Different cover crops may affect how plants can defend themselves against pests - including attracting more insects that eat harmful caterpillars, according to a new study led by researchers at Penn State's College of Agricultural Sciences.
The research, published in the journal Basic and Applied Ecology, analyzed the effects and resulting trade-offs of different cover crops, which are plants grown between growing seasons to help protect and enrich the soil.
For example, the team found that plants grown in soil previously used by pea cover crops emitted more of the chemical indole, which primes neighboring plants to defend themselves against insects by enhancing the production of stress hormones and defensive chemicals. However, these plants also had lower levels of benzoxazinoid - a natural toxin present in some plants that can act as a pesticide, antifungal and feeding deterrent.
The researchers also found that predator insects preferred to eat caterpillars from corn grown in soil previously occupied by pea cover crops.
Jared Ali, associate professor of entomology, director of the Center for Chemical Ecology and corresponding author on the paper, said the findings offer practical solutions for growers using techniques they're already familiar with.
"This allows me to go into the field and tell growers that the cover crops they select changes its potential resistance to pathogens and insects, and that each insect species may respond differently to each crop," he said. "That's an insight we didn't have before. There was anecdotal evidence, maybe some farmers had noticed a difference, but now we have the science behind why that's really possible and what mechanism allows that to happen."
The study is part of Ali and his lab's broader work examining the evolution of how plants defend themselves against pests. He explained that as humans domesticated plants and bred them to grow faster, produce more and taste better, we also unintentionally weakened some of their natural defenses. He compares it to the domestication of wolves into dogs: in selecting for the traits we wanted, we also bred out some of the "fangs" that served a protective purpose. As a result, growers now need to use strategies such as pesticides and herbicides to protect crops.
"What my group has been doing is looking for ways that some of the plants still have the ability - still have some of those metaphorical fangs to fight off pests and disease," Ali said. "If we can put them in the right context, what defenses still remain in our domesticated plants?"
Cover cropping has been a popular agricultural practice for thousands of years and has significant effects on soil, plants and insects. While these benefits are well known, Ali said less was known about how different plants used as cover crops would affect future generations of plants in the same plot of soil.
For the study, the researchers planted corn plants in soils conditioned by three cover crops: pea, radish and a wheat-rye hybrid called triticale, along with a control of fallow soil - soil not previously used by a cover crop. They raised fall armyworm larvae, which grow into moths, on the plants in each type of soil.
During plant growth, the team measured defensive chemistry levels and volatile compounds - chemicals released to attract pollinators, defend against pests and communicate with other plants - emitted by each plant. They also tested from which type of plant lady beetles preferred to eat larvae.
The researchers found that when lady beetles were given the choice between corn plants grown in different soils, they showed a significant preference for larvae on plants grown in pea-conditioned soils. One possible reason for this preference, Ali said, could be the quality of the larvae. Because cover crops are known to affect plant quality, they may also influence the quality of pests feeding on these plants.
Additionally, the researchers found that plants growing in pea-conditioned soil emitted almost twice the amount of indole than plants growing in fallow soil. Because indole contains nitrogen, the researchers said, this effect may be tied to the availability of nitrogen in the soil, which may be influenced by previous cover crops.
"Finally, we found that the corn plants' benzoxazinoid levels were approximately twice as high in fallow soil compared to any other soil, suggesting the absence of a prior cover crop may promote stronger expressions of certain direct chemical defenses," Ali said. "This could reflect differences in how plants allocate resources, which may affect plant growth and development."
The researchers said that in the future, additional studies could examine the roles of soil biota - the community of organisms living in the soil - as well as nutrients and multi-species interactions in the connection between these above- and below-ground effects in both natural and managed ecosystems.
They added that incorporating physiological traits, such as yield and development in these studies will be especially important for determining whether these shifts in defense allocation are accompanied by trade-offs in plant performance.
Sujay M. Paranjape, previous Penn State doctoral student currently a postdoc at Cornell University; Sahil V. Pawar, doctoral student at Penn State; Nathaniel McCartney, research technologist at Penn State; Gary W. Felton, Ralph O. Mumma Professor of Entomology and department head of entomology at Penn State; Annett Richter, Boyce Thompson Institute; and Georg Jander, Boyce Thompson Institute, also co-authored this research.
This research was supported by the Foundation for Food & Agriculture Research under grant number FF-NIA19-0000000058, the United States Department of Agriculture under grant number 2021-67014-342357 and the United States National Science Foundation under grant number 2019516. This content is solely the responsibility of the authors and does not necessarily represent the official views of the funders.
