Life history and habitat explain variation among insect pest populations subject to global change

Aerospace-foraging bats eat seasonably across varying habitats

Recent research has confirmed the efficiency of insectivorous bats as pest suppressors, underlining the ecological services they offer in agroecosystems. Therefore, some efforts try to enhance bat foraging in agricultural landscapes by acting upon environmental factors favouring them. In this study, we monitored a Miniopterus schreibersii colony, in the southern Iberian Peninsula. We intensively sampled their faeces and analysed them by metabarcoding to describe how the bent-winged bat diet would change with time, and to test whether their most-consumed prey would seasonally depend on different landscapes or habitats. Our results confirm that M. schreibersii are selective opportunist predators of moths, dipterans, mayflies, and other fluttering insects, shifting their diet to temporary peaks of prey availability in their foraging range, including both pest and non-pest insects. Supporting our hypothesis, throughout the year, M. schreibersii consume insects linked to diverse open habitats, including wetlands, grassland, diverse croplands, and woodland. The importance of each prey habitat varies seasonally, depending on their insect phenology, making bats indirectly dependent on a diverse landscape as their primary prey source. Bats' predation upon pest insects is quantitatively high, consuming around 1610 kg in 5 months, of which 1467 kg correspond to ten species. So, their suppression effect may be relevant, mainly in patchy heterogeneous landscapes, where bats' foraging may concentrate in successive outbursts of pests, affecting different crops or woodlands. Our results stress that to take advantage of the ecosystem services of bats or other generalist insectivores, keeping the environmental conditions they require to thrive, particularly a heterogeneous landscape within the colony's foraging area, is crucial.

Spatiotemporal dynamics of forest insect populations under climate change

Effects of climate on forest insect populations are complex, often involving drivers that are opposing, nonlinear, and nonadditive. Overall, climate change is driving an increase in outbreaks and range shifts. Links between climate and forest insect dynamics are becoming clearer; however, the underlying mechanisms remain less clear. Climate alters forest insect population dynamics directly through life history, physiology, and voltinism, and indirectly through effects on host trees and natural enemies. Climatic effects on bark beetles, wood-boring insects, and sap-suckers are often indirect, through effects on host-tree susceptibility, whereas climatic effects on defoliators are comparatively more direct. We recommend process-based approaches to global distribution mapping and population models to identify the underlying mechanisms and enable effective management of forest insects.

Cold winters drive consistent and spatially synchronous 8-year population cycles of cabbage stem flea beetle

Population cycles have been observed in mammals as well as insects, but consistent population cycling has rarely been documented in agroecosystems and never for a beetle. We analysed the long-term population patterns of the cabbage stem flea beetle Psylliodes chrysocephala in winter oilseed rape over 50 years. Psylliodes chrysocephala larval density from 3045 winter oilseed rape fields in southern Sweden showed strong 8-year population cycles in regional mean density. Fluctuations in larval density were synchronous over time across five subregional populations. Subregional mean environmental variables explained 90.6% of the synchrony in P. chrysocephala populations at the 7-11 year time-scale. The number of days below -10°C showed strong anti-phase coherence with larval densities in the 7-11 year time-scale, such that more cold days resulted in low larval densities. High levels of the North Atlantic Oscillation weather system are coherent and anti-phase with cold weather in Scania, Sweden. At the field-scale, later crop planting date and more cold winter days were associated with decreased overwintering larval density. Warmer autumn temperatures, resulting in greater larval accumulated degree days early in the season, increased overwintering larval density. Despite variation in environmental conditions and crop management, 8-year cycles persisted for cabbage stem flea beetle throughout the 50 years of data collection. Moran effects, influenced by the North Atlantic Oscillation weather patterns, are the primary drivers of this cycle and synchronicity. Insect pest data collected in commercial agriculture fields is an abundant source of long-term data. We show that an agricultural pest can have the same periodic population cycles observed in perennial and unmanaged ecosystems. This unexpected finding has implications for sustainable pest management in agriculture and shows the value of long-term pest monitoring projects as an additional source of time-series data to untangle the drivers of population cycles.

