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Influence of Agronomic Management Strategies on Hemp-Insect Interactions
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Agronomic management decisions are made to maximize plant growth and crop yields but can also be used as tools to manage insect pests. Different management practices, such as nitrogen fertilization, cultivar selection, and planting date choice can influence the foliar traits of a crop. Plant foliar traits play important roles in mediating plant-herbivore interactions. Herbivores balance consumption of plant tissue for nutritional gains while contending with plant traits that may deter herbivory or are toxic. Agronomic management practices have been widely studied in many crops, however, for a new or re-emerging crop, there is less information on the influence management has on insect herbivory. Industrial hemp (Cannabis sativa L.) is a re-emerging crop in the United States that is gaining interest, but the current literature has not clearly defined the influence different management practices can have on hemp foliar traits and the role foliar traits play on hemp-insect interactions. The goal of this study was to understand the influence management strategies and temporal changes have on hemp morphological and chemical foliar traits and how these differences alter herbivore performance.
To understand the influence of management practices on hemp foliar traits and insect performance, I conducted both greenhouse and field herbivory studies using field grown hemp in both cases. In 2017 leaves from three different field-grown hemp cultivars, were planted on two different dates, and received two different rates of nitrogenous fertilizer and were then used for a no-choice feeding bioassay using fall armyworm (Spodoptera frugiperda). Foliar nitrogen, C:N, LMA, THC, and CBD were determined and insect performance was measured as growth, consumption, and frass production. Management practices influenced the foliar traits of hemp, specifically fertilization rate, cultivar, and the interaction of fertilization rate with cultivar and with planting date altered foliar traits. Fertilization rate had the greatest influence on herbivore performance, driven by higher foliar nitrogen. Foliar nitrogen had a strong positive influence on insect performance and the ratios of carbon to nitrogen and total cannabinoids to nitrogen had strong negative influences on insect performance.
To measure foliar trait changes over the course of a growing season in different cultivars and to determine herbivore damage, three hemp cultivars were planted in 2018, and leaves were collected at five time points to measure changes in foliar traits. To measure herbivory, leaves were collected at the end of the 2018 growing season and percentage of tissue removed by herbivores was calculated. Foliar traits changed across the growing season, with a decrease in foliar nitrogen and an increase in the ratio of carbon to nitrogen, leaf thickness, THC, and CBD. The foliar trait that had the strongest positive influence on herbivore damage was foliar nitrogen, and higher nitrogen was associated with more herbivore damage. C:N, LMA, and ratio of total cannabinoids to nitrogen had strong negative influences on herbivore damage. Both studies can give us insight into the foliar traits that are driving hemp-insect interactions. Because foliar nitrogen had the greatest positive influence, while the ratios of carbon to nitrogen and total cannabinoid to nitrogen had the greatest negative influence on both performance and field herbivory, focusing on management that affects these three foliar traits is important for pest control. Since foliar nitrogen, C:N and total cannabinoids:N were all affected by nitrogen fertilization, reducing the amount of nitrogenous fertilizer applied while still considering yield could be and important management tool to reduce insect pests.