Drought stress increases the expression of barley defence genes with negative consequences for infesting cereal aphids

Crops are exposed to myriad abiotic and biotic stressors with negative consequences. Two stressors that are expected to increase under climate change are drought and infestation with herbivorous insects, including important aphid species. Expanding our understanding of the impact drought has on the plant-aphid relationship will become increasingly important under future climate scenarios. Here we use a previously characterised plant-aphid system comprising a susceptible variety of barley, a wild relative of barley with partial-aphid resistance, and the bird cherry-oat aphid to examine the drought-plant-aphid relationship. We show that drought has a negative effect on plant physiology and aphid fitness and provide evidence to suggest that plant resistance influences aphid responses to drought stress, with the expression of aphid detoxification genes increasing under drought when feeding on the susceptible plant but decreasing on the partially-resistant plant. Furthermore, we show that the expression of thionin genes, plant defensive compounds that contribute aphid resistance, increase ten-fold in susceptible plants exposed to drought stress but remain at constant levels in the partially-resistant plant, suggesting they play an important role in modulating aphid populations. This study highlights the role of plant defensive processes in mediating the interactions between the environment, plants, and herbivorous insects.

Oscillation, synchrony, and multi-factor patterns between cereal aphids and parasitoid populations in southern Brazil

In different parts of the world, aphid populations and their natural enemies are influenced by landscapes and climate. In the Neotropical region, few long-term studies have been conducted, maintaining a gap for comprehension of the effect of meteorological variables on aphid population patterns and their parasitoids in field conditions. This study describes the general patterns of oscillation in cereal winged aphids and their parasitoids, selecting meteorological variables and evaluating their effects on these insects. Aphids exhibit two annual peaks, one in summer–fall transition and the other in winter-spring transition. For parasitoids, the highest annual peak takes place during winter and a second peak occurs in winter–spring transition. Temperature was the principal meteorological regulator of population fluctuation in winged aphids and parasitoids during the year. The favorable temperature range is not the same for aphids and parasitoids. For aphids, temperature increase resulted in population growth, with maximum positive effect at 25°C. Temperature also positively influenced parasitoid populations, but the growth was asymptotic around 20°C. Although rainfall showed no regulatory function on aphid seasonality, it influenced the final number of insects over the year. The response of aphids and parasitoids to temperature has implications for trophic compatibility and regulation of their populations. Such functions should be taken into account in predictive models.

Direct and Knock-on Effects of Water Stress on the Nutrient Contents ofTriticum aestivum(Poales: Poaceae) and Population Growth ofRhopalosiphum padi(Hemiptera: Aphididae)

To ascertain the direct effects of water stress upon wheat plants (Triticum aestivum L.) and how these effects, in turn, influence the population growth of the bird cherry-oat aphid (Rhopalosiphum padi L.), we conducted a physiological analysis of wheat seedlings grown under three different watering regimes and subsequently determined the population parameters of the aphid using the age-stage, two-sex life table. A significantly higher content of free amino acids and soluble sugars were observed in wheat seedlings exposed to drought stress compared to seedlings that were well-watered and those that were grown under waterlogged conditions. Extended phloem salivation and stylet penetration with shorter duration of sustained ingestion from phloem was observed in an electrical penetration graph (EPG) of R. padi on drought-stressed wheat seedlings. This suggested that the aphid’s feeding activity, as well as nutrient intake, were impeded. The significantly higher percentage of essential amino acids found in wheat seedlings grown under waterlogged conditions promoted significantly higher fecundity and intrinsic rate of increase in R. padi populations compared to aphids fed on drought-treated or well-watered wheat seedlings. Our findings suggest that wheat seedling responses to water stress involve changes in sap composition that are responsible for altering the aphids’ nutrient intake and consequently affect their population growth. From a grower’s perspective, extending wheat cultivation in a rice–wheat rotation paddy field during the winter season may not be economically profitable if the fields are chronically waterlogged, since this may potentially lead to a higher infestation of cereal aphids.