The causes and consequences of pest population variability in agricultural landscapes

Variability in population densities is key to the ecology of natural systems but also has great implications for agriculture. Farmers' decisions are heavily influenced by their risk aversion to pest outbreaks that result in major yield losses. However, the need for long-term pest population data across many farms has prevented researchers from exploring the drivers and implications of pest population variability (PV). Here, we demonstrate the critical importance of PV for sustainable farming by analyzing 13 years of pest densities across >1300 Spanish olive groves and vineyards. Variable populations were more likely to cause major yield losses, but also occasionally created temporal windows when densities fell below insecticide spray thresholds. Importantly, environmental factors regulating pest variability were very distinct from factors regulating mean density, suggesting variability needs to be uniquely managed. Finally, we found diversifying landscapes may be a win-win situation for conservation and farmers, as diversified landscapes promote less abundant and less variable pest populations. Therefore, we encourage agricultural stakeholders to increase the complexity of the landscapes surrounding their farms through conserving/restoring natural habitat and/or diversifying crops.

Growth and development of an invasive forest insect under current and future projected temperature regimes

Temperature and its impact on fitness are fundamental for understanding range shifts and population dynamics under climate change. Geographic climate heterogeneity, behavioral and physiological plasticity, and thermal adaptation to local climates make predicting the responses of species to climate change complex. Using larvae from seven geographically distinct wild populations in the eastern United States of the non‐native forest pest Lymantria dispar dispar (L.), we conducted a simulated reciprocal transplant experiment in environmental chambers using six custom temperature regimes representing contemporary conditions near the southern and northern extremes of the US invasion front and projections under two climate change scenarios for the year 2050. Larval growth and development rates increased with climate warming compared with current thermal regimes and tended to be greater for individuals originally sourced from southern rather than northern populations. Although increases in growth and development rates with warming varied somewhat by region of the source population, there was not strong evidence of local adaptation, southern populations tended to outperform those from northern populations in all thermal regimes. Our study demonstrates the utility of simulating thermal regimes under climate change in environmental chambers and emphasizes how the impacts from future increases in temperature can vary based on geographic differences in climate‐related performance among populations.

Effects of Soil Temperature and Moisture on the Development and Survival of Grasshopper Eggs in Inner Mongolian Grasslands

Grasshopper eggs overwinter in soil for almost half a year. Changes in soil temperature and moisture have a substantial effect on grasshopper eggs, especially temperature and moisture extremes. However, the combinatorial effect of temperature and moisture on the development and survival of grasshopper eggs has not been well studied. Here, we examined the effects of different soil moistures (2, 5, 8, 11, 14% water content) at 26°C and combinations of extreme soil moisture and soil temperature on the egg development and survival of three dominant species of grasshopper (Dasyhippus barbipes, Oedaleus asiaticus, and Chorthippus fallax) in Inner Mongolian grasslands. Our data indicated that the egg water content of the three grasshopper species was positively correlated with soil moisture but negatively correlated with hatching time. The relationship between hatching rate and soil moisture was unimodal. Averaged across 2 and 11% soil moisture, a soil temperature of 35oCsignificantly advanced the egg hatching time of D. barbipes, O. asiaticus, and C. fallax by 5.63, 4.75, and 2.63 days and reduced the egg hatching rate of D. barbipes by 18%. Averaged across 26 and 35°C, 2% soil moisture significantly delayed the egg hatching time of D. barbipes, O. asiaticus, and C. fallax by 0.69, 11.01, and 0.31 days, respectively, and decreased the egg hatching rate of D. barbipes by 10%. The hatching time was prolonged as drought exposure duration increased, and the egg hatching rate was negatively correlated with drought exposure duration, except for O. asiaticus. Overall, the combination of high soil temperature and low soil moisture had a significantly negative effect on egg development, survival, and egg hatching. Generally, the response of grasshopper eggs to soil temperature and moisture provides important information on the population dynamics of grasshoppers and their ability to respond to future climate change.

