Genotypes from lower latitudes are less susceptible to insect herbivory
Diverse accessions of the warm-season bioenergy grass Panicum virgatum growing in Texas
Defending leaves and other tissue against herbivores plays an important role in the success of plants. These defenses are costly—nutrients and carbohydrates that the plant uses to defend tissue can’t contribute to growing new leaves, stems, and roots, or to producing flowers, fruits, and seeds. Defense against herbivory should be strongest in areas where herbivores have the largest potential to harm plant populations. Regions closer to the equator often support more herbivores than regions closer to the poles, which could result in a latitudinal gradient in plant defense, with plants better-defended against herbivores closer to the equator.
When herbivores are abundant, plants increase their defenses regardless of their genetic predisposition toward defense. One common way to separate these genetic and environmental effects is to grow plants together in the same environment so observed differences between plants result primarily from genetic differences. In this study, researchers gathered hundreds of plants of the widespread grass, switchgrass (Panicum virgatum), from across its native range, from Texas to Minnesota, to grow them in a shared location in central Texas. In fall 2018, an outbreak of the voracious insect herbivore, fall armyworm (Spodoptera frugiperda), occurred within this garden, providing an opportunity to test whether latitude of plant origin impacted herbivory.
The next summer, researchers performed controlled, lab-based follow-up studies to assess plant defenses and determine how quickly fall armyworms developed when fed leaves of different switchgrass plants. Fall armyworms are expected to gain less weight when feeding on well defended plants than when feeding on poorly defended plants. They found that plants of southern origin experienced less herbivory in the field and fall armyworms gained less weight when feeding on these leaves in the lab. This was likely due, at least in part, to higher defenses against herbivory in the leaves of southern plants—these southern leaves were harder for fall armyworms to chew and digest. These results are consistent with the hypothesized latitudinal defense gradient and suggest that herbivory is an important force driving evolution in this widespread species.