This year in California’s San Joaquin Valley (the southern arm of the Great Central Valley) it’s an outbreak year
for Beet Curly Top Virus (BCTV). The virus is vectored by the beet leafhopper, Circulifer tenellus, an invasive insect which was probably introduced to the western USA in the early 20th century. The beet leafhopper is a desert-adapted migratory insect which feeds and breeds on a wide variety of plant species. In the San Joaquin Valley C.tenellus migrates back and forward between winter breeding grounds in the hills on the western side of the valley and summer breeding grounds among the crop lands on the valley floor; BCTV travels back and forward along with its insect vector and probably also persists on the valley floor in infected weed hosts that colonize field and orchard margins where there is available water.
The California Department of Food and Agriculture has run a leafhopper population survey and control program in the foothills for several decades, supported by funds raised through a levy on crop production. The aim of the program is to reduce the winter breeding population of hoppers to non-damaging size and to warn growers of years, like this one, when the population is large. In spite of these management efforts, the beet leafhopper and BCTV have sporadic outbreak years. The dynamics of the leafhopper population have never been studied quantitatively, but detailed field studies in the 1950s and 1960s mean that we have a wealth of information about factors that are known to play a role. For example, early and higher than average autumn rainfall encourages the germination and vigorous growth of winter annual weeds in the foothills supporting survival and egg laying by female hoppers migrating back from the valley bottom. Conversely dry late winter and spring weather helps with survival of the new generation of hoppers (being a desert insect, C.tenellus does not thrive in cold, wet conditions) and encourages them to migrate out of the foothills to the valley. So, sequences of wet year-ends followed by dry year beginnings tend to be associated with outbreaks. The end of 2012 was very much wetter than average and the start of 2013 was among the driest on record.
The point here is not
that climate change caused this year’s BCTV outbreak; in as much as the outbreak depends on particular patterns of weather, what’s happening this year is not especially abnormal. However the connection between the outbreak and atypical patterns of weather gives us an indication of the type of effect that we are likely to see globally as the weather becomes more variable. The BCTV outbreak is a timely reminder, not that one was needed, that as difficult as it is to predict the dynamics of pest populations sometimes, it is only likely to get more so in future in many parts of the world. With increased uncertainty comes increased risk for farmers and all of the knock-on effects on innovation and willingness to try new ideas a more risky environment entails.
We are using modeling and data analysis to investigate how the long-term control strategy interacts with the intrinsic population dynamics and environmental effects to generate the oscillations in leafhopper population size and virus prevalence which are observed. Generic lessons learned from studying this well-known pest and disease complex should be useful in studying less well characterized systems elsewhere.