Host-mediated interactions between soybean aphid (Aphis glycines) and soybean mosaic virus (SMV) Buyung A.R. Hadi & Kelley J. Tilmon Plant Science Department, South Dakota State University Abstract Soybean aphid was introduced in 2000 to North America and was found to be a competent vector of Soybean mosaic virus (SMV). Outbreaks of SMV were thus anticipated with the introduction of an aphid vector species that regularly colonizes soybean. Yet, no SMV outbreak has been reported after more than a decade of soybean aphid introduction. In this study, SMV was not found to cause host modification that negatively effects soybean aphid host search behavior or population growth. These findings imply that SMV infection of soybean plants has at least neutral effects on the field spread of the virus, yet other aspects of SMV-soybean aphid interaction need to be further investigated. For example, the effect of SMV infection on soybean aphid feeding period potentially bears a bigger impact on the virus’ epidemiology. Introduction Results Discussion Since its first introduction to North America in 2000, soybean aphid has made a significant economic impact on the region’s soybean production (Ragsdale et al. 2007. Ragsdale et al. 2004). Feeding injuries account for most of the soybean yield loss due to soybean aphid colonization (Ragsdale et al. 2004). Soybean aphid is known to be a competent vector of soybean mosaic virus (Clark and Perry 2002). Outbreaks of SMV in soybean were anticipated. Yet no such outbreak has been reported. In this study we investigated the effects of SMV infection in soybean plants on the host-search behavior and population growth of soybean aphid. Recent general hypotheses predict that the transmission mechanism of a particular virus-vector complex shapes the host-mediated interactions between the virus and the insect vector in a way that optimize the virus spread in the field (Mauck et al 2012). Thus, new data on host-mediated interactions between SMV and soybean aphid may contribute to our understanding of SMV epidemiology. Our behavioral assay showed that, based on olfactory cues, soybean aphid does not prefer SMV-infected leaves over healthy ones (Figure 2). Fereres et al. (1999) observed that two other SMV vectors, Myzus persicae and Rhopalosiphum maidis did not show preference between SMV-infected and healthy soybean based on visual stimuli. The same study also reported that neither aphid species showed odor-mediated attraction to leaf extracts of SMV-infected leaves. Our observation and those of Fereres et al. (1999) support a newly-minted general hypothesis that “(plant) viruses will influence host-derived cues in ways that have positive (or neutral, but rarely negative) effects on vector attraction” (Mauck et al. 2012). During the five weeks of our observation there was no statistically significant difference in aphid population sizes on SMV-infected or healthy plants (Figure 3). A field study in Wisconsin reported by Donaldson & Gratton (2007) showed that the end-of-season aphid population sizes on SMV-infected plants were about half of those on healthy soybean. Their study took into account a season long observational period not captured in our study. It may thus take a long-term exposure to SMV-infected host plant for a sufficient reduction in the aphid population growth to occur. Alternatively, since the Wisconsin field study examined non-caged plants naturally colonized by soybean aphids, it might inadvertently measured the long term effect of SMV-infected host plant repellency on soybean aphid. Our results indicate that, at least within the observed period, SMV does not cause any host modification that negatively effect soybean aphid host search behavior or population growth. These findings have neutral effect on the field spread of SMV. Another aspect of aphid behaviors potentially affected by viral infection of the host is the length of aphid feeding period. Indeed Wang and Ghabrial (2002) found that when soybean aphid was given only one minute to feed on SMV-infected hosts, its SMV transmission rate was about 30 times higher than when it was given overnight feeding period. Thus, shorter feeding period has a net positive effect on the field dispersal of the virus. The natural progression of this work is to investigate the effect of SMV infection of soybean plant on soybean aphid feeding length and dispersal behavior. Indeed the key to explain the absence of SMV outbreaks amidst soybean aphid invasion may partly be found there. 