Emerald ash borer has been detected in three additional towns since the last Newsletter update in March: Stoddard, Dublin, and Sharon. All of these towns were adjacent to known infested towns and represent the continued gradual range expansion of the infestation in New Hampshire. With the southeastern portion of the state now entirely infested, detection efforts are being focused on the northern and western regions. 40 purple prism traps have been deployed throughout this Cheshire, Sullivan, Grafton, and Coos Counties with additional visual surveys scheduled to be conducted this coming fall and winter.
Biological controls for emerald ash borer are being released in two additional sites this year in Croydon and Rindge, representing the first releases in Sullivan and Cheshire Counties Respectively. These releases bring the total number of sites throughout the state receiving a full slate of parasitoid releases to 22, with an additional 15+ sites receiving reduced numbers in a research capacity. Evaluations of parasitoid establishment at past release sites continue to bear fruitful results. We are continually finding evidence of establishment of the larval parasitoid Tetrastichus planipennisi from all release sites that have been evaluated and have now documented several instances of this insect being recovered up to five miles from the nearest release location, indicating that it is readily dispersing long distances across the landscape. The larval parasitoid, Spathius galinae, which is the most recent addition to the biological control arsenal, is also proving to be capable of readily establishing throughout the state. This parasitic wasp, which was first released in 2017, has already been found establishing at high rates at four of six evaluated sites and in one instance was recovered one mile from a release site just one year following release.
Yellow pan traps are used to evaluate the establishment of parasitic wasps released through the biological control program. Adult parasitic wasps, which feed on the nectar of flowers, are attracted to the yellow color of the bowls.
Overall, biological control remains the most promising management strategy for preserving ash as a component of our forests, and recent evaluations of parasitoid success coming out of the Midwest are encouraging. This region of the country has been infested for over a decade longer than New Hampshire and most of the landscape in that part of the country is in a stage known as an “aftermath forest”. In these forests, emerald ash borer has killed off nearly all of the mature standing ash trees and only seedling and saplings that were too small to be attacked in the initial wave of EAB invasion remain. Ash borer populations in aftermath forests are sustained on lingering ash and by smaller trees that slowly grow into susceptible size classes, resulting in an EAB population that is considerably smaller than peak-infestation populations. With lower ash borer populations the parasitic wasps released through the biological control program are able to inflict meaningful mortality. The three species of parasitic wasps released through the biological control program: Oobius agrili, Spathius galinae, and Tetrastichus planipennisi have observed parasitism rates of up 30%, 70% and 80% respectively in small-scale studies. If these rates of mortality can be maintained across the entire landscape it will likely be adequate to suppress ash borer populations to levels that are sub-lethal to our ash trees, promoting ash regeneration and maintaining ash as a component of our forests into the future.
During the winter while parasitic wasps are dormant, ash trees are felled from biological control release sites and surrounding areas and placed into cardboard rearing tubes. If parasitic wasps are present in/on the felled trees they will emerge during the summer and can easily be collected from jars attached to the tubes.
Biological control for emerald ash borer is currently being conducted at six locations throughout the state to go along with over 25 retired sites where releases have been completed. Efforts to recover the parasitic wasps that have been released through this program have been encouraging so far. The larval parasitoid Tetrastichus planipennisi has successfully been recovered all retired release sites. We are also starting to recover the egg parasitoid Oobius agrili from more sites from around the state, in one case several years since the most recent release. These results indicate that both parasitoid species are establishing well in New Hampshire.
June 1 was the cut-off date for moving infested ash material. After June 1 it should only go to local outlets or primary processing facilities that debark immediately.
