Aphid Alert: A research/outreach initiative addressing potato virus problems in the northern Midwest

Robert Suranyi, Ted Radcliffe, David Ragsdale,
Ian MacRae, Department of Entomology & Ben Lockhart,
Department of Plant Pathology, University of Minnesota, St. Paul, MN

The Aphid Alert Initiative

The 1998 Minnesota State Legislature provided funding to the University of Minnesota College of Agricultural, Food, and Environmental Sciences to address urgent issues challenging the state's agricultural sector (Special Initiatives and Rapid Response Projects). One of the first projects selected for support addresses aphid transmitted viruses in seed potatoes. Additional support has been provided by a USDA, North Central IPM grant. Project leaders are entomologists Dave Ragsdale, Ted Radcliffe, Ian MacRae and plant virologist Ben Lockhart. Aphid identifications and on-site project management are the responsibility of post-doctoral fellow, entomologist Robert Suranyi.

Potato Viruses Plague Northern Midwest

Seed potatoes from the northern Midwest have earned over the years a reputation for outstanding quality. At one time, almost 40% of seed potatoes shipped interstate in the U.S. originated in Minnesota and North Dakota. Recently, aphid-transmitted viruses have unexpectedly devastated the seed potato industry in the northern Midwest (Minnesota & North Dakota). Our situation is not unique; other seed potato production regions have also experienced increased inicidence of virus diseases, particularly potato leafroll virus (PLRV). However, in our region, we've had to contend with increases of both PLRV and potato virus Y (PVY), and the pressure has been severe.  The losses experienced since 1995 have contributed to a 40% decline in acreage entered into certification programs.  In 1997 and 1998, the proportion of seed lots rejected for virus levels exceeding tolerance in winter tests was unprecedented in the history of the seed certification programs of the two states.  To save this industry and assure supply of disease-free seed for a rapidly expanding regional potato processing industry, it is essential to provide seed growers with the biological monitoring and knowledge needed to end this cycle of aphid-transmitted virus spread.

Green Peach Aphid Outbreaks

Concomitantly, with this most recent epidemic of virus diseases, many commercial potato growers have experienced exceptional green peach aphid outbreaks.  Severe aphid pressure can cause premature senescence of plants ("aphid holes’) and yield losses up to 7 tons/acre.

Green peach aphid is the most efficient vector of both PLRV and PVY, but this aphid is especially important in the epidemiology of PLRV because of its ability to colonize potato.  As a consequence of severe green peach aphid pressure and resultant within field spread of  PLRV, net necrosis (a tuber condition caused by PLRV) has become a common problem for the first time in this region.  Since 1997, growers have had loads of potatoes downgraded or rejected by processors because of the presence of net necrosis. Similar problems have been experienced in Manitoba.

Factors Contributing to Perpetuation of Current Situation

For an epidemic to persist, as this one has, a number of factors must contribute. Of course, there is now much inoculum of both PLRV and PVY present in the area. Without sources of virus there could be no spread.  Because PLRV and PVY are aphid-transmitted viruses, an understanding of the population dynamics and behavior of these vectors is essential to implementating management.

It has long been recognized that green peach aphid outbreaks can be induced by insecticides targeted against other pests, e.g., Colorado potato beetle or potato leafhopper. This occurs because green peach aphid tends to be highly resistant to most broad spectrum insecticides, but these insecticides are devastating to parasites and predators that otherwise hold this pest in check.

More recently, we have demonstrated that intensive fungicide use can also contribute to increased green peach aphid pressure. The emergence and now predominance of a metalaxyl (Ridomil)-resistant strain (US8, A2 mating type) of Phytophtora infestans, causal agent of potato late blight, has precipitated major changes in fungicide use patterns in commercial and seed potato (both in chemistries used and frequency of application). Fungicides can be devastatingly disruptive to a group of beneficial fungi that are key mortality factors regulating the populations of green peach aphid (Lagnaoui and Radcliffe).

