Doctoral Dissertations
Date of Award
5-2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Entomology and Plant Pathology
Major Professor
Jerome F. Grant
Committee Members
Mitchell D. Richmond, Feng Chen, Kimberly D. Gwinn
Abstract
Industrial hemp, Cannabis sativa L., production has been revived in the United States following its legalization through the Agricultural Improvement Act of 2018 (a.k.a. 2018 Farm Bill). The corn earworm, Helicoverpa zea (Boddie), has been considered one of the most harmful insect pests in outdoor hemp production as larvae feed on the marketable grain and inflorescences where cannabinoid concentrations are highest. However, in Tennessee, a Heliothine complex, including corn earworm and the tobacco budworm, Chloridea virescens (F.), occurs on hemp. To investigate the seasonal incidence of these hemp pests, larvae were collected from hemp grown in different regions of Tennessee during the 2022 and 2023 growing seasons and reared to adulthood or parasitoid emergence. Differences in the distribution of these two pest species were observed among locations in both years. Hemp variety consistently influenced larval abundance for both years with Hillbilly Hash having the most larvae and Sweetened having the least, highlighting the importance of varietal selection as a management strategy. Differences in parasitic fauna and abundance varied by location and year with two species, Campoletis sonorensis Cameron and Winthemia rufopicta Bigot, targeting larvae of both species at higher parasitism rates than other parasitoid species.
To determine the influence of hemp variety and the role of chemical volatiles in the host-plant selection process of gravid moths, multi-choice and two-choice oviposition assays were performed under laboratory conditions. In the multi-choice assay, variety significantly influenced the ovipositional preference of C. virescens and H. zea, and the attraction to different varieties varied among pest species. The two-choice assay complimented the results of the multi-choice assay indicating that ovipositional preference by gravid H. zea can be stimulated in response to headspace odors of female inflorescences. Ten volatile organic compounds were identified from headspace collections of flowering hemp. Mean egg counts of C. virescens were moderately correlated to the percentages of β-caryophyllene and humulene, but no relationships were found between any of the volatile compounds identified and the ovipositional preferences of H. zea. Thus, the primary olfactory cues from floral hemp employed by these Heliothine pests during host discrimination remain to be clarified.
To investigate the influence of host species and hemp variety on parasitoid development and the role of chemical volatiles from hemp inflorescences in the foraging strategy of the parasitoids, Campoletis sonorensis and Microplitis croceipes were tested in several experiments. The developmental parameters of C. sonorensis did not vary between the two larval host species. Unfortunately, the varietal influence of the host diet on the development of C. sonorensis could not be determined due to high mortality among treatment diets. The odors from inflorescences infested with larvae of C. virescens were preferred by M. croceipes, but most C. sonorensis failed to make a choice. None of the volatiles identified were unique to the larval herbivory of either pest species. However, the emission rates of several volatile chemicals increased in response to the severity of larval damage indicating the relative abundance of certain compounds may be exploited by foraging parasitoids. Understanding the phenology of pest species, varietal preference by gravid moths, and tritrophic interactions between H. zea, C. virescens, and associated parasitoids will inform pest management decisions to increase the productivity of hemp and similar host crops in Tennessee and the southeastern United States.
Recommended Citation
Cosner, Julian B., "Phenology, Olfactory Response, and Tritrophic Interactions of Helicoverpa zea and Chloridea virescens on Hemp, Cannabis sativa. " PhD diss., University of Tennessee, 2024.
https://trace.tennessee.edu/utk_graddiss/10109