27%(15%)b 13% (7%)bc 6% (6%)c 54% (15%)a Percentage of aphids (standard deviation) found under SMV-infected leaf found under healthy leaf found in the initial release area Percentage of aphids (standard deviation) found on the rest of the arena Methodology We conducted laboratory-based choice tests to examine the effect of SMV infection on the host-search behavior of soybean aphid. The tests were conducted in a petri dish arena setup described in figure 1. . Several layers of fine-mesh opaque screen were used to separated the aphids in the petri dish from the leaves above to block visual stimuli but allow volatiles to seep in. The leaves were placed on raised foam padding to avoid stylet contact. For each test, 20-30 alatae were placed on the starting platform and allowed to choose between leaves from 3-week-old plants corresponding to the two treatments (SMV-infected vs. healthy). The number of aphids present below each leaf was recorded every 15 min for 1 h (subsamples). The total number of aphids responding over the entire 1-h period was determined and divided by 4 (the number of time periods evaluated) to obtain an average number of responders for each treatment in each test replicate. The experiment was replicated five times. The distribution of aphids among the two treatments was analyzed using generalized linear model with Duncan’s multiple range test as the post-hoc test (SAS 9.2). A greenhouse study was performed to measure the effects of SMV infection of the host plant on soybean aphid population growth. The experiment had three treatments: SMV-infected, mock-infected and untouched healthy plants. Ten 3-week-old caged soybean plants were assigned to each treatment. A starting population of 10 apterous soybean aphids were assigned to each plant. Plants were placed randomly on a greenhouse bench with supplemental lighting to provide a 16:8-h (light:dark) photoperiod, and counts were conducted once a week for five weeks. The count data were analyzed for each week using the Kruskal-Wallis test which does not rely on the assumption of normality (SAS 9.2). Figure 2. Aphid response to a choice test between SMV-infected and healthy host plants. Superscripted letters distinguish treatments with statistically significant difference (p<0.05). Total number of aphids (both apterae and alatae) References Clark A.J., and K.L. Perry. 2002. Transmissibility of field isolates of soybean viruses by Aphis glycines. Plant Dis. 86:1219–22. Donaldson, J. R., and C. Gratton. 2007. Antagonistic effects of soybean viruses on soybean aphid performance. Environ. Entomol. 36: 918-925. Fereres, A., G. E. Kampmeier, and M.E. Irwin. 1999. Aphid attraction and preference for soybean and pepper plants infected with potyviridae. Ann. Entomol. Soc. Am. 92:542–548. Mauck, K., N.A. Bosque-Pérez, S.D. Eigenbrode, C.M. De Moraes, and M.C. Mescher. 2012. Transmission mechanisms shape pathogen effects on host–vector interactions: evidence from plant viruses. Funct. Ecol. 26:1162–1175. Ragsdale D.W., B.P. McCornack, R.C. Venette, D.B. Potter, I.V. Macrae, E.W. Hodgson, M.E. O’Neal, K.D. Johnson, R. J. O’Neil, C.D. DiFonzo, T.E. Hunt, P.A. Glogoza, and E.M. Cullen. 2007. Economic threshold for soybean aphid (Hemiptera: Aphididae). J. Econ. Entomol. 100:1258–67. Ragsdale D.W., D.J. Voegtlin, and R.J. O’Neil. 2004. Soybean aphid biology in North America. Ann. Entomol. Soc. Am. 97:204–8. Wang, R., and S. Ghabrial. 2002. Effect of aphid behavior on efficiency of transmission of Soybean mosaic virus by the soybean-colonizing aphid, Aphis glycines. Plant Dis. 86: 1260–1264. SMV-infected leaf Healthy leaf Foam padding Weeks after aphid infestation Opaque screen Petri dish arena Figure 3. Soybean aphid population size on SMV-infected, mock inoculated and healthy control host plants. No statistically significant difference was found between treatments at each observation period. Figure 1. Choice test arena setup.