1) The Risks of Reliance on Biological Control
An article published lately indicates yet another complication that might undercut reliance on biocontrol to counter mortality of eastern ash populations caused by the emerald ash borer (EAB) (See my blogs from November — here and here) regarding APHIS’ proposal to eliminate EAB quarantines in favor of relying chiefly on biocontrol – with little data to back up the change.)
Olson and Rieske (full citation at the end of this blog) found that one of the principal biocontrol agents now in use, and on which APHIS proposes to rely, Tetrastichus planipennisi, does not parasitize EAB larvae living in white fringetree, Chionanthus virginicus. While this tree is a suboptimal host for EAB – lower numbers of the beetle survive – the white fringetree would support survival of some EAB – thereby undermining efficacy of the biocontrol program.
Since white fringetree grows a cross much of the eastern range of ash trees — from New York to Texas, as shown by the map posted here, the presence of this reservoir that can be exploited by EAB will challenge the efficacy of biocontrol.
Olson and Rieske believe the reason that T. planipennisi does not attack EAB living in white fringetree is that the fringetree’s wood is so dense that the wasp cannot detect the presence of EAB in the tree (T. planipennisi apparently relies on tactile and vibratory clues to find its prey).
2) A Possible New Biopesticide to Suppress EAB?
A presentation at the 81st Northeastern Forest Pest Council by Mark Ardis of C.D.G. Environment described tests in the United States and Canada of methods for killing EAB by contaminating the beetles with the fungus Beauveria bassiana. The company is testing traps in which male beetles enter, become covered by fungal spores, then they fly out. The males not only become ill themselves, they also contaminate females during mating. Average overall beetle mortality from several test sites is 25%.
Given the terrible impact of the EAB invasions, I find it exciting to contemplate development of additional tools to be used in suppressing the beetles. However, I worry about possible impacts on non-target insects which might also be exposed to the fungal spores. A decade ago, David Wagner identified 21 species of insects that were specialists on Fraxinus, and said he expected additional species would also be associated with ash trees (full citation at end of blog). Mr. Ardis assured me that they had detected no insects other than EAB in the traps. I wish to see additional research on this issue.
The US Environmental Protection Agency would have to approve use of this biopesticide. I suggest that we all keep an eye on this process.
3) Citizen Scientists Searching for EAB- resistant “Lingering Ash”
Jonathan Rosenthal and Radka Wildova of the Ecosystem Research Institute have established a citizen science program to find ash trees that have survived the EAB invasion. These trees will become the foundation of efforts to breed more trees resistant to the EAB, which could be used to restore our forests.
The program is called “MaMa” – Monitoring and Managing Ash. So far, about 30 plots have been set up in New York, New Jersey, and Vermont where citizens are monitoring ash trees that have apparently survived the EAB invasion. The program seeks additional partners from other areas.
Searches for lingering ash must be strategically timed to ensure that the trees identified are truly resistant to EAB – not just late to become infested. But you can’t wait too long after the infestation wave has gone through an area, because the tree might die due to wind throw or human activity. Or, if a tree has just partial resistance (an important attribute for breeding!), it might eventually succumb. It is also imperative to avoid confusing stump sprouts with truly lingering ash. The conclusion: monitor the infestation and search for lingering trees two years after 95% of ash have been killed, or four years after 50% have been killed.
The MaMa program provides guidance, maps, electronic reporting systems … so you can help!
If you wish to participate – or to learn more – contact the program at monitoringAsh.org or 845-419-5229.
SOURCES
Olson, D.G. and L.K. Rieske. 2019. Host range expansion may provide enemy free space for the highly invasive emerald ash borer. Biol Invasions (2019) 21:625–635
Wagner, D.L. Emerald Ash Borer Threatens Ash-feeding Lepidoptera. 2007. News of the Lepidopterists’ Society. Volume 49, No. 1 (Spring 2007)
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
Faith, I imagine and hope you know a good deal about Beauveria bassiana (note spelling of genus).
It has been in the literature for some decades as a potential biological control agent for destructive insects and there have been many attempts to use Beauveria to control forest beetles (e.g. scolytine bark beetles). I am not aware of success of these attempts. I am not aware of Beauveria becoming part of best practices to combat beetles. Could be I am just out-of-the-loop. This is a fungus that has been trialed for many years.
Concerning the efficacy of EAB biocontrol. I did a lot of research on EAB in the early years, 2002-2010. Over this period, and since, Asian natural enemies have captured a lot of attention and research effort. In part, this effort is driven by a compelling logic, that natural control can be recovered by re-uniting pests with natural enemies from the common area of origin. Empirically, that is sometimes true (klamath weed and many others), though many other factors (such as host resistance) may be at play. Beyond the empirical case, there is a strong psychological impetus to biocontrol investigations. Entomologists love insects, and would always prefer to solve a problem by using natural mechanisms. It is exciting to journey to China, tease out the ecology of little known species, and then initiate high profile releases that capture public attention, and potentially solve a problem sustainably and permanently. I am a huge fan of biocontrol, and hope that it is useful for EAB. But the data are mixed. Studies that quote percent parasitism, or “up to __%parasitism” can be misleading. I have spent a lot of time in enormous contiguous ash stands, with every tree dead or nearly dead, where the predation mortality of EAB was 50-90%. I also studied ash in Michigan parks, and found up to 71% parsitism by a native braconid, though other studies show that to be an outlier. The most promising of the Asian biocontrols, T. planipennisi is recorded as causing 36-85% mortality in one study. At the upper end, control (negative growth rate) might be within reach. But is the upper end an outlier? And there is an enormous asterisk: T. planipennisi is limited to trees <12cm diameter.
There is enough data, I think, to surmise that imported natural enemies are not a magic bullet for EAB. So "APHIS’ proposal to eliminate EAB quarantines in favor of relying chiefly on biocontrol" would seem to be based on wishful thinking or a desire to avoid other tough choices. Meantime, biocontrol is certainly worth further investigation. Either for the limited help it currently provides, or for the possibility that conditions some place at some time might favor actual population regulation of EAB.
* I am not current in the field, but I use the 2018 publication (https://www.mdpi.com/1999-4907/9/6/296/htm) by long-term biocontrol researchers as a source for current state of knowledge on EAB.