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Soybean aphids benefit from potassium deficiency

Potassium deficiency makes soybean plants more nutritious for soybean aphids, but the threshold at which the effect occurs has not been pinpointed. Photo courtesy of Horst Bohner, OMAFRA.

Crop yields are reduced by both nutrient deficiencies and pests, so one can only imagine what the reduction in yield might be if nutrient deficiency caused increased pest levels. The individual effects on soybean yields of the combination of potassium (K) deficiency and soybean aphids still need to be teased apart. However, researchers do know that soybean aphids produce more offspring faster on K-deficient plants than on K-sufficient plants, and they have some idea why.

When soybean aphids first arrived in North America, people began to notice that there seemed to be more aphids on yellowish soybean plants than on healthy plants. What was causing this phenomenon? Large-scale field surveys of soybean aphids and soil and tissue chemistry by University of Wisconsin-Madison entomologist Dr. Claudio Gratton and his postdoctoral researcher Dr. Scott Myers, and by Michigan State University entomologist Dr. Christina DiFonzo and her graduate student Abigail Walter, indicated a correlation between aphid population growth and soil and soybean-tissue K. “The aphid populations tended to grow the fastest when leaf K tended to be the lowest and soil K tended to be the lowest in a particular field,” explains Gratton.

The presence of soybean aphids increased soybean cyst nematode infection on roots of resistant varieties.Photo courtesy of Dr. Felicitas Avendaño, Grand View University, Iowa.

To test the effects of K deficiency, both groups performed field experiments in which they added K to fields that were K-deficient. Gratton and Myers used a field in south-central Wisconsin that had been planted previously to alfalfa and corn. “We had no addition, moderate addition, and high addition of K fertilizer so that we would create low, medium and high levels of K in the soil,” says Gratton. These corresponded to soil-test K levels of about 60, 110 and 140 ppm. Gratton and Myers then monitored aphid growth in two ways. “We followed the ambient level of aphids that got there on their own and then grew on their own.” They also put individual aphids in small clip cages on individual leaves. “We started them as babies, we put them on the plant, and we monitored them every day for 30 days.”

The results confirmed their hypothesis that K-deficiency was beneficial for soybean aphids. “When we looked at the aphids in these little plots and in the clip cages where we knew nothing else had been changed except that we added K or not, there was a very strong and significant effect of those K additions on aphid population growth. The effect was such that if you didn’t add K, the aphids did a lot better than if you did add K,” says Gratton. The aphid population growth was better on K-deficient plants, i.e., in plots that received no K amendment, because many more offspring were produced than in plots that received K amendment, whether the resultant soil-test K levels were moderate or high.

In addition to faster aphid population growth on K-deficient plants, the plants produced half the yield of plants that received a K amendment. However, there were no differences between plots that received moderate or high K addition. The critical range of soil-test K for aphids is still undetermined. “We don’t know where that cutoff is,” says Gratton. “We just know that at 60 ppm, you’re probably going to be in trouble.”

It is important to note that soil-test K values may not be exactly comparable among studies and regions, depending on the measurement methods used.

Michigan results showed similarities and differences
In a similar experiment, DiFonzo and Walter amended a K-deficient, sandy soil in a Michigan field. “The K levels were extremely low to the point where the soil scientist here said it would take perhaps three years to bring levels up to what he would consider to be good,” says DiFonzo. “They were all well below the Michigan State University recommendation of 150 ppm.” In fact, their amendments only increased the soil-test K levels slightly, so that K-deficient soybean plots had, on average, 29 and 42 ppm soil-test K in two consecutive years, and amended plots had on average 34 and 53 ppm soil-test K in the same two years.

However, when DiFonzo and Walter monitored the aphid populations, they found differences between the K-deficient and K-amended areas. “In the areas that were K-deficient, the aphid populations increased faster. They actually crossed the economic threshold of 250 aphids per plant sooner than in places where we had applied potash,” states DiFonzo. In following individual aphids, they found that “aphids that were on the leaves in the more deficient areas became mothers sooner and had more babies per day.”

