MN Fruit & Vegetable IPM News - Vol. 1, No. 1

The incidence and severity of mosaic viruses in cucurbits were much greater than normal in several local areas of northern Illinois, northwestern Indiana, and southern Michigan in 2003. Outbreaks in pumpkins and squash developed earlier than usual and caused much greater losses in yield. Cucumber mosaic virus outbreaks were severe in mid- to late-season snap beans grown for processing in northwestern Indiana as well.

These viruses are carried by several different aphid species, and the species responsible for transmitting the viruses are not limited to those that colonize the crops (that is, reproduce and remain on the crop, producing colonies or groups of aphids on leaves or stems). Aphids move on winds as they search for the crop or weed plants that serve as good hosts for their development. As they do so, they land and insert their needle-like stylet (piercing beak-like mouthparts) into leaves or stems to take up a little sap and determine whether or not they will stay or move on. Many of the mosaic viruses of cucurbits and other vegetable crops are transmitted by aphids when they land on an infected plant (often a perennial or winter annual weed in which the virus has overwintered), probe it with their stylet and pick up the virus, then fly to a crop plant, probe it with their stylet, and inoculate it with the virus. Most of the common viruses carried to vegetables by aphids are transmitted in a nonpersistent manner - they are carried by the aphid from an infected plant to the next one or two or three plants that the aphid probes, but the virus is lost from the mouthparts very rapidly. This is key in virus management because by the time an insecticide applied to the crop can kill the aphids that carry a virus into a field from surrounding weeds, all the virus transmission is accomplished anyway.

Why the greater problem in 2003 than in other recent years? The answer is not clear. I suspect that the number of overwintered weed hosts infected with the viruses that caused problems in 2003 was not significantly different than in previous years, although it may have been (as a result of soybean aphid ... see below). It is more likely that flights of aphid vectors were earlier and/or heavier in 2003 than in previous years. Two scenarios could have produced earlier and greater numbers of aphids moving through vegetables in 2003. One is that flights of aphids from southern states resulted in earlier and greater numbers of vectors in the affected areas. This is quite possible. Several aphid species (and other insects) that commonly infest Illinois crops do not survive the winter here, and instead the survivors in southern states fly up into convection currents and are carried hundreds of miles on storm fronts before dropping to the ground. (No, it's not validation of Chicken Little's "the sky is falling" hysteria, but it is somewhat like microscopic plankton moving thousands of miles in ocean currents.) Observations of the deposition of larger insects (potato leafhoppers and corn earworm moths in particular) from storm cells suggests that large numbers may reach a certain area, while a few miles away none of the insects are "dropped off." Insects may "swirl" into northern Illinois and miss central Illinois (think of the cloud patterns and rain patterns you see on weather maps). If this "storm cell transport" occurred a few weeks earlier than average in 2003, it might explain why virus problems were more severe in some areas than others, and it might account for greater problems in northern instead of southern portions of Illinois and Indiana. This is not really a predictable phenomenon, and there is no way to forecast whether or not it will occur in 2004 or any other year in the future.

There is, however, another possible explanation for the greater number of vectors moving through vegetables in 2003 - the soybean aphid does overwinter successfully in Illinois (and in IA, MN, WI, MI, and IN ... and elsewhere) and is more prevalent in the north than the south, at least so far. This insect is of Chinese origin and was first discovered in the US in 2000. In the Midwest, populations have been greatest in southern Minnesota and Wisconsin, northern Illinois, northern Indiana, and southern Michigan. Winged adults of the soybean aphid fly from soybeans to buckthorn in the late summer and fall to lay eggs. Eggs overwinter on buckthorn and two generations (all female) develop on buckthorn in the spring. (In a large portion of their life cycle, most aphid species are all female and they reproduce without mating, giving birth to live young that also are all female ... sort of explains their rapid population growth, right?) Adults of the next generation fly to soybeans. Flights across weedy areas and vegetable fields occur when aphids move from buckthorn to soybeans, from too-heavily infested soybeans to a new patch of soybeans, and from soybeans back to buckthorn in August and September. Large suction traps that draw in samples of air at a height of 25 feet (operated as part of a survey project by Dr. David Voegtlin, Center for Economic Entomology, Illinois Natural History Survey) are used to monitor soybean aphids at several locations around Illinois. Counts from the Dekalb trap were hundreds of times greater in 2003 than in 2001 or 2002. Scouting of soybean fields resulted in reports of earlier and heavier infestations in 2003 as well.

So did the soybean aphid serve as the virus vector and account for all the problems in 2003? Again, the answer is evasive. The soybean aphid has been shown to transmit at least some of the viruses that infected cucurbits and snap beans in 2003 (including cucumber mosaic virus), and it is possible if not likely that it transmits most of them. Unfortunately, it's difficult to determine after-the-fact whether long-distance migrants or soybean aphids were the culprits, and therefore it's hard to predict what might happen in 2004. If soybean aphid is an efficient vector of several viruses of vegetables, one has to expect that their incidence and severity will increase as the soybean aphid becomes an established and naturalized citizen pest of the Midwest ... and it will, just as the European corn borer, alfalfa weevil, imported cabbage worm, and countless other "exotic" organisms have. If the 2003 problem resulted from an aberrant early arrival of migrant vectors from southern states, no pattern can be predicted.

Planning for 2004: The steps that growers can take to reduce losses have to be taken before aphid movement can be detected or quantified. So for 2004, steps that might provide some benefit include:

Plant resistant varieties ... talk with your seed supplier about what varieties have virus resistance.
Plant as early as possible. Plants that get a head-start on growth before they are infected will yield more fruit and higher quality fruit.
Make multiple plantings over a series of dates and at different locations on the farm ... hoping that the odds will favor some plantings escaping inoculation by migrating aphids.
Plant on reflective mulches ... reflective mulches reduce aphid landing rates.
Use row covers if practical to prevent aphid access to young plants. Keep the row covers on as long as possible.
Use high-volume, thorough-coverage applications of stylet oils (such as JMS Stylet Oil or Glacial Oil), especially on seedlings and small plants and before any aphids have reached the plants. Stylet oils do not kill aphids; instead they interfere with virus infection. Results of trials that have evaluated styelt oils on a range of crops are HIGHLY variable ... they do not always work.

For viruses that are transmitted in a nonpersistent manner, insecticides applied to the crop (whether applied as soil or seed treatment systemics or as foliar sprays) WILL NOT PREVENT VIRUS TRANSMISSION AND ARE A WASTE OF MONEY. Virus transmission is completed before insecticides kill the aphid vectors.

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@umn.edu
Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Division,jeanne.ciborowski@state.mn.us
Suzanne Wold-Burkness, Department of Entomology, University of Minnesota, woldx018@umn.edu

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Tuesday to Jeanne Ciborowski, 651-297-3217, jeanne.ciborowski@state.mn.us , MDA, 90 W. Plato Blvd., St. Paul, MN 55107-2094. You can access the Newsletter at the U of MN web site in htm. format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf. format at: www.mda.state.mn.us/biocon/fruitreports/default.htm

Partial funding for this publication is provided through partnership agreements with the Minnesota Fruit and Vegetable Growers Association and the United States Department of Agriculture – Risk Management Agency (RMA) and the RMA Community Outreach and Assistance Partnership Program. These institutions are equal opportunity providers.

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Last Revised May 7, 2004.
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