Intermittent Sequential Herbivory Affects Herbivore Growth Traits but Not Plant Fitness in Soybean

ABSTRACT Plants are often attacked by multiple herbivores. Most studies on plant–herbivore interactions focus on pairwise comparisons, examining one or two herbivores attacking a single species, or a single herbivore attacking multiple species. Although sequential herbivory is prevalent, there has been limited investigation on plant and herbivore response to sequential attacks, especially in soybean ( Glycine max , L.). To examine this, we imposed sequential and intermittent herbivory with two herbivores—fall armyworm, Spodoptera frugiperda (FAW), and soybean looper, Chrysodeixis includens (SL)—on two soybean varieties, “Magellan” and “Black Hawk”. A comprehensive evaluation of plant growth, physiology, and fitness traits, alongside herbivore growth and development, was conducted. No significant effects of the sequential attacks were observed on soybean growth traits. However, after the first attack, physiological traits were upregulated but remained unchanged post sequential herbivory. FAW exhibited higher mass gain compared to SL during the first herbivory. In sequential attacks, FAW that fed on SL‐primed plants gained more mass, whereas both herbivores that fed on FAW‐primed plants gained the least. The frass weight and the number of frass pellets collected from FAW were also higher in both the first and sequential attacks compared to SL. However, yield and fitness were unaffected by sequential herbivory by either FAW or SL. Taken together, these findings suggest that FAW, although a minor but polyphagous pest, was able to feed more and induce higher defenses compared to SL, a major pest of the crop with limited host range. More importantly, we show that sequential and intermittent herbivory has differential effects on host and herbivore traits, but not on yield, suggesting tolerance—an area that should be explored further.