Altered precipitation dynamics lead to a shift in herbivore dynamical regime

Long-term population dynamics of an insect in a simple food web under a changing environment

Weather conditions are important for the population dynamics of "cold-blooded" animals like insects, with both direct and indirect effects (via the food web). How weather, in combination with other factors, generates population change, and how such effects change over time, are important questions in times of climate change. We monitored an insect seed predator population during a 36-year period of changing weather patterns. The insect is part of a simple food web dominated by seed consumption and lacking natural enemies. Environmental conditions were relatively stable during the first half of the study, but patterns changed during the latter half. Areas of host plant patches increased and seed production entered a strong bi-annual pattern. Insect abundance was measured twice during the yearly life cycle, before and after summer reproduction, and seed resources and competitor densities were measured at the end of the summer. We fit a population model to abundance data to investigate the population dynamics of the insect in relation to changing patterns in weather conditions and food resources. There were both direct and indirect effects of weather, operating at multiple time scales. Abundant sunshine during summer resulted in increased population growth during the same period, but it also resulted in increased survival the following winter. Population growth further depends on seed set the previous summer, which in turn depends on summer rainfall and is likely affected by climate change. This implies indirect weather effects at both short-term and decadal time scales. The new pattern of seed production seems to have led to increased average insect abundance but did not otherwise lead to clear changes in the dynamics of the population. This can be explained by weak regulation of the dynamics of the insect such that short-term environmental variation leads to long unstable population fluctuations. Our study illustrates how insect responses to drastic changes in their environment can be subtle, slow, and hard to detect, manifested by long-term fluctuations. This highlights the importance of long-term data and mechanistic understandings of population dynamics to assess the consequences of changing weather and climate on insects.

Aphids and their parasitoids persist using temporal pairing and synchrony

The study analyzed the population dynamics of aphids and their parasitoids in winter cereals in southern Brazil, using wavelet transform (WT) to detect patterns of periodicity and synchronization over a decade (2011 to 2020). The wavelet analysis revealed different patterns of population peaks between aphid species and their parasitoids. Aphids, such as Rhopalosiphum padi L., Sitobion avenae (Fabricius), Schizaphis graminum (Rondani), and Metopolophium dirhodum (Walker), showed varied peak frequencies, with M. dirhodum consistently exhibiting a shortening interval between outbreaks. In contrast, parasitoids maintained more-constant patterns, with peak frequencies predominantly around 12 mo. Cluster analysis identified 4 highly synchronized aphid-parasitoid pairs: S. graminum-Diaeretiella rapae (MacIntosh), R. padi-Aphidius platensis Brèthes, S. avenae-Aphidius uzbekistanicus Luzhetzki, and M. dirhodum-Aphidius rhopalosiphi De Stefani-Perez. The wavelet coherence (WC) showed significant correlations between the time series of these pairs, ranging from in-phase to anti-phase relationships over time. The results indicate that wavelet analysis is a viable tool for characterizing non-stationary time series, such as aphid and parasitoid populations. Understanding these dynamics and synchronization patterns can support integrated pest-management strategies, enabling more effective and sustainable agricultural interventions.

Spatial habitat heterogeneity influences host-pathogen dynamics in a patchy population of Ranchman's tiger moth

Host-pathogen dynamics are influenced by many factors that vary locally, but models of disease rarely consider dynamics across spatially heterogeneous environments. In addition, theory predicts that dispersal will influence host-pathogen dynamics of populations that are linked, although this has not been examined empirically in natural systems. We examined the spatial dynamics of a patchy population of tiger moths and its baculovirus pathogen, in which habitat type and weather influence dynamics. Theoretical models of host-baculovirus dynamics predict that such variation in dynamics between habitat types could be driven by a range of factors, of which we predict two are likely to be operating in this system: (1) differences in the environmental persistence of pathogens or (2) differences in host intrinsic rates of increase. We used time series models and monitored infection rates of hosts to characterize population and disease dynamics and distinguish between these possibilities. We also examined the role of host dispersal (connectivity) and weather as important contributors to dynamics, using time series models and experiments. We found that the population growth rate was higher, delayed density dependence was weaker, and long-period oscillations had lower amplitudes in high-quality habitat patches. The infection rate was higher on average in high-quality habitat, and this was likely to have been driven by higher mean population densities and no differences in pathogen persistence in different habitats (delayed density dependence). Time series modeling and experiments also showed an interactive effect of temperature and precipitation on moth population growth rates (likely caused by variation in host plant quality and quantity), and an effect of connectivity. Our results showed that spatial heterogeneity, connectivity, climate, and their interactions were important in driving host-baculovirus dynamics. In particular, our study found that connected patches and spatial heterogeneity generated differences in dynamics that only partially aligned with theoretical predictions.