What is it about K-deficient soybean plants that makes them so much better for aphids? To get at this question, DiFonzo and Walter analyzed the composition of the phloem sap, which is the aphids’ food source. “We found a couple of amino acids in the plant that were correlated with the K deficiency,” says DiFonzo. She explains that when the soybean plant is K deficient, certain amino acids that are beneficial for aphids are released into the phloem in greater amounts than normal. “The bottom line is, a K-deficient plant has phloem sap in it that is more nutritious for the aphid. The aphids reproduce better on those plants, and as a result, their populations build faster. In the end, the number of aphids per leaf on those plants is higher.”

In a larger-scale study, Gratton and colleagues from Michigan, Wisconsin, Iowa and Minnesota looked at how the size of soybean aphid populations is related to aspects of the landscape such as the size and number of areas with different vegetation types, e.g., corn fields, soybean fields, wetlands and forests. “One of the really interesting things that we found was that when you looked over this entire upper Midwest area, the aphid populations tended to be the highest in areas that tended to be dominated by corn/soybean rotations,” states Gratton. And those landscapes tended to have the lowest K levels in the soybean leaves. Gratton thinks that this might indicate that the corn/soybean rotation tends to deplete soil K, particularly if growers do not do regular soil tests and amend their fields as needed. “If you are in that kind of a rotation, then you may be at increased risk for having K deficiency and therefore have higher aphid populations. So you should be a little more vigilant in what those values are for your own fields,” he suggests.

Pest management implications
As researchers learn more about how soil fertility affects various insects and diseases, nutrient management could become an important aspect of integrated pest management. The research suggests that by ensuring soil K levels are adequate, growers may be making conditions less favourable for soybean aphid reproduction. This could delay or reduce the risk of soybean aphid levels exceeding the economic threshold.

However, “soybean aphids cannot be controlled with the addition of K in low-testing fields since it’s only one of many factors influencing reproduction,” states Horst Bohner, soybean specialist with the Ontario Ministry of Agriculture, Food, and Rural Affairs. “Also, adding potash to fields with medium or high levels of K will have no impact on aphids.”

Growers who know that they have a soybean field or area of a field that is at risk of low K, perhaps because of the rotation, soil type or lack of K amendment, should be particularly vigilant for soybean aphids, suggests DiFonzo. “I would scout those more frequently, and even in a year that’s not a good aphid year. Scout those fields first and keep scouting them, even through early to mid-August, because sometimes those pop up late as being the fields that go over threshold.”

Pest control approaches may benefit from increased attention to bottom-up effects of plant quality, in addition to top-down effects of natural enemies. “Most of the time, when people think about aphid populations, they think about things that are related to the natural enemies,” says Gratton. “There’s no doubt that they play a really important role. But I think what this particular work shows is that there are other aspects of the aphid–plant interaction that might also be contributing to the aphid populations that could be of relevance to management. We should try to take those into account simultaneously.”

Potassium deficiency can be highly localized and has several potential causes. Photo courtesy of Horst Bohner, OMAFRA.

Managing K deficiency
Symptoms of K deficiency can result from poor fertility caused by rotation, crop, soil type and fertilization practices. However, plants can also exhibit K deficiency, even when soil K levels are good, under conditions of drought or soil compaction. “Just because you may have seen K deficiency in a field, it does not necessarily mean that you need to apply fertilizer,” says Horst Bohner, soybean specialist with the Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA).  “The only real way to know is to do a soil test.”

Dr. Richard Wolkowski, extension soil scientist at the University of Wisconsin-Madison, works with compaction- and drought-induced K deficiency issues. “Compaction is probably the most significant cause of induced K deficiency,” says Wolkowski. Compaction creates wetter soil by increasing the number of small soil pores that prevent water from draining, causing limited soil aeration. “The plant roots are not able to respire, take in oxygen, to create the energy they need to actively take up K. So, it’s not uncommon that you can induce K deficiency where you have compaction either from heavy wheel traffic or, in some cases, long-term no till, especially where there are some wheel tracks.” In contrast, drought limits the diffusion of K through the soil to the plant roots, “so it’s possible that it would not be supplied to meet the crop’s need.”

