Posted by Sara Tanis and Bert Cregg
NOTE: Last week I gave an update on the 10-year anniversary of the discovery of the emerald ash borer in the U.S. In response to the post, Sonia John raised a couple of points:
“Please share your knowledge about the status of efforts to: 1. introduce insect species or other biocontrols that predate the EAB, and 2. evaluation of the degree of resistance to EAB of Asian (and European) Fraxinus spp. as well as Asian/N.American hybrids)”
Sonia is right on the mark because these two areas of investigation probably represent the best long-term solution to the EAB issue. As I mentioned last week, EAB was a relatively obscure insect in its native range in Asia, rarely causing any significant damage. This suggests that native (Asian) ashes are resistant, native biocontrols keep it in check, or both.
In order to reply to Sonia’s post I enlisted the aid of Sara Tanis. Sara is a Ph.D. student in the Department of Forestry at MSU (and a former MS student of mine) and is studying variation in susceptibility to EAB among ash species for her dissertation.
Almost from the moment that EAB was first identified, USDA and US university scientists have been working with their Asian counterparts to identify diseases or other insects that keep EAB under control in Asia. From this work, researchers have identified several parasitic wasps that affect EAB. Some of the wasps lay their eggs on EAB eggs – when the wasp larvae hatch they feed on the EAB eggs. Other wasps lay eggs on the EAB larvae and then the wasp larvae feed on the EAB larvae – yeah, pretty gross. (for more information visit http://www.emeraldashborer.info/biocontrol.cfm). A state of the art bio-control research facility has been established by USDA-APHIS in Brighton, Michigan to rear these parasitoids and to date they have been released in several locations throughout infested areas of the United States. We know the wasps are highly effective in parasitizing EAB eggs and larvae in China. The question is whether we can establish viable and self-sustaining populations in the US.
In addition, a native parasitoid Atanycolus cappaerti, a Braconid wasp described in 2009, is also proving to be successful in curbing EAB densities. At the MSU Tree Research Center, we have seen more than 70% parasitism on EAB in black ash and green ash trees. Hopefully turn-about will be fair play and lack of co-evolution will back-fire on EAB and our all-American native wasp can do a number it. All together now: USA! USA! USA!
Everybody’s All-American? Atanycolus cappaerti Photo: David Cappaert
Variation in resistance among species.
One of the main points to hit home here is the fact that even though Asian ash are considered “resistant” in their native habitat, in North America where EAB population densities are extremely high, they do succumb to EAB attack despite their co-evolutionary history. Here at MSU, Andrea Anulewicz has done extensive host preference work using a variety of ash species including those found west of the Rockies (F. anomala), south in the Carolinas (F. caroliniana), Europe (F. excelsior and F. ornus), and an evergreen species found in Central America (F. uhdei). All of these ash species have proven to be viable hosts for both adults and larvae. However, EAB do prefer some ash species over others. At the top of the menu are green ash and black ash, less preferred is white ash, and least preferred is blue ash, our hope for the future. In the field, of the five ash species that EAB has encountered to date, blue ash appears to be the most resistant or the least preferred ash species native to North America. In order to quantify differences in EAB related mortality between blue ash and white ash, we inventoried all living and dead trees in two woodlots in Southeast Michigan in 2010
-2011. We found approximately 68% (402 out of 590 trees) of the blue ash trees in these sites were alive, and most (60%) appeared healthy! We calculated that these trees were subject to at least 178,000 beetles (calculated from dead ash phloem area) over the course of a 15+ year invasion. That is a lot of beetles. In contrast, only 29 of the 312 white ash trees survived, and these were likely too small to be infested during the peak of EAB invasion (around 2005). Blue ash survival has also been reported in Ontario and Ohio. Many of us assumed that there would be 99% ash tree mortality across North America. Don’t get me wrong, many forests and urban areas DO have mortality this high because of the ash species found there. However, we now have something positive to report. Evidence suggests blue ash trees will persist in forests ravaged by EAB. In addition, blue ash resistance might also enhance bio-control efforts. The combination of a less susceptible host and a specialized parasitoid could maintain EAB populations at densities blue ash trees could sustain and possibly aid in slowing the spread of EAB across the US. At this time, we don’t know the exact mechanism of blue ash resistance, but we are working on it. Bert serves on my graduate advisory committee, so I’m sure he will keep you up to date as we learn more. There are likely several factors that contribute, once isolated, we hope to incorporate these mechanisms into other ash species and introduce less susceptible ash tress back into the landscape.
Variation in susceptibility to EAB between blue and white ash
USDA and Ohio State researchers are working with the Dawes arboretum to hybridize North American and Asian ashes. This work is based on the model that the American Chestnut Foundation has used to breed blight-resistant chestnuts; starting with an Asian – American hybrid (1/2 Asian; ½ American) and then making repeated back-crosses with the American species to increase the desired horticultural characteristics while maintaining the resistance from the Asian parent. Clearly this is a long-term proposition and so far the efforts have been slowed by poor crossibility among species and insect damage to seed. Moreover, earlier hybrids that have been produced crossing Manchurian ash have not exhibited any resistant qualities. Breeding work, of course, is always a numbers game. As a point of comparison, the USDA National Arboretum’s program to select Dutch elm resistant American elms found only 11 survivors out of 21,000 seedlings screened for tolerance.