Progress Report to the Maine Potato Board Research Subcommittee
Project Title: Soybean Aphid as a Potential Vector of Potato Virus Y in Maine Potatoes
Investigators:
Andrei Alyokhin, Gary Sewell, and Randy Alford, Department of Biological
Sciences, University of Maine
Grant received: $3,500
Background: Soybean aphid, Aphis glycines Matsumura, is an exotic pest of Southeast Asian origin. It was first discovered in the U.S. in nine mid-Western states in 2000. Since then, it has spread to 21 U.S. states, sometimes producing considerable outbreaks and damaging soybean crops. In Maine, a small population of soybean aphid was first detected in the summer of 2004. Additional populations were reported in 2005, indicating that this pest has most likely established breeding and overwintering populations. As in other affected areas, soybean aphid populations observed in 2005 in Maine fields were very high, commonly exceeding 250 individuals per plant.
Soybean aphid does not colonize potato plants. However, rejection of non-host plants by non-colonizing aphid species usually does not take place until aphids probe them with their mouthparts. As a result, dispersing winged adults of non-colonizing species commonly land on potato plants, insert their stylets into plant tissue, and then leave in search of a more appropriate host. Direct damage caused by probing is negligible. However, probing may also result in the transmission of non-persistent viruses to healthy plants. Potato Virus Y (PVY) is one such non-persistent virus. The mouthparts of the aphid may get contaminated with PVY inoculum in the brief process of probing the epidermal tissues of infected plants. There is no latent period between acquisition and inoculation, and the entire transmission process takes just minutes.
Soybean aphid has been shown to be physiologically capable of transmitting a number of non-persistent plant viruses, including Potato Virus Y (PVY). In the study conducted by Davis, Radcliffe, and Ragsdale in Minnesota, up to 75% of plants that had infected aphids caged on their leaves contracted the PVY. However, being capable of transmitting a virus does not necessarily mean actually transmitting it under field conditions. If soybean aphids do not leave soybean fields, they will have no impact on PVY transmission in potato fields. Similar situation will exist if they do leave soybean fields, but are capable of recognizing potato plants as non-hosts without probing them.
Accomplishments: Experiment was conducted on ten 50-feet long and 4 row wide plots set up on Aroostook Research Farm. Five plots were planted to certified seed potatoes. Other five plots were planted to both potatoes (the inside two rows) and soybeans (the outside two rows). Plots were arranged in a randomized complete block design. To prevent virus transmission between the plots, they were separated from each other by strips of small grain (20-feet wide between the blocks, and six-feet wide between the plots within each block). The plots were sprayed with Bt-based insecticides as needed to prevent potato defoliation by the Colorado potato beetles.
Before furrow closure, ten certified seed pieces in the central two rows of each plot were manually replaced with seed pieces deliberately infected with PVY. Infected pieces were marked at planting, so that the tubers produced by infected plants would not be sampled at harvest. Unmarked plants developing from accidentally infected seed pieces and showing PVY symptoms after emergence from the soil were rogued out.
Twenty potato plants and 20 soybean plants were randomly selected at weekly intervals and visually examined for the presence of soybean and potato-colonizing aphids. In four-row potato plots, only the plants in the middle two rows were inspected. Winged and wingless aphids were recorded separately. In addition, we also counted aphids on 24 soybean plots that were part of the Potato Ecosystem Project (20 plants per plot).
First soybean aphids appeared on the scouted soybean plants in the end of July, and then their populations gradually built up as the season progressed (Fig. 1). However, aphid densities were very low. Compared to the 2005 growing season, their numbers decreased about 1000-fold. Furthermore, unlike the previous growing season, we found virtually no winged soybean aphids. At this point, we do not no the reason for the observed decline. Possibly it was caused by the natural enemies. We did find two lady beetle species (Coccinella septempunctata and Harmonia axyridis) and a number of unidentified spiders associated with soybean plants. However, at this point we cannot say this for sure.