Unfortunately, there is not much that can be done once the plants are showing symptoms of K deficiency.  “There certainly are some foliar K products to apply,” says Bohner. “But so far, the trial results have been quite variable in success in terms of economic response. You can get some response, but whether it’s economic, on average, is unproven in Ontario to date. We are continuing to do trials to answer this question.”

Wolkowski says that the best thing to do is to avoid compaction in the first place. “The keys there are to stay off wet soils, try to limit your load weights whenever possible, and manage your traffic so that you drive over as little of the surface area of the field as you can. Multiple passes in one area is much better than spreading that traffic out over the field.”
If care is taken to maintain fertility and control wheel traffic, the no-till system can be as, or more, advantageous than tillage, says Wolkowski. He describes a 10-year study of chisel, strip and no-till for soybeans planted in 30-inch rows. “In eight out of 10 years, the cost of production for growing the soybean in no till was less than the cost of production for chisel plowing.  In seven out of ten years, it was lower compared to strip till. You may get a slight yield hit, but the money you save by not burning up the fuel to do those tillage operations is offsetting any yield loss.”

When to apply potash
If a soil test indicates adequate levels of K, potash application is not necessary. According to OMAFRA’s Agronomy Guide for Field Crops, ammonium acetate soil-test K results of 0–60 and 61–120 ppm in Ontario indicate low and medium levels of K, respectively. These soils should receive potash amendment. Amendment is not recommended for soils with levels greater than 120 ppm. 

Potash can be banded or broadcast. “Should you be putting on a 2-by-2-inch band in the springtime, and is that more efficient than broadcast and incorporation in the spring?” muses Bohner. “We have data to suggest that a band will give more yield than broadcast in the spring, but only when soil tests are low.”

Rather than think of it as a soybean feeding issue, Bohner advises growers to think in terms of a long-term, field maintenance strategy. “If your potash level is low and you can get a reasonable cost on the fertilizer, you should think about fixing that field, not only for the sake of the soybeans, but also for the sake of the following crops, including soybeans the next time around. I think that is the best approach to fertilizing phosphorus and K.”

Microplots were independently infested with soybean cyst nematode (orange flags) or left uninfested (yellow flags). Five soybean varieties were grown inside exclusion cages, with one plant per cage. Soybean aphids were added to half of the cages and monitored regularly for one month. Photo courtesy of Dr. Felicitas Avendaño, Grand View University, Iowa.

Soybean aphids may compound soybean cyst nematode problems
Soybean aphids may reduce the efficacy of soybean cyst nematode (SCN)-resistant soybean varieties, says Dr. Felicitas Avendaño, assistant professor at Grand View University in Iowa. As a postdoctoral researcher working with nematologist Dr. Greg Tylka and entomologist Dr. Matt O’Neal at Iowa State University, she performed experiments by putting soybean aphids, SCN, or both on soybean varieties that were either resistant or susceptible to SCN. 

The aphids and SCN did not seem to directly affect each other’s life cycle, and aphid population growth did not seem to be affected by the presence or absence of the nematode.

Interestingly though, “soybean aphids affected the level of resistance of the plant to the nematode,” says Avendaño. Few nematodes infected the resistant varieties when aphids were not present. But when aphids were there, more nematodes were able to colonize the resistant varieties. 

This interaction also affected the soybean yield.  The SCN-resistant plants had similar yield with and without the nematode. “But when you had the nematode and the aphid together, the yield was reduced,” says Avendaño. To maintain soybean yields, people who know that they have SCN in their field and are planting SCN-resistant varieties might be wise to keep a sharper eye out for aphids.