Another factor that appears important in our region is the increase and southern expansion of canola production. Canola is a very favorable host for green peach aphid. Green peach aphid does not overwinter outdoors in our area so populations must be reestablished each spring. We suspect that the presence of large expanses of canola favors the survival and increase of spring migrants of this aphid from the south.

Many of the best management tactics available to growers to prevent virus spread are preventative or prophylactic, e.g., early planting, use of clean seed, isolation from sources of inoculum, early season roguing, use of systemic insecticides at planting, and use of crop borders (DiFonzo et al.).   However, once the growing season begins the only management tactics available to growers are therapeutic , e.g., broadcast or border application of aphid-specific insecticides, application of mineral oils, and early vine kill.   Without systematic biological monitoring, growers and their consultants cannot have sufficient   information to make informed decisions in using these therapeutic measures.   Ending the present cycle of virus infection is essential to restore economic viability to the northern Midwest seed potato production and to assure adequate supplies of disease-free seed for a rapidly expanding regional potato processing industry.

How Aphid Alert Works

The Aphid Alert operates a trapping network to monitor aphid flight activity in seed potato production areas in Minnesota and North Dakota.  In 1998, we monitored aphid flight activity at 12 locations (1998 summary).  In 1999, we are operating 1 suction trap and 4 pan traps at each of 14 locations, 8 in Minnesota, 6 in North Dakota.  The present Aphid Alert trap sites were selected to include all major seed potato producing regions of the two states. A similar program was initiated by Agriculture and Agri-Food Canada in Manitoba in 1999 (McLaren).

We believe that a regional approach to monitoring is essential because the problem is shared and no respecter of state lines or international boundaries. We have agreed that future U.S. and Canadian aphid monitoring activities will be coordinated and a common advisory newsletter for growers/crop consultants developed. If sufficient funding can be generated, we intend to operate the region-wide, aphid-trapping network and continue associated research for 3 more years.

We believe continuation of this project is appropriate because the spatial and temporal dynamics of potato virus epidemiology are extremely complex and variable across years and locations. This is evident from comparison of 1998 and 1999 population trends of green peach aphid and the other aphid species that are potential PVY vectors. Our research goals are longterm, but, there is immediate payoff from data collected. Advisories and recommendations generated in the first 2 years of this project have been closely followed by growers and crop consultants in Minnesota, North Dakota and Manitoba, and are used in making management decisions. Indeed, more than 83% of growers and crop consultants (n=42) responding to a recently conducted survey indicated they read the advisories cover to cover and used the information in making management decisions.

Temporal Trends in Aphid Flight Activity

In future years, we plan to deploy two suction traps at each location. Until now, we have used 1 suction trap and 4 pan traps at each location. We have found no obvious differences in the selectivity of the two trap types. Suction traps are preferable because they are operated on timers and intake flow can be adjusted to only catch weak fliers. We now have strong evidence that for most of the season our traps are capturing winged adults originating from sources outside the potato field. This is exactly the information needed, since this provides warning of potential introduction of virus by in-coming aphids. Later in the season, as winged aphids begin to be produced on potatoes, trap captures may be of aphids that originated within that field. However, once winged aphids are developing on potatoes, immediate vine-kill is advisable since aphids from potato are likely to carry virus and inter-field movement is inevitable.

The aphids captured in the traps are sorted and identified to species on a real-time, i.e., within 48 h of collection and results reported on our WWW website, Aphid Alert and by e-mail to Aphid Alert subscribers. Printed copy of Aphid Alert is mailed weekly to all seed potato growers in Minnesota and North Dakota and to other individuals who request it. To subscribe to the e-mail version of our newsletter write to us at radcl001@umn.edu. Unfortunately, although electronic communication provides real-time communication, most of our growers must, or do, rely on printed copy.

