The history of soil cultivation can alter how corn distributes resources between direct and indirect defense mechanisms against insects. Experiments with soils subjected to pea, radish, triticale, or fallow land showed differences in benzoxazinoid production and indole emission after attack by the fall armyworm. The effect also extended to the interaction between the pest and a generalist predator.
Researchers evaluated maize grown in soils with legacy crops of pea (Pisum sativum), radish (Raphanus sativus), and triticale. A fallow area served as a control. The team investigated the plants' response to attack by Spodoptera frugiperda, as well as the interaction of the caterpillars with the predator Harmonia axyridis (doi 10.1016/j.baae.2026.04.003).
The results indicated a trade-off between different defensive strategies. Corn grown after peas showed a higher proportion of indole in the mixture of volatile compounds released after herbivory. This compound is part of the indirect defense mechanisms of corn. Its action participates in plant signaling and may favor responses related to attracting natural enemies.
At the other extreme, plants grown in fallow soil showed a higher concentration of benzoxazinoids in their leaves. These metabolites act in direct chemical defense against herbivores. After 48 hours of feeding by the fall armyworm, the total amount of these compounds in the fallow corn reached approximately double the values recorded in the other soil legacies.
The response to indole followed a different direction. Herbivory increased the emission of this compound, while soil type also showed an independent effect. Plants grown after peas registered almost double the proportion of indole in the airspace compared to plants from fallow land. The difference between soils, however, appears in a specific compound, and not in the set of volatiles: on the overall profile of the mixture, the effect of soil type was only marginal (P = 0,0549). The researchers did not detect a similar response, associated with the soil legacy, for other compounds evaluated, including green leaf volatiles, monoterpenes, and sesquiterpenes.
According to the scientists' interpretation, the results point to a possible redistribution of metabolic resources between direct and indirect defense. Indole participates in pathways related to benzoxazinoids, although different enzymes produce indole reservoirs with distinct functions within the plant. In soil with a legacy of legumes, corn directed the response towards greater participation of the indirect mechanism. In fallow soil, there was greater investment in certain compounds of direct defense.
The researchers also analyzed the expression of a gene related to protease inhibitor. Feeding with Spodoptera frugiperda induced this response in all treatments. However, soil type did not cause a significant difference in the level of induction. The result indicates the absence of a uniform effect of the legacy of cover crops on all maize defense mechanisms.
The effect reached the relationship between the caterpillar and Harmonia axyridis. In a no-choice trial, the predator consumed caterpillars at a similar rate, regardless of the soil treatment. In the trial with prey choice, a different response occurred. Adults showed a significant preference for caterpillars fed on corn grown after peas. These caterpillars received more than twice as many choices compared to individuals from treatments with triticale or fallow.
The chemical composition of the caterpillars also changed. Individuals fed on corn grown in soil with pea legacy accumulated higher concentrations of different benzoxazinoids compared to caterpillars from the radish treatment. The fallow soil showed intermediate values for these compounds. The triticale treatment was excluded from this analysis: the caterpillars raised in this soil were smaller than expected, which hindered the detection of the compounds. The study recorded concentrations of benzoxazinoids in the body of Spodoptera frugiperda much higher than those found in plant tissue. However, the researchers emphasize the need for further studies to clarify the role of this accumulation in the interaction with predators and parasitoids.
The experiment used soil collected in 2022 and 2023 from a cover crop research area in Pennsylvania, United States. The site had been organically managed without chemical inputs since 2014. After the cover crops were finished and incorporated into the soil at the end of spring, the team collected topsoil up to twenty centimeters deep. The samples underwent composition analysis within each treatment and were used for growing corn in a greenhouse.
For the researchers, nutrients and microbial communities are among the possible components of the legacy effect. The study did not separate the participation of each factor. The team observed trends in nutrient profiles, but did not assess changes in the structure of the microbial community. For this reason, the work does not attribute the results to a specific mechanism.
The data broaden the discussion on the use of cover crops in integrated pest management. The choice of the preceding species can produce effects beyond soil structure, nutrient cycling, and water conservation. The legacy left in the soil can also modify the chemical defense pathways of corn, the quality of herbivores as prey, and the behavior of natural enemies. According to the scientists, aligning the choice of cover crop with the desired defensive outcome could serve as a natural tool for pest suppression. They add that future studies integrating microbiota, nutrient flow, plant growth, and species interactions will be important to consolidate this approach.
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