Paper Reviewed
Chen, Y., Martin, C., Mabola, J.C.F., Verheggen, F., Wang, Z., He, K. and Francis, F. 2019. Effects of host plants reared under elevated CO2 concentrations on the foraging behavior of different stages of corn leaf aphids Rhopalosiphum maidis. Insects 10: 182, doi: 10.3390/insects10060182.

Writing to introduce their work, Chen et al. (2019) say that "studies dealing with the behavior of aphids, exposed to volatile organic compounds (VOCs) of host plants, grown under ambient and elevated atmospheric CO2, are not widespread." Nevertheless, one such aphid, the corn leaf aphid (Rhopalosiphum maidis), annually causes a considerable amount of cereal crop damage on barley, corn, wheat and broad bean. Thus, the team of seven researchers felt it prudent to examine the effects of elevated CO2 concentrations on the foraging behavior of corn leaf aphids.

Their work was conducted in a controlled-environment setting where barley plants (Hordeum vulgare, cv. Etincel) were grown for two weeks under concentrations of either 450 ppm or 800 ppm of atmospheric CO2. At the end of the two week period, a colony of corn leaf aphids (nymphs, winged and wingless tested separately) were subjected to different olfactory stimuli from the ambient vs elevated CO2 grown plants to determine if there was a measurable CO2 effect.

Results indicated that both "winged and wingless aphids were more attracted to the VOCs of barley seedlings cultivated under ambient CO2 concentrations than barley seedlings cultivated under elevated CO2 concentrations." Additionally, the authors note that "nymphs were not attracted by the VOCs of elevated CO2 barley seedlings either."

Analysis of volatile compositions emanating from the barley plants revealed a total of 16 VOCs that were identified in ambient CO2 grown plants and only 9 in elevated CO2 grown plants. Six of the volatiles overlapped the two CO2 treatments, but were present in reduced abundances in the elevated CO2 treatment, which helped to explain the preferential treatment of the aphids for the ambient CO2 barley plants. What is more, the VOC with the largest concentration in the elevated CO2 treatment, linalool, has been identified in previous studies as having a repellant effect on aphids, whereas the VOC with the largest concentration in the ambient CO2 treatment (2-hexenal) has proved to be attractive to aphids. Thus, elevated CO2 not only reduced the effectiveness of the most attractive barely-emitted VOC for the corn aphid by reducing its concentration, it also increased the concentration of its most effective VOC for repelling aphids. And that is great news for future cereals like barley, which often sustain significant yield losses from aphid-introduced plant viruses.