Pest stability in agricultural landscapes

Daniel Paredes, Wildlife, Fish and Conservation Biology, University of California, Davis, Davis, CA, Jay A. Rosenheim, Department of Entomology, University of California, Davis, CA and Daniel Karp, Wildlife, Fish, and Conservation Biology, University of California, Davis, CA

Background/Question/Methods
The study of stability is foundational to the ecology of natural systems but also has great implications for agriculture. Farmers’ decisions regarding pest management (e.g., insecticide applications) are heavily influenced by their strong risk-aversion. That is, farmers fear very rare but catastrophic pest outbreaks that can result in total crop failure. Beyond catastrophic outbreaks, many farmers also choose to apply insecticides based on when pests exceed economic injury thresholds. Unstable pest populations may be more likely to exceed economic thresholds and trigger insecticide applications. To date, the need for long-term data collected across many sites has prevented researchers from exploring pest population stability and catastrophic outbreaks. Here, we leverage an expansive government database of ~1300 farms monitored for ~13 years across Southern Spain. Our focus is on three important crop pests, two within olive systems (Olive moth- Prays oleae; Olive fly- Bactrocera oleae) and one in vineyards (Grapevine moth- Lobesia botrana). We supplemented this government database with information on topography, landscape composition, landscape productivity, and climate. We then explored how mean pest abundances and population variability affect the likelihood of pests exceeding hypothetical and real thresholds. Finally, we identify key environmental factors that affect pest stability and mean pest abundances.
Results/Conclusions
For pests whose densities are usually below hypothetical tolerance thresholds, higher levels of instability are associated with greater likelihoods that pests reach damaging densities. Paradoxically, for pests whose densities are usually above threshold levels, greater instability can be beneficial, creating temporal windows when densities reach below innocuous levels. We then explored how stability affected thresholds for spraying insecticides, for downgrading quality (i.e., from extra virgin to virgin olive oil), and for causes catastrophic losses (i.e., >20% of fruits damaged). Volatile populations were associated with olive oil quality downgrade as well as catastrophic losses of both olives and grapes. Critically, we found that environmental factors like interannual variation in landscape productivity and climate increased variability in pest populations but tended to have more muted effects on pest abundances. Nonetheless, landscape simplification (i.e.,monocultures) increased pest volatility as well as mean levels of abundance, thus increasing the likelihood that farmers experience catastrophic damage. Increasing landscape complexity may thus represent a promising management strategy to stabilize pest populations. More broadly, our work highlights the critical importance of stability for sustainable farming and suggests that stability has to be uniquely studied as the ecology of variability differs from the ecology of mean population densities.