Analysis of soybean aphid feeding behavior on drought-stressed plants using electrical penetration graphs

In the U.S., the worst drought conditions in more than 50 years have ravaged major crops such as soybean. Drought stress not only impacts crop yield but also promotes outbreaks of insect pests and pathogens. The physiological and molecular changes occurring in the plant during drought stress is well understood. On the other hand, the impact of drought stress on phloem-feeding insects and the phytopathogenic viruses they vector is relatively undefined. Our study system consisted of soybean plants under a defined drought regime exposed to a specialist phloem feeder, the soybean aphid (Aphis glycines Matsumura), and Soybean mosaic virus (SMV), which it vectors. The drought regime consisted of soybean plants exposed to 25%, 75% and 100% field capacity (FC). The electrical penetration graph (EPG) technique was used to determine the feeding behaviors of non-viruliferous soybean aphids on drought stressed plants. A significant reduction in the time spent on feeding from the sieve element was observed in non-viruliferous aphids under drought stress conditions which was coupled with an increase in time spent in probing. Interestingly no significant reduction in time spent on ingesting xylem sap was observed. EPG analysis indicated that feeding behavior of viruliferous aphids differs from that of non-viruliferous aphids at each drought stress level. These results suggest that drought stress may cause reduced palatability of host plants. Knowledge of the feeding behavior of aphids during drought stress will aid in either breeding for and/or genetically engineering soybean cultivars resistant to aphid feeding and drought stress.