trees larger than 82 cm d.b.h. may require treatment in two successive years or use of an imidacloprid formulation that permits higher dosages to be applied.   

Environmental risk can be mitigated by applying the minimum effective dosage in forests. A combination of optimum dosing of trees and adoption of the tablet formulation when treating trees in sensitive habitats should minimize the risk of contaminating aquatic resources with imidacloprid. This approach will provide satisfactory multiple-year suppression of adelgids where an immediate reduction in adelgid populations is not needed.

A field test of a new systemic insecticide, dinotefuran (Safari), injected into the soil near the base of the trunk or sprayed directly on the bark of hemlock trees, revealed extraordinarily rapid translocation in the trees. Applications were made in early October; adelgids were already dying approximately 2 weeks after the application. Therefore, dinotefuran may be suitable where an immediate response to protect the health of trees is needed. The complementary nature over time of these two insecticides for protecting trees from adelgids has led to Section 24C registrations (Special Local Need) that permit both the CoreTect tablets (imidacloprid) and Safari (dinotefuran) to be used in southern Appalachian forests.

For quarantine purposes or where rapid and short-term suppression of adelgids is needed, adelgid populations can be suppressed with a foliar spray of horticultural oil and/or bifenthrin (not appropriate near aquatic resources) or a trunk application of dinotefuran.  The trunk spray of dinotefuran may be especially important where the tablet formulation is being used, because it takes approximately one year more for the tablet-based application to be effective than when using soil injection.  The multiple-year benefits obtainable with soil application of imidacloprid  could be combined with these shorter term options to realize the benefits of quick and long-lasting treatments.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlife if not handled or applied properly. Use all pesticides selectively and carefully. Follow recommended practices given on the label for use and disposal of pesticides and pesticide containers. Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) requires all chemical controls to be used in accordance with their label or with a 24C registration. Even though recent research may indicate that they are effective, doses lower than specified on the label cannot be implemented until the label is changed allowing it.

The use of trade, firm, or corporation names is for information only and does not constitute an endorsement by the U.S. Department of Agriculture or the Connecticut Agricultural Experiment Station.

Cowles, Richard S.; Lagalante, Anthony F. 2009.  Activity and persistence of systemic insecticides for managing hemlock woolly adelgids. In: McManus, Katherine A; Gottschalk, Kurt W., eds. Proceedings. 20th U.S. Department of Agriculture interagency research forum on invasive species 2009; Gen. Tech. Rep. NRS-P-51. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 17-18.

Cowles, R. S. 2009. Optimizing dosage and preventing leaching of imidacloprid for management of hemlock woolly adelgid in forests. Forest Ecology and Management. 257: 1026.

Cowles, R. S., M. E. Montgomery, and C. A. S. J. Cheah. 2006. Activity and residues of imidacloprid applied to soil and tree trunks to control hemlock woolly adelgid (Hemiptera: Adelgidae) in forests. Journal of Economic Entomology. 99: 1258.

Research Participants

Principal Investigators

  • Michael Montgomery, Emeritus Research Entomologist, U.S. Forest Service, Northern Research Station
  • Richard Cowles, Connecticut Agricultural Experiment Station
  • Anthony F. Lagalante, Villanova University

More Information

  • R. Talbot Trotter III, Research Ecologist, U.S. Forest Service, Northern Research Station

Last Modified: 05/05/2015