Historical data helps explain recent outbreaks of a vector-borne plant pathogen
Zebra Chip disease has recently become a serious problem for potato and tomato production in the Midwest and West coast of US. While outbreaks of Zebra Chip disease—caused by the bacterium Candidatus Liberibacter solanacearum—have exploded only within the last 20 years, the primary insect vector—Bactericera cockerelli—is native to western US and Mexico. Much speculation has occurred on the factors causing recent outbreaks of Zebra Chip but these hypotheses have remained untested.
Disease ecology theory predicts that increases in vector populations can dramatically enhance the spread of vector-borne pathogens. We hypothesize that Zebra Chip outbreaks have been caused by range expansion and changing phenology of the vector B. cockerelli. We further hypothesize that such changes in vector populations could be due to either changes in climate, host plant availability, or both.
We are taking an eco-informatics approach to testing these hypotheses. Natural history collections and agricultural censuses have provided a rich historical data set on B. cockerelli and wild host plant occurrence, and on extent of crop host production in California over the last century. We are using occupancy models and lists of related species collected through time to control for the opportunistic nature of museum data and estimate changes in B. cockerelli distribution.
Preliminary results show that the phenology of B. cockerelli populations in California has changed over the last century; the probability of B. cockerelli occurrence in springtime has significantly increased in recent decades. Such results are consistent with recent hypotheses that recent climatic warming has enhanced over-wintering survival. Nonetheless, changes in host plant availability—particularly through agricultural expansion—could also play an important role in B. cockerelli phenology. We are currently incorporating climatic and host plant data into our models to assess their relative importance.
In the face of anthropogenic climate change, the challenge for pest risk assessment is in developing rigorous predictions of which pests will become worse and which will not. At the same time, changes in biotic interactions through time—particularly with wild and cultivated host plants—are likely to influence pest populations as well. Our study will contribute to developing frameworks for assessing pest outbreaks risks from climate change while simultaneously incorporating biotic interactions.