Stemming the spread of Russian olive
Russian olive is a significant invasive weed in North America but is perceived as a useful and attractive tree by some stakeholders. It is especially a problem in western parts of the USA where it affects many natural habitats, altering the ecosystem and its functions. Biological control is a useful approach in such circumstances because scientists can look for natural enemies that damage reproduction, and thus future spread, without damaging established trees.
So, what’s the problem
Russian olive (Elaeagnus angustifolia) was introduced to North America in the late 19th century as an ornamental plant , for erosion control, and as a windbreak and shade tree. It has become a significant invader of natural habitats, particularly along riverbanks, and to date has been classified as a noxious weed in four states of the western USA, a figure that is likely to increase in the near future.
Although Russian olive competes with native species and alters ecosystem functions, some still value it as an ornamental plant and windbreak. To avoid conflicts of interest, the biological control project for Russian olive is therefore concentrating on natural enemies that specifically attack the flower buds, flowers or seeds in order to slow its spread without harming established trees.
What is this project doing?
CABI’s centre in Switzerland and the Biotechnology and Biological Control Agency (BBCA, Rome, Italy) initiated surveys in Eurasia in 2007 to determine whether biological control by introducing natural enemies from Russian olive’s area of origin was a feasible option. Out of 72 insect and mite species found associated with the tree in its native range, the shoot- and flower-attacking eriophyid mite Aceria angustifoliae and the shoot- and fruit-boring moth Anarsia eleagnella were prioritised for in-depth studies to assess their host range (ie. whether they could attack non-target plants if introduced in North America) and their impact on Russian olive.
The host specificity of the mite, Aceria angustifoliae, was assessed in an outdoor experiment in Iran and at CABI’s centre in Switzerland. We did this by attaching infested leaves to closely related test plant species and monitoring for damage. Results so far suggest that A. angustifoliae has a very narrow host range and is likely to be restricted to Russian olive under natural field conditions. We submitted the petition for field release to the USDA-APHIS (Animal and Plant Health Inspection Service) Technical Advisory Group (TAG) and the Canadian Food Inspection Agency in Canada in November 2019.
The moth Anarsia eleagnella has at least two generations per year: the first attacks shoot tips, while the second feeds predominantly on the fruit and damages the seeds. With regard to host specificity, published records suggest A. eleagnella attacks Elaeagnus and Hippophae species. No-choice tests (where the insect is only exposed to one plant) and the open-field test in Iran revealed that the host range of this species is wider than initially anticipated and appears to include several species within the family Elaeagnaceae. Since we suspended further work in Iran, we are currently attempting to locate this species in other countries.
Field surveys in Kazakhstan are ongoing to find additional potential biological control agents, including the stem mining weevil, Temnocerus elaeagni.