S-B-A and K….what’s the link?
Editor’s note: This article is from the archives of the MSU Crop Advisory Team
Alerts. Check the label of any pesticide referenced to ensure your use is
included.
With the high cost of fertilizer this season, many producers are making
decisions about how much to purchase, how much to apply, and which
fields to treat. As you make these decisions, consider that insects as
well as plants are affected by fertilization.
In 2000, when soybean aphid (SBA) was first found in Michigan, growers
and extension educators reported severely yellowed plants in some
fields. These plants were described as “dripping” with aphids. The
yellowing resembled potassium (K) deficiency, although the symptoms were
top-down, not bottom-up. Plant tests confirmed that the plants were
indeed K-deficient. Yellowed, heavily infested plants were reported in
southwest Michigan in areas with sandy soil, on sandy knolls and in
soybean fields planted after alfalfa. All clues pointed to a link
between K deficiency and aphids, but which came first? The deficiency or
the dripping?
In 2004 and 2005, graduate student Abby Walter sampled plants from
symptomatic (yellow) and nearby green areas in commercial fields in
southwest Michigan. She also conducted a series of experiments in a
severely K-deficient field (soil test: 37 ppm) in southwest Michigan,
measuring how fast aphids reproduced in meter-square field cages in
replicated potash-fertilized and unfertilized plots. She then used tiny
clip cages to follow the reproduction of individual aphids on plants in
the same K-fertilized and unfertilized plots. Finally, she sampled
phloem sap from plants to determine the proportion of various important
amino acids. Here is a summary of her findings:
Hypothesis #1: Soybean aphid numbers are higher on K-deficient plant
Answer: Yes. In commercial fields, the
average number of aphids per leaf was 50 percent greater on the plants
with severe yellowing compared to plants growing in green areas of the
same field. Plants from deficient and non-deficient areas had the same
aphid number per plant. However, the K-deficient plants were stunted and
had fewer leaves per plant, so that aphid density was higher. This is why aphids appeared to “drip off” of symptomatic plants.
Hypothesis #2: Soybean aphid populations increase faster on K-deficient plants
Answer: Yes. In field cages infested with
soybean aphids on June 1, populations went over the economic threshold
and injury level (EIL) earlier, and increased to a much higher level, on
plants growing in unfertilized plots (dashed line) compared to plants
growing in K-fertilized plots (solid line). (View accompanying graph.)
Hypothesis #3: Soybean aphids reproduce faster on K-deficient plants
Answer: Yes. Aphids on plants growing in
unfertilized (deficient) plots reproduced at an earlier age
(approximately nine days) and produced more nymphs (nearly 90 per
female) than aphids on plants growing in K-fertilized plots (first
reproduced at 11 days, 60 nymphs total).
Hypothesis #4: K-deficient plants are nutritionally “better” for aphids
Answer: Probably. Abby found that the
proportion of the amino acid asparagine was greater in phloem sap from
deficient plants. Aphids feed on phloem sap, a poor source of nutrition
because it is low in nitrogen in the form of amino acids. Asparagine is a
non-essential amino acid that is an important nitrogen source in the
aphid diet; aphids can convert asparagine into essential amino acids
lacking in their diet! How do aphids do this? They have friendly
symbiotic bacteria living in their guts that do the job for them. So,
K-deficient plants do appear to be nutritionally better (i.e. aphid
candy), which may partially explain the quicker and greater reproduction
on such plants.
So what? First, this research demonstrates that crop fertility can
effect the population of, and damage from, an insect pest. If you need a
reason to spend time and money soil sampling and following
recommendations for applying potash to build soil K levels, reducing the
impact of soybean aphid is a good one. Second, even if you don’t plan
to apply potash, this research still provides you with valuable
information to select which fields to scout first and more often. Fields
with a history of K deficiency, or a soil type susceptible to
deficiency, or fields planted after alfalfa are all at greater risk for
an aphid outbreak.
In 2007, a very low aphid year in Michigan, the only fields that went
over threshold in August were K-deficient fields in southern Michigan.
The link between potassium deficient and soybean aphid is real, and can
lead to yield loss!
If you are a glutton for punishment, and want to read a journal article about this research, visit:
http://www.ingentaconnect.com/content/esa/envent/2007/00000036/00000001/art00004