Plants are routinely subjected simultaneously to different abiotic and biotic stresses, such as heat, drought, and insect infestation. Plant–insect interactions in such complex stress situations are poorly understood. We evaluated the performance of the grain aphid (Sitobion avenae) in wheat (Triticum aestivum L.) exposed to a combination of heat and drought stresses. We also performed assays of the relative water content, nutritional quality, and responses of phytohormone signaling pathways. Lower relative water content and accumulation of soluble sugars and amino acids were observed in plants exposed to combined heat and drought stress. These conditions increased abscisic acid levels in the absence of aphids, as well as leading to higher levels of jasmonate-dependent transcripts. The grain aphid infestation further increased abscisic acid levels and the abundance of jasmonic acid- and salicylic acid-dependent defenses under the combined stress conditions. Aphids reared on plants grown under drought stress alone showed lower net reproductive rates, intrinsic rates of increase, and finite rates of increase compared with aphids reared on plants in the absence of stress. The heat-treated plants also showed a decreased aphid net reproductive rate. These findings demonstrate that exposure to a combination of stresses enhances plant defense responses against aphids as well as altering nutritional quality.

Elevated atmospheric CO₂ (eCO₂) and increased nitrogen (N) fertilization significantly change the nutritional quality of plants and influence the growth and development of insects. However, little is known about plant metabolism and plant-insect interactions under eCO₂ and increased N fertilization, especially C₄ plants. Thus, the combined effects of eCO₂ and increased N fertilization on maize-Ostrinia furnacalis interactions were tested in this study. Our data demonstrated that both eCO₂ and increased N fertilization increased starch content, while increased N fertilization promoted the N content in maize. The combined effects of eCO₂ and increased N fertilization did not influence the total non-structural carbohydrates (TNC):N ratio in maize. The jasmonic acid level of maize was enhanced by increased N fertilization and O. furnacalis infestation. The total phenolics content and defensive enzyme activities of maize increased under eCO₂, increased N fertilization and O. furnacalis infestation. Protective enzyme activities were enhanced, while digestive enzyme activities, mean relative growth rate, body mass and efficiency of conversion of ingested food decreased for O. furnacalis feeding on maize grown under eCO₂ and increased N fertilization. Therefore, eCO₂ and increased N fertilization increased starch and N accumulation, and did not influence the TNC:N ratio, however, eCO₂ and N promoted the resistance-related secondary metabolites (with or without O. furnacalis induced) of maize, which ultimately decreased the fitness of O. furnacalis to the host. These results will help to better understand the metabolic mechanisms of plants and the plant-insect interaction under eCO₂ and increased N fertilization in the context of future climate change scenarios.

The growing resistance of Tetranychus urticae to chemical pesticides highlights the need for alternative solutions for its sustainable control. To develop botanical acaricides and reduce the usage of chemical pesticides, this study assessed the acaricidal activity of AB extract and its combinations with four commercial insecticides: imidacloprid (IMI), acetamiprid (ACE), thiamethoxam (TMX), and bifenthrin (BF). Results showed that the AB extract suppressed detoxification enzymes and downregulated their corresponding genes in T. urticae. Bioassays identified hyoscyamine and genistein as key bioactive compounds. Field trials revealed that treatments with AB + IMI, AB + TMX, and AB + BF significantly enhanced the corrected efficacy against T. urticae compared with AB treatment, with the highest corrected efficacy observed for AB + BF. Notably, the AB + BF treatment did not influence the stability of the natural enemy community, as indicated by the characteristic indices of the natural enemy community, which suggests that the integration of AB extract with BF may represent a sustainable pest management strategy. Therefore, the AB extract represents an environmentally benign alternative to chemical pesticides for the sustainable control of T. urticae.

Due to rising concentration of atmospheric CO₂, climate change is predicted to intensify episodes of drought, however, our understanding of how combined environmental conditions will influence crop-insect interactions is limited. The direct effects of elevated CO₂ and drought stress on wheat (Triticum aestivum L.) nutritional quality, insect resistance and their indirect effects on the grain aphid (Sitobion avenae) performance are reported here. Elevated CO₂ was able to alleviate low water content in wheat caused by drought stress. Both elevated CO₂ and drought promoted soluble sugar accumulation in wheat. However, elevated CO₂ decreased and drought increased the amino acid content in wheat. Elevated CO₂ induced the down-regulation of jasmonic acid (JA) -dependent defense, but up-regulated the salicylic acid-dependent defense. Drought enhanced abscisic acid accumulation that promoted the JA-dependent defense in wheat. Aphid-induced phytohormone resistance in wheat was not influenced by elevated CO₂ and drought. The negative effects of drought on the performance of the aphid population was offset by positive effect of elevated CO₂. In conclusion, elevated CO₂ can alleviate the effects of drought stress on wheat nutritional quality and resistance, which results in unchanged damage to wheat from aphid populations under future elevated CO₂ and drier conditions.