Phenotypic plasticity of corn leaf aphid Rhopalosiphum maidis under elevated temperature and CO2

In the current context of global climate change, the world average temperature and carbon dioxide (CO2) concentration in the atmosphere has steadily increased. Climate change is also predicted to increase the frequency and magnitude of extreme heat events, which will involve great challenges for most of the ectotherm organisms, such as aphids. As a worldwide pest insect, corn leaf aphid, Rhopalosiphum maidis caused significant damage on cereal crops such as barley, corn, and wheat. R. maidis is also a vector of plant viruses including sugarcane mosaic virus, maize dwarf mosaic virus, which result in economic losses. Similar to all ectotherms, aphids have acute sensory capability for detecting temperature variations. Their small size and high mass-specific metabolic rates makes them sensitive to energy and water balance during exposure to extremes. In this study, thermal tolerance of R. maidis under extreme high temperatures across differential life stages were tested. The critical high temperature (CHT) of R. maidis was between 39.0 to 40.0 °C according kinds of feeding treatments. The upper lethal temperatures (ULTs) of R. maidis were significantly different between feeding and no feeding treatments. In addition, the ULTs varied significantly across life stages with highest ULTs values for 4th instars. Feeding significantly increased the thermal tolerance of aphids.R. maidis exhibit wing polyphenism in response to poor habitat quality. Polyphenism is an important form of adaptation in an adverse environment. Five developmental stages of aphids with increased population densities were investigated under two kinds of temperature patterns. Crowding was found to directly impact winged induction. The 1st and 2nd nymphs were more sensitive for alate morphs induction under high density. In addition, temperature played a significant role in wing production, with the 26/39 °C temperature setting inducing higher alate morphs and survival.The increase in concentrations of CO2 not only affects plant growth and development, but also impacts the emission of plant organic volatile compounds. During the dual choice bioassays, the winged and wingless aphids were more attracted by the VOCs of barley seedlings cultivated under ambient CO2 concentrations (aCO2; 450ppm) than barley seedlings cultivated under elevated CO2 concentrations (eCO2; 800ppm), nymphs were not attracted by the VOCs of eCO2 barley seedlings. While 16 VOCs were identified from aCO2 barley seedlings, only 9 VOCs were found from eCO2 barley seedlings. The effect of elevated CO2 on feeding behavior of R. maidis on barley seedlings was tracked using electrical penetration graph (EPG). The nutrient content of host plant and the developmental indexes of aphids under eCO2 and aCO2 conditions were examined. Barley seedlings under eCO2 concentrations had lower contents of crude protein and amino acids. EPG analysis showed plants cultivated under eCO2 influenced the aphid feeding behavior, by prolonging the total pre-probation time and the ingestion of passive phloem sap. Moreover, fresh body weight, fecundity and intrinsic population growth rate of R. maidis was significantly decreased in eCO2 in contrast to aCO2 condition. Our reasearch provide a new perspective on understanding plant-insect herbivore interactions under climate change.