Author ORCID Identifier:
Date of Graduation
5-2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Entomology (PhD)
Degree Level
Graduate
Department
Entomology
Advisor/Mentor
Kariyat, Rupesh
Committee Member
Joshi, Neelendra
Second Committee Member
Bateman, Nick
Third Committee Member
Burgos, Nilda
Keywords
Gut Microbiome; Oryza sativa; Phenology; Plant defenses; Spodoptera frugiperda; Trichomes
Abstract
The state of Arkansas is the largest producer of rice (Oryza sativa L.) in the United States, contributing approximately 50% of the total rice production. Extending beyond the state, globally, maintaining rice production and productivity is critical as rice is the primary stable food crop, sustaining half of the world’s population. Like other crops, rice is also threatened by many insect herbivores. Fall armyworm (FAW; Spodoptera frugiperda (J.E.Smith)), native to the Americas, is considered as one of the highly polyphagous transboundary pests infesting rice. Although the pest is currently considered as a minor pest on rice in the United States, changes in the climatic conditions, especially global temperature fluctuations, can alter the pest biology and survival, making it a serious concern to the rice producers in the state and across the world. As the pest has strong dispersal and rapid insecticide resistance development attributes, alternate pest management strategies, such as deploying the plant’s innate defenses, including structural and chemical defenses, are being explored. Although there are numerous studies investigating such defenses in dicots, only a few have examined the integrated defense phenotype in rice against FAW. To the best of our knowledge, this is the first study that extensively examined the structural and chemical defenses in rice, and how phenology affects them, with the long-term goal of developing sustainable management strategies. This was the focus of my literature review in the first two chapters of this dissertation. In the third chapter, I tried to address this knowledge gap by characterizing the major structural defenses, trichomes, epicuticular wax, and lignin across 4 phenological stages- V6, V9, R4, and R8, in 3 rice cultivars- Diamond, Lynx, and RT 7301. The results revealed that the trichome density and epicuticular wax were more abundant in V6 compared to all other stages, whereas lignin was more abundant in the R8 stage, highlighting the variation of structural defenses across phenology. Among the trichomes, we found only non-glandular types, with micro trichomes being the predominant type, followed by papillae, macro trichomes, and prickles. The trichomes were also denser on the adaxial side compared to the abaxial side of the leaf. This study established the baseline, validating the critical role of phenology in shaping plant structural defenses. In my 4th chapter, I examined the response of FAW, at different phenological stages-V5: tillering; V11: maximum tillering; R3: panicle exsertion as well as towards the general commercial seed treatment, CruiserMaxx plus Vibrance package (thiamethoxam as the active ingredient) in 4 different rice cultivars- Diamond, Jupiter, Lynx, and Jewel. The results demonstrated a significant effect on the FAW response due to seed treatment at different phenological stages. Although seed treatment negatively affected the survival rate of FAW, the epicuticular wax was also reduced by approximately 20% in seed-treated rice compared to untreated rice. I also observed that the effect of seed treatment diminished as the larvae reached the adult stage, highlighting the ability of FAW to detoxify or degrade these chemicals. One of the potential mechanisms underlying resistance development is probably the alteration or shift in the gut microbiome of FAW. In the 5th chapter, I tested this hypothesis by examining how phenological differences and seed treatment (CruiserMaxx plus Vibrance package) affect the gut microbiome of FAW. In this experiment, I exposed FAW to 3 rice cultivars- Diamond, Jupiter, and RT 7301 at both vegetative and reproductive stages for 96-hours of feeding. Contrary to my expectations, I didn’t observe any differences in alpha and beta bacterial diversity across seed-treated and untreated samples. Interestingly, I found significant differences in the gut bacterial composition across phenology, with Enterococcaceae (60-70%) as the most abundant family, irrespective of the stages, underscoring the importance of understanding phenological variation in plants and how it mediates changes in the insect gut microbiome for developing effective management strategies. In my 6th chapter, I followed up on the defense traits and focused on volatile-mediated defenses in Arkansas rice-FAW interactions. I collected the foliar volatiles from 3 rice cultivars-Diamond, Lynx, and RT 7301 at vegetative (V7) and reproductive (R4) stages, after exposure to FAW herbivory for 48 hours. I found a similar, strong effect of phenology, with the highest volatile emission in the vegetative stage compared to the reproductive stage. A common garden experiment was also conducted to record the arthropod communities under the same treatments, by setting up three different types of traps: piepan, pitfall, and sticky traps. The level of attraction was in line with the volatile emission, with more arthropods attracted towards the vegetative stage. Diptera was found to be the predominant order collected from the traps, irrespective of the stages. Choice assays were also conducted to observe the FAW preference towards the volatiles, and as predicted, FAW preferred vegetative stage and undamaged volatiles when compared to the reproductive and FAW-damaged volatiles. These findings also confirmed that the phenology is a significant driver in mediating volatile emission in rice, with consequences for the field arthropod community and trophic interactions. In my 7th chapter, I followed up on an interesting observation that the trichomes in rice spikelets trap early instar FAW during anthesis. Here, I showed that the trichomes in spikelets act as a strong barrier, and the larvae, when trying to feed on spikelets during anthesis, become trapped post-anthesis, thus preventing escape, and leading to their death inside the spikelet. This finding provides preliminary evidence on the ability of spikelet trichomes to function as a defense barrier to herbivores feeding on reproductive structures and can be potentially used to develop varieties with more trichome density without compromising the yield. In my 8th chapter, I examined the role of Sucrose non-fermenting related Kinase 1 (SnRK1), against FAW. SnRK1 is an important protein kinase in plants that has shown to provide tolerance against abiotic as well as biotic stress. Although there are many studies investigating the role of SnKR1 in tolerance against rice diseases, only a few have focused on insect pests. In this study, I explored the role of SnRK1 by using SnRK1 mutants and Wild type (O. sativa japonica cv. Kitaake) against FAW using a series of experiments, including exogenous application of phytohormone, Jasmonic acid (JA), and the JA blocker, Ibuprofen, under short and long-term leaf feeding experiments. Contrary to our hypothesis, we failed to observe any significant difference between the SnRK1 mutants and the wild type in all the experiments in their defense against FAW. This study hence highlights that SnRK1, although provides tolerance to rice diseases, has a limited role in stress modulation against the insect herbivore, FAW. Taken together, results from my dissertation collectively suggest the importance of phenology and considering it as a keystone trait while designing pest management strategies, as well as in the role of structural and chemical defenses, FAW microbiome, and field arthropod community in shaping rice-FAW interaction, with potential avenues for future research with the long-term goal of developing sustainable pest management strategies.
Citation
BALAKRISHNAN, D. (2026). Phenological and Cultivar-Specific Defense Strategies Shaping Fall Armyworm Interactions in Arkansas Rice. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/6171
