UC Davis

Yield losses in crop production due to insect pests and plant diseases are a major threat to food security worldwide. Furthermore, some insect pests transmit plant pathogenic viruses, which are more difficult to control than other pests because of complex interactions among insect vectors, plant viruses, and host plants. Hemipterans (e.g., aphids, leafhoppers, mealybugs, planthoppers, psyllids, soft scales, and whiteflies) are the most common and efficient insect vectors because of their piercing-sucking mouthparts. Effective management of insect-vectored plant viruses requires integration of diverse management practices and knowledge of their complex interactions. In this project, beet leafhopper, Circulifer tenellus (Baker) (Hemiptera: Cicadellidae), and beet curly top virus (BCTV) were studied as a model insect vector-plant virus system. Beet leafhopper and BCTV have wide host ranges and cause yield losses in economically important crops such as sugar beet, tomato, pepper, melon, and common bean. For instance, BCTV caused ~$100 million in losses to the California processing tomato industry in 2013. In California, beet leafhoppers overwinter in the foothills located on the western side of the Central Valley, and their progeny acquire BCTV from non-agricultural host plants and migrate to agricultural fields in spring. The Curly Top Virus Control Program run by the California Department of Food and Agriculture monitors populations using sweep net assessment and sprays malathion insecticide using airplanes to reduce beet leafhopper density in the foothills. Due to geographical scale, the program only covers small portions of the foothills and more efficient management strategies are needed to reduce BCTV incidence in crops. This dissertation addresses questions to improve management of beet leafhopper in California through multiple approaches. Chapter 1 describes how BCTV affects beet leafhopper to enhance its propagation in tomato fields with barley or ribwort plantain as trap crops. BCTV effects on host preference of beet leafhoppers were tested in dual- and no-choice experiments and simulation modeling predicted how they alter BCTV spread in tomato fields. Non-viruliferous beet leafhoppers preferred to probe on barley and ribwort plantain compared with tomato but viruliferous beet leafhoppers showed no probing preference. Simulation modeling revealed that this alteration may increase BCTV infection rate in tomato fields with trap crops. Chapter 2 describes oviposition of beet leafhopper on four common non-agricultural host plants [Erodium cicutarium (L.) L’Hér. (Geraniaceae), Kochia scoparia (L.) Schrader (Amaranthaceae), Plantago ovata Forsskál (Plantaginaceae), and Salsola tragus L. (Amaranthaceae)] at two temperatures (30 and 35 °C). In addition, oviposition models for each non-agricultural host plant were constructed and validated under fluctuating temperature conditions. K. scoparia was the most suitable host plant (highest number of eggs laid) followed by P. ovata, E. cicutarium, and S. tragus, and the optimal temperature for oviposition was 30.6 °C. Since beet leafhoppers in non-agricultural areas are the most important targets to prevent BCTV spread, this information is crucial for developing effective beet leafhopper management strategies. Chapter 3 describes beet leafhopper migration time and its association with BCTV outbreaks. Plant greenness effects on flight propensity of beet leafhopper were determined with two host plants, sugar beet and redstem filaree, under greenhouse conditions. In addition, spring migration of beet leafhoppers was monitored in the foothills for 2 years and vegetation greenness of study sites was calculated using satellite imagery. As plants in the foothills became dry in spring, beet leafhoppers started migrating to agricultural fields. Based on vegetation greenness, a spring migration model was developed to estimate beet leafhopper migration time. In addition, the spring migration model was implemented in a web-based system as a decision support tool for beet leafhopper management. Severe BCTV outbreaks were reported in the San Joaquin Valley and Sacramento Valley in 2013 and 2021, respectively. In these years, early spring migration was estimated from the spring migration model, which supports the possibility of early spring migration as a key factor in causing BCTV outbreaks. The web-based mapping system not only aids in effective beet leafhopper management, but also provides valuable insight into BCTV epidemiology.