Wide Diurnal Temperature Amplitude and High Population Density Can Positively Affect the Life History of Sitobion avenae (Hemiptera: Aphididae)

Diurnal temperature amplitude is known to have a large influence on insect life history. Population density affects intraspecific competition and many other aspects of insect life history. However, there is limited information on the interactive effects of these factors on insects. Here, we tested the interactive effects of three diurnal temperature amplitudes (22 ± 0°C, 22 ± 6°C, and 22 ± 12°C) and three population densities on the development, survival, longevity, and fecundity of the English grain aphid Sitobion avenae (Fabricius) (Homoptera: Aphididae). At a constant temperature, increasing population density reduced the growth and survival of early-instar nymphs, increased longevity, and reduced fecundity. At a low population density, increasing temperature amplitude inhibited nymph development. However, even at a high temperature amplitude, nymph survival rate was higher than expected, and reproduction was possible because the recovery of the lower night-temperatures eliminated thermal stress. Increasing the population density reduced, and even reversed, the negative effects of the wide temperature amplitude. This may reflect synergistic interactions between population density and wide temperature amplitude as these stressors each incur energetic costs. These findings emphasize the importance of temperature amplitude and population density for improving prediction accuracy and damage assessment during pest control modeling.

Prevalence of Early- and Late-Season Pest Damage to Corn in Cover Crop-Based Reduced-Tillage Organic Systems

In organic agronomic cropping systems, the use of synthetic insecticides and transgenic varieties are prohibited and producers rely mainly on biological control, tillage, crop rotation, and other cultural practices to manage pests. We measured damage to organic corn (Zea mays L.) from multiple invertebrate pests, including slugs (Gastropoda: Mollusca), European corn borer (Ostrinia nubilalis Hübner), corn earworm (Helicoverpa zea Boddie), and fall armyworm (Spodoptera frugiperda Smith), early and late in the growing season in four cropping systems that varied in tillage frequency and intensity and in winter cover crop species. Specific management tactics included two cover crop mixtures preceding corn, the use of a roller-crimper or tillage to terminate cover crops preceding corn, and the establishment of interseeded cover crops after corn emergence. Prevalence of early-season damage was high, but severity of damage was very low and unrelated to corn yield. The proportion of corn plants affected by chewing pests early in the season was lower in plots in which tillage compared to a roller-crimper was used to terminate cover crops. Cropping system did not affect the numbers of late-season caterpillar pests or corn yield. Predation by natural enemies appeared to effectively maintain damage from chewing pests below yield-damaging levels. These results support the inclusion of winter and interseeded cover crops in organic agronomic crop rotations to gain environmental benefits without increasing risks of damage by insect pests.

Changes in Major Insect Pests of Pine Forests in Korea Over the Last 50 Years

Understanding the occurrence patterns of forest pests is fundamental for effective forest management from both economic and ecological perspectives. Here, we review the history of the occurrence patterns and causes of outbreaks and declines of pests in Korean pine forests over the last 50 years. During this period, the major pests of pine forests in Korea have shifted from pine caterpillar (Dendrolimus spectabilis Butler) to the pine needle gall midge (PNGM, Thecodiplosis japonensis (Uchida and Inouye)) and finally to pine wilt disease (PWD) caused by the pine wood nematode (Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle). Outbreaks of pine caterpillar, a native species in Korea, have been recorded as far back as 900 years, and it was the most relevant forest pest in Korea until the 1970s. The decline of its importance has been attributed to reforestation and higher levels of subsequent natural enemy activity. The PNGM is an invasive species, first discovered in Korea in 1929, that became widely distributed by 1992 and the major forest pest in the 1980s and 1990s. A suite of parasitic wasps attacking the PNGM contributed at least partially to the decline of PNGM densities. Following the decline of the PNGM, damage from PWD has increased since 2003. These shifts in major forest pests might be related to changes in forest composition and interactions among forest pests. Therefore, a new management strategy for controlling forest pests is required to mitigate the decline of pine forests in Korea.