Detecting Virus in Aphids

PCR primers for PLRV and PVY are available. Immunocapture-PCR (IC-PCR) assays are so sensitive that we (Lockhart or R. P. Singh in Canada) can identify one viruliferous aphid in a sample of 50 aphids (even after capture in propylene glycol and storage in ethanol). PLRV positives can be detected even when diluted 100,000 fold. Unfortunately, only a small number of aphids are captured and tested. Thus, unless the proportion of positives is high (as it was much of 1998) detection of aphids carrying virus can substantially lag onset of spread (as has been the case in 1999). Information on the incidence of virus is nevertheless critical to understanding the epidemiology of virus spread. Thus, we will continue in future years to assay all green peach aphids captured for both PLRV and PVY.

Spatial Aspects of Managing Virus Epidemics

Spatial components are as important as temporal considerations in the understanding of epidemics.  State seed potato inspectors collaborate with us by providing global positioning (GPS) to record the coordinates of all seed potato fields in Minnesota and North Dakota. They also record adjacent crops in all directions that might serve as reservoirs of virus and vector aphids. Results of summer field inspections and winter indexing for virus are also provided to the PIs by the state agencies. This spatial data is being analyzed (by MacRae) using Geographical Information System (GIS) software. Two Ph.D. students, Matt Carroll, who joined our laboratory this past month, and an M.S. meteorologist who is expected to join our laboratory next spring, will explore the spatial and temporal aspects of potato/virus epidemiology for their dissertations.

Effective geographic isolation of potato seed lots from potential virus inoculum is not possible in Minnesota and North Dakota; the distances required (estimated by some European workers to be approximately 80 km) are far greater than the physical separation feasible in this production region. However, our preliminary data indicate that immediate proximity to sources of inoculum and aphid pressure are actually the key determinants of risk.

Aphid species that vector PVY and PLRV may increase in crop fields other than potato, resulting in a dispersal of these insects into adjacent potato fields. Dispersal of vectors from these alternative hosts tends to be associated with increased occurrence of the viral diseases in adjacent potato fields. Aphid populations are greatly influenced by their host plants, e.g., canola can support large populations of green peach aphid and turnip aphid. The recent southward expansion of canola production has provided vast tracts of an exceptionally suitable host to green peach aphid arriving from southern overwintering sites.  Green peach aphid was not a significant factor in a previous PVY epidemic in the Red River Valley in the early 1990s when little canola was being grown.

Rotational cropping systems present a landscape mosaic of different host plant species that impact aphid population dynamics. Crops in which aphid species readily increase favor dispersal into nearby potato fields. Flight ability and local weather conditions will determine dispersal capability. We propose to combine laboratory and field data to develop predictions of aphid vector population growth, and dispersal into and within potato fields.

Selected fields and surrounding regional cropping systems will be mapped using Geographic Information Systems (GIS) and aphid movement into the potato fields monitored with a system of traps. Influence of wind, temperature and precipitation will be incorporated into the GIS models. Aphid populations in selected neighboring crops will be monitored and numbers and life stages recorded. This information will be combined with the trap catches to evaluate the influence of other crops on aphid populations immigrating into potato fields.

The critical local distance over which aphid vectors can disperse will be established by combining laboratory wind tunnel experiments and mark/recapture field trials. Wind tunnel experiments will involve flying tethered aphids in a frictionless flight mill. Mark/recapture experiments will be conducted by marking aphids with fluorescent dust and monitoring dispersal between other crops and potato and within potato fields. These critical distances will be used to establish a threat radius within which seed potato lots would be subjected to unacceptable risk of high populations of the aphid vectors of PVY and PLRV.

Application of Global Information System technology

Mid and late season establishment of aphid populations within potato fields will be described by weekly sampling and GIS mapping of the distribution and density of aphid vectors in commercial fields. Preliminary data suggest that aphids first settle at field margins and subsequently disperse across the field as the season progresses (image below). This spatial-temporal distribution of aphids in potato fields provides insight into the local dispersal of aphid vectors and can be used to develop site specific treatment recommendations.

Increasingly darker colors denote increasing aphid populations. A significant edge effect is shown with highest aphid populations being found at the edge of the field while populations at the center of the field are much lower. Map constructed with the GIS, ArcInfo©.