Changes in defense-related enzymes and phenolics in resistant and susceptible common wheat cultivars under aphid stress

The cereal aphids Rhopalosiphum padi L. are serious pests on grain crops, reducing the quality and yield by direct feeding damage and virus transmission. The changes in the physiological and biochemical parameters of two wheat cultivars under aphid stress were investigated to understand aphid-resistance mechanisms. The activity levels of phenylalanine ammonia-lyase (PAL), peroxidase (POD), tyrosine ammonia-lyase (TAL), Ca2+-ATPase and Mg2+-ATPase in resistant cultivar W0923 increased during aphid feeding, and most of them were significantly higher than those in the susceptible cultivar GN21. However, these enzyme activities increased and then decreased in GN21. The polyphenol oxidase (PPO) activity in W0923 was maintained longer than in GN21. The total phenol contents of the two cultivars exhibited significant increases on the 15th day compared with the untreated controls, and the content in W0923 was significantly higher than in GN21 by the 30th day. Most of the POD, PPO, PAL, TAL and Ca2+-ATPase activities and phenolic metabolite contents were higher in W0923 than in GN21 under the untreated control conditions. The Ca2+-ATPase and PAL activities positively correlated with POD, PPO and Mg2+-ATPase activities. Ca2+-ATPase and PAL may be key biochemical markers for evaluating aphid resistance. W0923 had a strong ability to maintain higher enzyme activities and synthesize more phenols and tannins than GN21, which contributed to aphid resistance.

Moderately decreasing fertilizer in fields does not reduce populations of cereal aphids but maximizes fitness of parasitoids

Examination of the tradeoff between the extent of decreasing nitrogen input and pest suppression is crucial for maintaining the balance between essential yield and an efficient, sustainable pest control strategy. In this study, an experiment with four manipulated nitrogen fertilizer levels (70, 140, 210, and 280 kg N ha−1 = conventional level) was conducted to explore the effects of decreasing nitrogen on cereal aphids (Sitobion avenae and Rhopalosiphum padi) (Hemiptera: Aphididae), Aphidiinae parasitoids (Hymenoptera: Braconidae: Aphidiinae), and body sizes of parasitoids. The results indicated that nitrogen application, in the range of 70–280 kg N ha−1, has the potential to impact the populations of cereal aphids and their parasitoids. However, both differences between densities of cereal aphids and their parasitoids in moderate (140–210 kg N ha−1) and those in high nitrogen input (280 kg N ha−1) were not significant, and the parasitism rate was also unaffected. A higher parasitism rate reduced population growth of the cereal aphid (S. avenae). Additionally, a moderate decrease of nitrogen fertilizer from 280 to 140–210 kg N ha−1 maximized the body sizes of Aphidiinae parasitoids, indicating that a moderate decrease of nitrogen fertilizer could facilitate biocontrol of cereal aphid by parasitoids in the near future. We conclude that a moderate decrease in nitrogen application, from 280 to 140–210 kg N ha−1, does not quantitatively impact the densities of cereal aphids or the parasitism rate but can qualitatively maximize the fitness of the parasitoids.

Lipopeptide producing endophytic bacteria of the genus Bacillus regulate wheat resistance to cereal aphids

We have shown the direct and indirect effect of lipopeptide-producing endophytic bacteria of the genus Bacillus on the viability of greenbug aphid Schizaphis graminum Rond. and the induction of systemic resistance in wheat plants.

Recombinant Bacillus subtilis strain deficient in production of surfactin loses ability to induce resistance of wheat plants against aphid Schizaphis graminum (Rond.)

An important role of surfactin synthesis by endophytic bacteria in protecting wheat against cereal aphids has been shown to manifest itself in a direct insecticidal effect and an indirect effect through the induction of systemic resistance in plants.

Instant Insights: Climate change, insect pests and invasive species

This specially curated collection features three reviews of current and key research on climate change, insect pests and invasive species. The first chapter reviews the impact of climate change on insect pests and how it has affected insect pest development and population dynamics, activity and abundance, diversity and geographical distribution. It also assesses insect-host plant interactions and the effectiveness of crop pest management techniques. The second chapter discusses the literature on the potential impact of climate change on the principal insect pests of wheat, including cereal aphids, Hessian fly, orange wheat blossom midge, cereal leaf beetle and cotton bollworm. It assesses the different methods used to assess likely impacts as well climate change effects on biological control in wheat systems. The final chapter surveys what we know about the ecology of invasive species and potential management strategies. In particular, it assesses how integrated pest management (IPM) needs to evolve to deal with invasive species, particularly in focussing more on monitoring, prevention and rapid response.