Close Encounters Plants & People

Should we be looking to parasitic plants to help control invasive species?

Biologists have been looking at biological controls like herbivores to control invasive plants. New research suggests we should also look at what parasitic plants can do.

Controlling invasive plants through biological means isn’t a new idea, but Jakub Těšitel, Robert Cirocco, José Facelli and Jennifer Watling suggest using a new tactic, deploying parasitic plants. In Applied Vegetation Science, they argue that including native parasitic plants in invasion management schemes could be both environmentally friendly and cost-effective.

Normally, when botanists discuss parasitic plants, they’re either interesting freaks or trouble. However, Těšitel points out that they make up around 1% of the angiosperms in species, and while they can harm crops, there’s also an increasing amount of evidence that they can harm invasive species too.

A tree with mistletoe, a hemiparasite. Image: Canva.

Back in 2008, Watling and Facelli with lead author Jane Prider examined the effects of Australian hemiparasite Cassytha pubescens on an exotic host, Cytisus scoparius, and a co-occurring native host, Leptospermum myrsinoides. They found that Cassytha was killing the invasive host, but not having the same effect on the native species.

Těšitel and colleagues draw on three other examples, one being Těšitel (and some other colleagues) work on Rhinanthus major and the grass Calamagrostis epigejos. This was a study to see if Rhinanthus major could help restore meadowland in the White Carpathians Protected Landscape area. Here they found that Rhinanthus worked as a parasite on the invasive grass, and worked particularly well when combined with a cut. In a similar way Rhinanthus minor has been called a meadow maker, when combined with cutting, to help create wildflower meadows.

Another example is Těšitel and colleagues use is work by Prider and colleagues on Cassytha in many articles. This is ongoing work. Těšitel’s colleagues, Cirocco, Facelli and Jennifer Watling also have a paper in this month’s Journal of Experimental Botany, examining the effect Cassytha pubescens, has on Ulex europaeus, an invasive shrub. They find that Cassytha hits hardest when the Ulex host plant is youngest, which correlates with Li and colleagues findings in AoB PLANTS, looking at the host Bidens pilosa and Cuscuta australis.

Finally Těšitel and colleagues look at the effect of Cuscuta (dodder) and a number of plants in China. Cuscuta australis and C. chinensis are both native parasites in China. However Těšitel and colleagues also note that some work in China has been using C. campestris on Mikania micrantha, a fast-growing clonal vine which is creating major problems in parts of Asia.

Těšitel and colleagues argue the work around the work in diverse habitats shows similar potential. Therefore parasitic plants should be considered part of the biotic resistance hypothesis, the idea that generalist enemies can be used to fight invasive species. The authors also add that, by using native parasitic plants, the control works both with alien and native invaders, such as plants expanding their range due to climate change.

One reason the authors are particularly interested in parasitic plants as a control is that they can be well adapted for targeting the traits that make invasive species a problem. “Remarkably, the parasitism seems to particularly affect invasive species displaying traits usually associated with fast growth or high competitive ability, such as symbiotic nitrogen fixation, tree growth form or clonal spreading by rhizomes or other vegetative means,” Těšitel and colleagues write in their article.

Whether or not small-scale trials can be applied to work in the field is, as yet, uncertain. Těšitel and colleagues point out that the success of the parasitic interaction will be affected by genetic variability in both the host and parasitic plants. Testing in greenhouses could, however, suggest potential sources for interactions between plants.

One new interaction reported this month in the Journal of Plant Pathology is between Solanum rostratum and Cuscuta campestris in Northeast China. In the Liaoning province, Wang and colleagues observed C. campestris spread through S. rostratum and reduce its growth.

Těšitel and colleagues don’t propose planting parasitic plants as a magic bullet against invasive species, but say it could complement other control measures. They also point out that we don’t have to stick to the parasitic plants we currently have today. “Testing of different genotypes may produce more effective control measures for certain populations of invasive species. Beyond the natural variability found in wild populations, novel strains or cultivars of parasitic plants may be developed using plant breeding techniques, which may further increase the application potential on additional target invasive species.”

While this might improve parasitic plants for use, it also carries risks. A bred parasite could reduce the genetic diversity of the natural population, Těšitel and colleagues note. They also recognise that the species are often ‘weedy’ and persistent, which means they too could become a problem to be tackled. This, they emphasise, underlines the importance of using native, non-weedy species, where possible.

However, while the parasitic plants will have a negative effect on some plants, they can also help create a rich ecosystem. In the case of work by Těšitel and others on Rhinanthus ssp. reducing grasses allows other plants to flourish, increasing the biodiversity of a location. While parasites are unlikely to replace herbicides in agricultural fields, they may prove a more eco-friendly solution to removing problem plants in nature reserves.

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