Phenology and seed treatment impacts surface defenses and fall armyworm (Spodoptera frugiperda) growth in rice (Oryza sativa)

Rice ( Oryza sativa L.) is considered as the most important staple food crop in the world, feeding half of the global population. One of the emerging concerns in rice is the fall armyworm ( Spodoptera frugiperda , FAW), a highly polyphagous insect herbivore. Although FAW has been well studied across many host systems, rice-FAW interactions—including the effects of phenology and seed treatment—are less understood. In this study, we examined the response of FAW to rice under a thiamethoxam seed treatment across three different phenological stages (V5: tillering; V11: maximum tillering; R3: panicle exsertion). We used four commonly grown cultivars in USA, exposed them to FAW for a short period of 48 ​h, and conducted a separate long-term feeding experiment using an artificial diet fortified with leaf material from treated and untreated plants. Data on FAW growth and development traits, rice surface defense levels, and FAW larval feeding behavior were collected. Our results showed that FAW response was significantly affected by seed treatment and rice phenological stages. Interestingly, maximum tillering, the most resistant stage against FAW was also found to have the highest levels of epicuticular wax and trichomes (after the V5 stage)—the two major surface defenses compared to the reproductive stage. We also found that seed treatment negatively affected wax content and increased FAW mortality. In light of the consequent changes in pest biology and emerging insecticide resistance, our data highlight the importance of rice phenology and seed treatment effects on innate rice defenses, as well as their consequences for herbivore feeding. • FAW’s response varied with rice seed treatment and phenology, affecting its normal growth, development, and survival. • Surface defenses of leaves were dynamic and changes with phenology and seed treatment. • Thiamethoxam seed treatment reduced rice epicuticular wax by 19.27 ​%. • Among growth stages, rice exhibited the highest defenses at maximum tillering and the lowest at panicle exsertion stage.