Cold winters drive 8-year population cycles for a beetle pest in agriculture

This blog post is provided by Sara Emery and tells the #StoryBehindthePaper for the paper “Cold winters drive consistent and spatially synchronous 8-year population cycles of cabbage stem flea beetle”, which was recently published in Journal of Animal Ecology. In their paper they explore regular cycles of the cabbage stem flea beetle in Sweden, investigating what impacts these cycles including climatic conditions in the winter.

For centuries, natural historians and scientists have observed that some species cycle very consistently between years in which the population is small and years in which it is large. The classic example many of us are taught when we learn about ecology is the 10-year boom and bust population cycles of snowshoe hares monitored from fur trapping records over the past 300 years in Canada and the Northern United States. Indeed, when the boom-and-bust cycles of snowshoe hares are plotted with lynx populations, a specialist predator of snowshoe hares, we see a remarkably consistent real-life example of the Lotka-Volterra predator-prey equation. Peaks and valleys in lynx populations follow shortly behind and are offset from those of the hare population. But ecological processes are hardly ever so consistent, and in other species for which population cycles are observed (lemmings/voles, moths in forests) scientists have argued about what causes cyclic population patterns, from climate oscillations, latitude, plant quality or specialist predator populations to self-regulation, in which a species reduces its reproductive output. The drivers of cyclicity have been hotly debated over the past 50 years, but research has been constrained by a lack of long-term data.

Data from insects monitored in agricultural fields over decades by government extension agencies and pest control advisers have been used, historically, to inform within-year management recommendations. These data also offer an expansion to the foundation of species being evaluated to assess the drivers of population cycles and may discount some hypotheses. If species have population cycles in annual agriculture crops planted every year, for example, it discounts the long-held hypothesis that changes in perennial plant quality drive population cycles of herbivores.

Fig 1. The mean annual cabbage stem flea beetle larval density across 3,045 winter oilseed rape fields over 50 years in Southern Sweden. The population peaks approximately every eighth year.

For decades crop advisors and farmers in southern Sweden had observed that an agricultural pest in their winter oilseed rape (canola) fields, the cabbage stem flea beetle (Psylliodes chrysocephala) had consistent boom-and-bust cycles, just like the snowshoe hare (Fig 1). This was surprising, ecologically, for two reasons: 1) winter oilseed rape (or canola) is an annual crop planted and harvested in different fields each year, meaning plant quality is an unlikely driver of the cycles, and 2) annual cropping systems are exposed to high levels of disturbance, making a tightly coupled predator-prey system (e.g. lynx/hare) harder to imagine. Cabbage stem flea beetle adults feed and oviposit at the base of newly emerged winter oilseed rape plants from September through October (Fig 2). The damage of greater concern, however, is caused by stem and leaf mining from overwintering larvae (Fig 3).

Fig 2. Adult cabbage stem flea beetle (L), photo credit: Dimitri Geystor. Winter oilseed rape seedling with characteristic stippling damage from adult cabbage stem flea beetle feeding (R), photo credit: Sara Emery.

We leveraged 50 years of data from over 3,000 winter oilseed rape fields in southern Sweden, where cabbage stem flea beetle larvae were monitored (Fig 4), to quantify synchrony and cyclicity. We find that when the population of cabbage stem flea beetle in one subregion is booming the others are too across all five subregions (high spatial synchrony). We verified farmer observations that there are 8-year population cycles of cabbage stem flea beetle (Fig 5).

Fig 3. Larva of cabbage stem flea beetle emerging from a winter oilseed rape stem, photo credit: Gille San Martin.

In addition to quantifying both the cyclicity and synchrony of cabbage stem flea beetle populations across Southern Sweden, our results show that cold winters drive these cycles. Cold winters, in turn, are affected by The North Atlantic Oscillation weather system. This represents a significant step forward in recognizing that population cycles persist, even in highly disturbed landscapes, despite resource variability in location and area planted and pesticide use. It also highlights that climatic oscillations are an important driver of population cycles.

Fig 4. Map of Sweden, highlighted in grey, and the southern Scania region of cabbage stem flea beetle larvae data collection, in red.

Knowledge of the regional cyclicity can be used to predict larval abundance in an individual year, which is important for predicting future boom and bust years, but not for understanding between-field variability. By including field-specific variables known to affect larval survival, we improved our predictions of the larval density on the field scale.  We show that warmer autumns increase larval densities, while later planting date of the crop and cold winters decrease larval densities.  

Fig 5. This analysis shows strong correlations of cabbage stem flea beetle larval population sizes on the 8-year timescale.  Warmer colors show stronger autocorrelation and black lines indicate significance. We can see that population sizes 8 years apart are strongly correlated with each other (timescale) across the entire sampling period (year). For more information on how to interpret this figure see the associated paper!

Cold weather synchronizes populations, drives cyclicity and explains subregional deviations from cycles in cabbage stem flea beetle larval densities in southern Sweden. It is difficult to predict how climate change, which is expected to result in warmer, wetter winters in Sweden, may alter these historically consistent population patterns.

Author bio: Sara Emery

I am an applied ecologist currently working as a postdoctoral researcher at UC Davis. In my research I seek to understand the influence of global climate change on insect phenology using long-term historic data sets.

Twitter: @saraeemery


Read the paper

Read the full paper here: Emery, S.E., Klapwijk, M., Sigvald, R., Bommarco, R. and Lundin, O. (2022), Cold winters drive consistent and spatially synchronous 8-year population cycles of cabbage stem flea beetle. J Anim Ecol. Accepted Author Manuscript.

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