Effects of intermittent and sequential herbivory on herbivore traits and plant fitness in soybean (Glycine max)

Plants are often attacked by multiple insect. Most studies on plant-herbivore interactions focus on pairwise comparisons; examining one or two herbivores attacking a single crop, or a single herbivore attacking multiple crops. 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 conducted an experiment with two herbivores- fall armyworm, Spodoptera frugiperda (FAW) and soybean looper, Chrysodeixis includens (SBL) on two soybean varieties, ‘Magellan’ and ‘Black-Hawk’. A comprehensive evaluation of plant growth, physiology, and fitness traits, along with herbivore growth and development was conducted. No significant effects of sequential attacks on the growth traits of soybean was observed. However, after the first attack, the physiological traits were upregulated but unchanged post sequential herbivory. FAW exhibited higher mass gain compared to SBL, during the first herbivory. In sequential attack, FAW that fed on SBL-primed plants gained more mass, whereas both herbivores that fed on FAW-primed plants gained the least. Leaf trichomes, a major surface defense was significantly increased upon dual attack by FAW. However, yield and fitness were unaffected by sequential herbivory. These findings suggest that FAW, although  minor, but polyphagous pest, was able to feed more and induce higher defenses as compared to SBL, a major pest of soybean with limited host range. Sequential and intermittent herbivory has differential effects on host and herbivore traits; yet the limited impact on yield suggests soybean tolerance to herbivory regardless of herbivore status.