Developing a Leafhopper Degree-day Spray Program for Cherry IPM
Author: Dr. Tobin Northfield
Published: 2026
Summary: Model development. Leafhopper counts from sticky cards (Figure 1) and vacuum sampling (Figure 2) show similar patterns during 2022–2024. The species C. reductus clearly had three distinct periods of adult peaks. Phenology of E. variegatus was more idiosyncratic, and C. geminatus were too uncommonly found to visualize clear patterns. For both methods, most of the leafhoppers collected did not belong to any of these species and were probably of no concern as potential vectors of Xdisease. The best fit model for the sticky traps suggests that the C. reductus base temperature for development was approximately 45ºF, with adult generations emerging onto traps from approximately 500 degree days (dd) to 1600dd in generation one, 1600dd to 3700dd in generation two, and 3700 to approximately 4500dd in generation 3 (Figure 3). There was significant variation between years, with 2022 typically occurring earlier in the season on a degree day scale than 2023 or 2024. This could be due to a number of reasons. First, it is possible that weather allowed greater movement in 2022 than in other years, allowing earlier movement. In addition, there could have been increased mortality in 2022 that reduced the amount of leafhopper capture later in the generations. Further research on movement and mortality, as well as evaluating counts in the 2025 data currently being collected will help identify variability in year-to-year variation. There were some differences between the two sampling methods. Vacuum sampling seemed to have more variation in counts between samples than sticky cards, and vacuum samples tended to catch relatively more E. variegatus than sticky cards. Similarly, it was more difficult to distinguish between generations with vacuum sampling (Figure 4) than with sticky cards. Because sticky cards are typically used to monitor C. reductus, we focused modeling efforts on predicting counts on traps, allowing growers to compare counts with the model. With both methods, the three Oregon orchards tended to have more E. variegatus. In Washington orchards, C. reductus was clearly the dominant species with sticky card monitoring, whereas in Washington vacuum samples, E. variegatus were often found in similar or greater numbers than C. reductus. Previous research suggests E. variegatus has a single generation in Italian vineyards (Bosco et al. 1997, Ann. Appl. Biol. 130:1), and this may also be the case in Pacific Northwest orchards. Simulation model. The simulation model found that contrary to our hypothesis, the optimal timing for insecticide treatments to reduce transmission was when adults numbers are increasing (Figure 5). This may seem counterintuitive, given that adults may be able to transmit prior to treatment. If coverage and mortality was 100%, earlier treatment would indeed be more effective. However, with more v2024 realistic coverage and mortality, we found that earlier sprays killed leafhopper nymphs that may have died naturally before emerging as adults anyway. By moving treatments later in the season, it targets only the leafhoppers with the greatest likelihood of transmission, reducing efforts on killing nymphs that may not have survived anyway. Thus, we recommend timing insecticides to occur when adults first start emerging on traps, which our predictive model can help detect. The PI from 2023–2024, Dr. Orpet, departed Washington State University but remained as a co-PI on the project, and Dr. Northfield started leading in 2025 to oversee the remaining work in leafhopper quantification and model development. The datasets will be augmented by additional data in 2025 related to a Washington State Department of Agriculture Specialty Crop Block Grant project entitled “Leafhopper phenology model development and habitat assessment to improve cherry X-disease management” awarded to PI Orpet and co-PI Northfield. That grant supports technical assistance and leafhopper sampling in three-point transects within orchards: point 1 is adjacent vegetation, point 2 is the orchard edge, and point 3 is the orchard center. This expanded sampling will help us understand the role of orchard-adjacent habitat as a potential leafhopper source and its role in phenology of the leafhoppers. Insecticide trials. Wrath, Sefina, and Pyganic, which were tested in associated with the current project in 2023 and 2024, each increased mortality to exposed C. reductus. A single publication that has all insecticide tests funded by WTFRC/OSCC and elsewhere has been published (Marshall et al. 2025) and information has been incorporated in the WSU Crop Protection Guide for Tree Fruits in Washington.
