To stop West Nile, go native

When contemplating the harm caused by invasive species, the imagination usually stops at fairly direct effects: an introduced predator decimates hapless prey; invasive weeds choke out native plants. But hacking around in the shrubbery a bit — literally — reveals that native and invasive species also have subtler pros and cons. Certain species of invasive plants may enhance transmission of mosquito-borne West Nile Virus, according to a new study, while some native ones could help to halt its spread [1].

West Nile virus can infect humans, birds, and other wild and domestic animals, causing serious neurological symptoms or even death. The illness is of increasing concern in the Northeastern and Midwestern United States, where it is spread mainly by the mosquito Culex pipiens.

In the new study, researchers from the University of Illinois at Urbana-Champaign set out containers of water and leaves from one of six plant species common in the area, and counted the number of egg rafts C. pipiens females laid in each. In the laboratory, they reared C. pipiens larvae in water infused with decomposing leaves of each of the six plants. The tests included three species of native shrubs – Allegheny blackberry, elderberry, and serviceberry – and three invasive ones – autumn olive, Amur honeysuckle, and multiflora rose.

The recipe for disease transmission by mosquitoes has a number of ingredients, including where females lay their eggs, how many larvae survive, how fast they develop, and the size they reach as adults. Leaves from two invasive shrubs, Amur honeysuckle and autumn olive, yield significantly more adult mosquitoes than those of other species, the researchers reported June 16 in Parasites & Vectors. Mosquitoes reared in honeysuckle-infused water are also the fastest to develop and the largest.

One of the native shrubs, in contrast, presents a thorny situation for mosquitoes. The researchers found the largest number of egg rafts in water containing Allegheny blackberry leaves. But fewer than 20 percent of larvae placed in blackberry-infused water survive to adulthood. Those that do survive take a long time to develop and are very small.

“What’s exciting about this is that it suggests that blackberry functions as a kind of ecological trap, enticing mosquitoes to lay their eggs in a place where the larvae are unlikely to survive,” says Brian Allan, professor of entomology at the University of Illinois.

Leaves from a second species of native shrub, elderberry, also draw lots of female mosquitoes to lay eggs but result in low larval survival, although the results are not as dramatic as they are with blackberry leaves.

The effects of some of the species are more mixed, the researchers found. The leaves of one invasive species, multiflora rose, appear to be toxic to mosquito larvae, as none of the larvae placed in this infusion survived to adulthood — though these leaves aren’t as attractive to egg-laying females as blackberry leaves are. Female mosquitoes also lay few eggs among the leaves of one of the native species, serviceberry, but this species yields a high mosquito larvae survival of 62 percent.

Still, the results provide one more entry on the list of ills caused by invasive plants, and one more reason to choose native ones for backyards and other landscaping. Other research published this year suggests that we’re missing out on many such opportunities. Non-native landscaping plants cover an estimated 75 to 100 million acres in the United States, an area almost as big as Wisconsin. Native plants, by contrast, account for only 13 percent of sales at nurseries [2]. But perhaps the realization that our own health may be at stake will spur us to go native more often. – Sarah DeWeerdt | 14 July 2015

Sources:

1. Gardner A.M. et al. 2015 Asymmetric effects of native and exotic invasive shrubs on ecology of the West Nile virus vector Culex pipiens (Diptera: Culicidae). Parasites & Vectors DOI: 10.1186/s13071-015-0941-z 

2. Wilde D.H. et al. 2015 State of the science and challenges of breeding landscape plants with ecological function. Horticultural Research DOI: 10.1038/hortres.2014.69

Header image:  Rubus allegheniensis near Fancy Gap, VA. Credit: Lindley Ashline via Flickr.

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