Unraveling the molecular mechanisms of insect resistance to Bacillus thuringiensis (Bt) toxins is of great significance for sustainable utilization of Bt-based biotechnology products. Increased titers of 20-hydroxyecdysone (20E) in the pest insect Plutella xylostella (L.) result from the epigenetic silencing of two 20E-degrading enzymes (EO and GLD), triggering a defense reaction against Bt Cry1Ac toxin. Here, we identified and characterized an enzyme (3-dehydroecdysone-3(3reductase, 3DE3(3R) that can complement the above reaction by increasing the synthesis of 20E and demonstrated its role in the defense mechanism. Specifically, silencing of Px3DE3/3R significantly decreased 20E titer, suppressed MAPK pathway activation, and decreased Cry1Ac resistance in P. xylostella. These results indicate that Px3DE3/3R upregulation enhances 20E production and promotes Bt resistance, which could be used for developing resistance management strategies.
Intercropping flowering plants is a habitat management strategy that seeks to enhance pest suppression by attracting natural enemies. The predators that are attracted often rely on plant-emitted volatiles to locate resource-rich patches for foraging for prey and nectar/pollen. In this study, a two-year field experiment was run in an insecticide-free apple orchard that was intercropped with the herb Cnidium monnieri (L.) Cusson. In olfactometer trials, both the leaves and flowers of C. monnieri were significantly attractive to two selected predators, i.e., the coccinellid Harmonia axyridis (Pallas) and the green lacewing Chrysoperla sinica (Tjeder). A total of 32 volatile organic compounds (VOCs) from flowers and 24 VOCs from leaves were identified from C. monnieri using gas chromatography-mass spectrometry analysis. Under field conditions, six predator species were recorded on flowering C. monnieri plants, including the ladybeetles Propylaea japonica (Thunberg), H. axyridis, and Hippodamia variegata (Goeze), the green lacewing C. sinica, the crab spider Misumenops tricuspidatus (Fabricius), and the syrphid fly Episyrphus balteatus (De Geer). Based on predator exclusion trials, the Relative Biocontrol Service Index was significantly higher in the C. monnieri plots than in the bare-ground control plots in both 2020 and 2021. We found that intercropping apple orchards with C. monnieri attracted six species of aphid predators and enhanced the biological control of the pest Aphis spiraecola Patch. These results support the integration of flowering plant intercropping into integrated pest management (IPM) programs to enhance pest suppression in apple orchards.
Drought can alter the chemical and physical characteristics of plants, which may trigger cascading effects on other trophic levels. Here, cotton plants were subjected to two watering regimens (high and low) and the plants' resistance to a generalist insect herbivore, Helicoverpa zea, were examined. Since plant nutritional and allelochemical properties are known to impact the outcomes of interactions between H. zea and a highly pathogenic baculovirus, H. zea single nucleopolyhedrovirus (HzSNPV), we exposed H. zea larvae to the virus on foliage from amply watered and water-restricted plants. We found that water stress conditions decreased plant growth and increased foliar protein content and peroxidase activity. Larval growth was significantly faster on water-restricted plants than on amply watered plants, while induction of plant defenses with prior larval feeding damage suppressed larval growth on amply watered plants but not on water-restricted plants. HzSNPV-induced mortality was significantly affected by both plant watering regimen and prior herbivory. Caterpillar-induced defenses in cotton reduced viral mortality when the virus was ingested on leaves from amply watered plants but increased mortality under water stress conditions. Moreover, HzSNPV-infected larvae feeding on leaves from water-restricted plants produced more viral occlusion bodies (OBs) and increased the efficiency of conversion of host tissue into OBs. Our findings demonstrate that water stress can reshape the impacts of herbivore-induced plant defenses on insect-pathogen interactions. As the frequency and severity of droughts change, the effects on plant physiology could have profound effects on pathogen-mediated insect population dynamics and the efficacy of microbial biocontrol of pest insects.
