Soybean Aphid (Hemiptera: Aphididae) Feeding Behavior is Largely Unchanged by Soybean Mosaic Virus but Significantly Altered by the Beetle-Transmitted Bean Pod Mottle Virus.

The soybean aphid (Aphis glycines Matsumura) is an economically important invasive pest of soybean. In addition to damage caused by soybean aphid feeding on the phloem sap, this insect also transmits many plant viruses, including soybean mosaic virus (SMV). Previous work has shown that plant viruses can change plant host phenotypes to alter the behavior of their insect vectors to promote virus spread, known as the vector manipulation hypothesis. In this study, we used electropenetography (EPG) to examine the effects of two plant viruses on soybean aphid feeding behavior: SMV, which is transmitted by many aphid species including the soybean aphid, and bean pod mottle virus (BPMV), which is transmitted by chrysomelid and some coccinellid beetles but not aphids. These two viruses often co-occur in soybean production and can act synergistically. Surprisingly, our results showed little to no effect of SMV on soybean aphid feeding behaviors measured by EPG, but profound differences were observed in aphids feeding on BPMV-infected plants. Aphids took longer to find the vascular bundle of BPMV-infected plants, and once found, spent more time entering and conditioning the phloem than ingesting phloem sap. Interestingly, these observed alterations are similar to those of aphids feeding on insect-resistant soybean plants. The cause of these changes in feeding behavior is not known, and how they impact virus transmission and soybean aphid populations in the field will require further study.

Feeding Behavior of Soybean Aphid (Hemiptera: Aphididae) Biotype 2 on Resistant and Susceptible Soybean.

Host plant resistance to the soybean aphid, Aphis glycines Matsumura, is an effective means of controlling populations of this introduced pest species in the United States. Rag (Resistance to Aphis glycines) genes identified in soybean germplasm have been incorporated into commercial cultivars, but differential responses by soybean aphid biotypes to the Rag genes have made understanding mechanisms underlying resistance associated with Rag genes increasingly important. We compared the behavior of biotype 2 aphids on the resistant soybean line PI243540, which is a source of Rag2, and the susceptible cultivar Wyandot. Scanning electron microscopy revealed that the abaxial surface of leaves from resistant plants had a higher density of both long and glandulartrichomes, which might repel aphids, on veins. Time-lapse animation also suggested a repellent effect of resistant plants on aphids. However, electropenatography (EPG) indicated that the time to first probe did not differ between aphids feeding on the resistant and susceptible lines. EPG also indicated that fewer aphids feeding on resistant plants reached the phloem, and the time before reaching the phloem was much longer relative to susceptible soybean. For aphids that reached the phloem, there was no difference in either number of feedings or their duration in phloem. However, aphids feeding on resistant soybean had fewer prolonged phases of active salivation (E1) and many more pathway activities and non-probing intervals. Together, the feeding behavior of aphids suggested that Rag2 resistance has strong antixenosis effects, in addition to previously reported antibiosis, and was associated with epidermal and mesophyll tissues.

Plant host range and leafhopper transmission of maize fine streak virus.

Maize fine streak virus (MFSV), an emerging Rhabdovirus sp. in the genus Nucleorhabdovirus, is persistently transmitted by the black-faced leafhopper, Graminella nigrifrons (Forbes). MFSV was transmitted to maize, wheat, oat, rye, barley, foxtail, annual ryegrass, and quackgrass by G. nigrifrons. Parameters affecting efficiency of MFSV acquisition (infection) and transmission (inoculation) to maize were evaluated using single-leafhopper inoculations and enzyme-linked immunosorbent assay. MFSV was detected in ≈20% of leafhoppers that fed on infected plants but <10% of insects transmitted the virus. Nymphs became infected earlier and supported higher viral titers than adults but developmental stage at aquisition did not affect the rate of MFSV transmission. Viral titer and transmission also increased with longer post-first access to diseased periods (PADPs) (the sum of the intervals from the beginning of the acquisition access period to the end of the inoculation access period). Length of the acquisition access period was more important for virus accumulation in adults, whereas length of the interval between acquisition access and inoculation access was more important in nymphs. A threshold viral titer was needed for transmission but no transmission occurred, irrespective of titer, with a PADP of <4 weeks. MFSV was first detected by immunofluorescence confocal laser scanning microscopy at 2-week PADPs in midgut cells, hemocytes, and neural tissues; 3-week PADPs in tracheal cells; and 4-week PADPs in salivary glands, coinciding with the time of transmission to plants.