Age-dependent and herbivory-induced epicuticular wax in Sorghum x drummondii has consequences for herbivore growth and development

In crop production, management strategies have historically relied upon chemical control to mitigate insect pressure. As resistance to pesticides in herbivores grows at an alarming rate, the search for novel pest management strategies has become more important, alongside a better understanding of host-plant defenses and mechanisms. Among the common physical defenses in plants, epicuticular waxes serve as an important first line of defense. Brown midrib (BMR) is a host-plant trait from a recessive gene that affects the monolignol biosynthetic pathway, reducing lignin, and is considered as a gene of interest for developing pest management strategies. Sorghum-sudangrass, Sorghum x drummondii, is an economically important crop with BMR; however, the effects of BMR have primarily been studied in animal agriculture and its effects on insects is largely unknown. In addition, how BMR affects epicuticular waxes, and how seed treatments affect is also unknown. This study is aimed at understanding how epicuticular wax, in Sorghum x drummondii is affected by chemical control (seed treatments), BMR, and plant age, and how wax affects a polyphagous herbivore, the fall armyworm, Spodoptera frugiperda. Through wax quantification and wax added artificial diet experiments, we show that while BMR and seed treatment does not affect wax content, plant age significantly affects wax, and has negative consequences for caterpillar growth. In addition, we also found that wax can be significantly induced via both fall armyworm, and sugarcane aphid (Melanaphis sacchari) suggesting the potential of wax as both a constitutive and herbivory induced defense trait, that should be explored further.