Since 2000 we (Dept. of Entomology, Virginia Tech, hemlock woolly adelgid group) have worked on managing HWA at Mt. Lake. Mt. Lake (elev. 4,000') is one of only two natural lakes in Va.  It is surrounded by hundreds of hemlocks, many up to 40 - 45 " dbh.  HWA  began infesting these trees in 1999.  Releases of  Sasajiscymnus tsugae  (Coleoptera: Cocinellidae)  (formerly Pseudoscymnus tsugae) were made in 1999. Several were found in 2000 but none since. No impact on HWA could be attributed to this species at Mt. Lake.  Recovery of this species at release sites from North Carolina to Massachusetts have been rare.  As far as I know Sasajiscymnus tsugae is no longer being released. A new bio control agent Laricobius nigrinus (Coleoptera: Derodontidae) shows more promise since it has been found 1- 2 years after release.  This species is not available for general public release.

Since 2001 we have treated hemlocks using soil injection (Kioritz) and stem injection (Mauget) of imidacloprid (Merit). Stem injection is more expense since it is more labor intensive than soil injection.  Stem injection was used on trees near open water or springs to avoid ground water contamination.  We have treated ~380 trees. As of 2005, 10% of the treated trees have died while 23% of the untreated trees have died.  We have not treated since 2004 due the decline and nearly complete absence of HWA in this area. HWA normally declines 2-3 years after the initial infestation due to the decline in tree health and was exasperated by the drought from 2000 - 2003. However, this year there has been a slight increase in HWA around Mt. Lake. Trees should be treated as soon as you find HWA. If you can save the tress during the first year or two of the initial infestation you have a better chance of saving trees. 
The area surrounding Mt. Lake is very rocking with the water table fairly close to the surface. As stated on the Merit label do not exceed 0.4 lb of active ingredient per acre. If the area is rocky and the water table is close to the surface I'd suggest using stem injection on trees within 50 yards of open water or springs. Especially if the soil is low in organic matter since imidacloprid binds with organic matter which reduces migration in the soil. So the more the organic matter the less chance of movement.
 
Imidacloprid has saved a significant number of trees at Mt. Lake.

The cost of imidacloprid has recently increased substantially.

Tom McAvoy

 

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Tom McAvoy
216A Price Hall MC 0319
Dept. of Entomology
Virginia Tech
Blacksburg, VA 24061
Phone: 540-231-6320
Fax: 540-231-9131