Water availability and plant-herbivore interactions

Water is essential to plant growth and drives plant evolution and interactions with other organisms such as herbivores. However, water availability fluctuates, and these fluctuations are intensified by climate change. How plant water availability influences plant-herbivore interactions in the future is an important question in basic and applied ecology. Here we summarize and synthesize the recent discoveries on the impact of water availability on plant antiherbivore defense ecology and the underlying physiological processes. Water deficit tends to enhance plant resistance and escape traits (i.e. early phenology) against herbivory but negatively affects other defense strategies, including indirect defense and tolerance. However, exceptions are sometimes observed in specific plant-herbivore species pairs. We discuss the effect of water availability on species interactions associated with plants and herbivores from individual to community levels and how these interactions drive plant evolution. Although water stress and many other abiotic stresses are predicted to increase in intensity and frequency due to climate change, we identify a significant lack of study on the interactive impact of additional abiotic stressors on water-plant-herbivore interactions. This review summarizes critical knowledge gaps and informs possible future research directions in water-plant-herbivore interactions.

The consequence of leaf life span to virus infection of herbivorous insects

Many herbivorous insects die of pathogen infections, though the role of plant traits in promoting the persistence of these pathogens as an indirect interaction is poorly understood. We tested whether winter leaf retention of bush lupines (Lupinus arboreus) promotes the persistence of a nucleopolyhedroviruses, thereby increasing the infection risk of caterpillars (Arctia virginalis) feeding on the foliage during spring. We also investigated whether winter leaf retention reduces viral exposure of younger caterpillars that live on the ground, as leaf retention prevents contaminated leaves from reaching the ground. We surveyed winter leaf retention of 248 lupine bush canopies across twelve sites and examined how it related to caterpillar infection risk, herbivory, and inflorescence density. We also manipulated the amount of lupine litter available to young caterpillars in a feeding experiment to emulate litterfall exposure in the field. Greater retention of contaminated leaves from the previous season increased infection rates of caterpillars in early spring. Higher infection rates reduced herbivory and increased plant inflorescence density by summer. Young caterpillars exposed to less litterfall were more likely to starve to death but less likely to die from infection, further suggesting foliage mediated exposure to viruses. We speculate that longer leaf life span may be an unrecognized trait that indirectly mediates top-down control of herbivores by facilitating epizootics.

Hilltopping influences spatial dynamics in a patchy population of tiger moths

Dispersal is a key driver of spatial population dynamics. Dispersal behaviour may be shaped by many factors, such as mate-finding, the spatial distribution of resources, or wind and currents, yet most models of spatial dynamics assume random dispersal. We examined the spatial dynamics of a day-flying moth species (Arctia virginalis) that forms mating aggregations on hilltops (hilltopping) based on long-term adult and larval population censuses. Using time-series models, we compared spatial population dynamics resulting from empirically founded hilltop-based connectivity indices and modelled the interactive effects of temperature, precipitation and density dependence. Model comparisons supported hilltop-based connectivity metrics including hilltop elevation over random connectivity, suggesting an effect of hilltopping behaviour on dynamics. We also found strong interactive effects of temperature and precipitation on dynamics. Simulations based on fitted time-series models showed lower patch occupancy and regional synchrony, and higher colonization and extinction rates when hilltopping was included, with potential implications for the probability of persistence of the patch network. Overall, our results show the potential for dispersal behaviour to have important effects on spatial population dynamics and persistence, and we advocate the inclusion of such non-random dispersal in metapopulation models.