Pest population dynamics are related to a continental overwintering gradient

Abstract

Overwintering success is an important determinant of arthropod populations that must be considered as climate change continues to influence the spatiotemporal population dynamics of agricultural pests. Using a long-term monitoring database and biologically relevant overwintering zones, we modeled the annual and seasonal population dynamics of a common pest, Helicoverpa zea (Boddie), based on three overwintering suitability zones throughout North America using four decades of soil temperatures: the southern range (able to persist through winter), transitional zone (uncertain overwintering survivorship), and northern limits (unable to survive winter). Our model indicates H. zea population dynamics are hierarchically structured with continental-level effects that are partitioned into three geographic zones. Seasonal populations were initially detected in the southern range, where they experienced multiple large population peaks. All three zones experienced a final peak between late July (southern range) and mid-August to mid-September (transitional zone and northern limits). The southern range expanded by 3% since 1981 and is projected to increase by twofold by 2099 but the areas of other zones are expected to decrease in the future. These changes suggest larger populations may persist at higher latitudes in the future due to reduced low-temperature lethal events during winter. Because H. zea is a highly migratory pest, predicting when populations accumulate in one region can inform synchronous or lagged population development in other regions. We show the value of combining long-term datasets, remotely sensed data, and laboratory findings to inform forecasting of insect pests.

Keywords: bollworm; corn earworm; dispersal; long-term monitoring; migration.

Fig. 1.

Overview of our modeling approach. We constructed overwintering zones based on laboratory cold tolerance studies with remotely sensed climatic reanalysis data. Then, we partitioned H. zea population dynamics based on the overwintering zones. Lastly, we projected the H. zea overwintering range into the future by relating current soil and air temperature data and predicting future soil temperature using future air temperature conditions.

Fig. 2.

Overwintering zone classification based on a 40-y averaged modeled soil temperature (0 to 28 cm). (A) How often year-to-year changes occur between the three overwintering zones. The scale represents the number of between-year changes between the three zones. For example, if a pixel switched between the northern limits and transitional zone between the years 2001 and 2002, we assigned a value of 1 and summed up all between-year changes for the available data. (B) The 40-y averaged zone classification. Grey dashed lines indicate the location of the widely accepted northern overwintering limit of the 40°N latitude.

Fig. 3.

H. zea population dynamics among (A) and within (B) years. Figures are final model predictions (model GS) for year and week of year, respectively. Solid black lines represent the global (or species range) and dashed lines represent the population dynamics within the overwintering zones. Gray shaded areas represent model uncertainty.

Fig. 4.

Projected overwintering zone change from historic and current averages to 2099. Area was estimated using NASA’s Earth Exchange Global Daily Downscaled Climate Projection under RCP 8.5 adjusted to soil temperatures. Because these data were derived from projected air temperatures, they only provide a coarse understanding of potential overwintering zone shift.