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How a crop nutrition problem becomes an insect problem

Potassium-stressed soybean plants – with their telltale yellowed leaf edges – can harbor large numbers of soybean aphids, insidious pests that can cause millions of dollars in damage to Wisconsin crops. In a study published in the April edition of the journal Environmental Entomology, University of Wisconsin-Madison entomologists examine the connection between potassium availability and aphid population growth, illustrating the important role that resource quality plays in the population dynamics of this emergent pest.

“There”s a lot of emphasis on understanding how aphid populations are affected by natural enemies – for example the Asian lady beetle,” explains Claudio Gratton, an entomologist with the UW-Madison”s College of Agricultural and Life Sciences. “In contrast to those “top-down” effects, this study shows that resources in the soil and plant quality also affect populations in a “bottom-up” way. I”m trying to build an understanding of how all of this ties together.”

The study, conducted by Gratton and his post-doctoral fellow Scott Myers, now with the USDA, began with a casual observation that soybean plants with yellowed leaves tend to have more aphids than nearby healthy plants. Stunted plants with leathery, yellowed leaves indicate potassium deficiency, which can occur at localized sites within a larger field, often the result of soil erosion or previous cropping rotations, or across entire fields in some parts of the state.

Plants regulate the concentration of potassium salt by secreting specific amino acids, which contain nitrogen – and soybean aphids, which feed on the sap from plants, thrive on nitrogen. Therefore, says Gratton, when soybean plants are deficient in potassium they secrete more nitrogen-containing amino acids to correct osmotic imbalances and become hotbeds for aphids, which damage crops and spread disease.

Gratton and Myers spent the summer of 2004 surveying 34 soybean fields across Wisconsin, taking soil and leaf samples and measuring aphid levels. They found that fields with the lowest soil and leaf potassium measurements had the fastest rates of aphid population growth. In a related experiment that summer at the CALS Arlington Agricultural Research Station, they added potassium to certain areas within a potassium-deficient soybean plot, and found that aphid populations doubled more quickly in the low-concentration areas than in the amended areas.

Aphids have been a concern in Wisconsin since the were first discovered in the Midwest in 2000, but some years seem to be worse than others. Because Gratton”s study was done in 2004, a low-aphid year, he cautions that additional work in a more severe year needs to be done. It”s still too early to say what 2006 will be like, but he does have some general advice for farmers.

“If you suspect you have areas subject to potassium deficiency, you should be on the alert,” he cautions. “It is possible that in a high-aphid year problems will likely be exacerbated. However, we still don”t know if small potassium-deficient patches within a field can create aphid hot-spots that are enough to eventually cause problems for the whole field.”