The current dominant hypothesis purported by the scientific community is that both the wtb and pathogen are native to the southwestern United States. The wtb has a long-term association with Arizona walnut (Juglans major), with the first report of the beetle made in New Mexico in 1929. The beetle tends to exhibit limited colonization of Arizona walnut, and cankers induced by the pathogen are smaller on Arizona walnut than on black walnut. The limited affect of both the pathogen and insect on Arizona walnut suggest the potential for a long-term coexistence of host, pathogen, and vector. Similarly, wtb were reported in the Los Angeles area in 1959 in the native range of California walnut (Juglans californica), another host exhibiting some level of resistance to the disease.

Over the next few years, genetic analyses of the wtb and G. morbida will likely elucidate the origin and distribution pattern of the disease within North America.

California walnut growers have facilitated nationwide research and monitoring efforts

Numerous walnut growers in California have cooperated with UCCE Farm Advisors to participate in a statewide wtb trapping program led by US Forest Service Entomologist, Dr. Steve Seybold. Since 2009, in Tulare County alone, five growers have permitted weekly trapping of wtb in orchards. The year-round monitoring of wtb activity provides valuable insight into the epidemiology of the disease in diverse walnut growing regions in California, and has provided a framework for the guidelines now utilized nationwide for early detection of new forest infestations. Early results of the wtb monitoring program demonstrate that the beetles are active in winter, particularly after a period of mild temperatures. Consequently, infected trees remaining in orchards over the dormant season may serve as primary inoculum for disease transmission by winter beetle flight.

Upcoming UCCE research efforts

With support from the UC ANR Competitive Grants program and the Specialty Crops Research Initiative (SCRI), collaborating Farm Advisors throughout the state will be conducting extensive local surveys for the disease in commercial orchards. Upcoming surveys will be conducted in September and October 2012. Orchards in both Tulare and Kings Counties will be included in the statewide survey, with the goal of understanding the distribution of the disease in diverse geographic orchard systems.