Effects of soil management and plant breeding on insect herbivore growth and consumption
Authors:   , Althoff Emily Rose 
Published: August 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] The benefits of sustainable soil management practices on soil health have been well documented for over a century. In 2017, 42,087,306 ha of farm land were cultivated with no-till practices, which reduce soil erosion. Crop rotation can also improve soil health by increasing soil nitrogen content. As nitrogen is a limiting factor to plant and insect growth, changes in soil nitrogen will likely affect plant-insect interactions. Furthermore, far fewer studies have examined long-term effects of soil management on insect communities. We examine the effects of longterm soil management practices (27 years) on soil nitrogen availability, foliar nitrogen content and secondary metabolites, and insect herbivory. We compare leaf-chewing insect growth and consumption rates feeding on tilled continuous, no-till continuous, tilled rotated, and no-till rotated plots containing soybean (Glycine max). While piercing-sucking insects were fed corn (Zea mays) kernels from rotated no-till or rotated tilled plots or soybean pods from rotated no-till or continuous till plots. Piercing-sucking insects experienced higher growth on continuous tilled corn in comparison to the soybean treatments, likely due to the senescence of the soybean pods. In 2018, soybean looper (Chrysodeixis includens Walker) feeding on continuously planted, tilled soybean had greater relative growth rates than those feeding on rotated, no-till soybean. However, in 2019, soybean looper consumption was higher on the rotated, no-till treatment than on continuous till soybean. Fall armyworm (Spodoptera frugiperda J.E. Smith) consumption and growth did not differ among treatments in both years. While percent soil nitrogen levels differed, our bioassay results led us to conclude that management practices that promote soil health such as no-tillage and crop rotation do not greatly influence insect herbivory. Additionally, for thousands of years, humans have artificially selected for yield in food crops. In recent years, this has become more critical as arable land has decreased while human populations have increased. Increase in yield influences plant physiology, photosynthetic rates, and nitrogen uptake. As plant growth has been selected for, plant defenses may have decreased, leading to potential increases in insect herbivore growth, consumption, efficiency of conversion of digested food, and efficiency of conversion of ingested food. We conducted bioassays feeding soybean looper (Chrysodeixis includens Walker) larvae leaves from 19 soybean cultivars released over the last 86 years. Larval growth was not affected by soybean release year, but consumption decreased on recent varieties. Efficiency of conversion of digested food and efficiency of conversion of ingested food also increased on more recent varieties, perhaps, because of differences in foliar nitrogen or terpenoids. This is important as selection for yield has not necessarily resulted in selection for higher insecticide application.