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What are Soybean Aphids Going to Do in 2008?

Tracey Baute - Field Crop Entomologist/OMAFRA Ridgetown
Creation Date: 07 May 2008

Many are already asking about soybean aphids this year, given the early infestations that occurred in some areas of Ontario last year. Though overwintering populations appear to be low this year, Ontario tends to experience soybean aphids almost every year, at least as hot spots in some regions, giving less strength to the "every other year" scenario. Michigan too is unsure as to what kind of year 2008 could be. Though they experienced early aphids last year, the rest of 2007 was reasonably quiet. Fall trap counts in Michigan in 2007 make predictions difficult, as mentioned in the article by Chris DiFonzo (attached). All we can say for sure is to scout fields weekly this year to determine what aphid populations are doing. Given how early they arrived last year, I would also suggest scouting early planted fields at least once before bloom to ensure aphids have not arrived and require management early.

Another article Chris DiFonzo wrote is with regards to research done at MSU on the relationship of potassium deficient plants and soybean aphids. It sums up all of the questions we have been getting about this relationship and possible strategies to manage it. Have a read.

Insects to watch 2008 - Part III - soybean aphid
Christina DiFonzo, Entomology
Source: MSU Field Crop Advisory Team Alert newsletter, Vol. 23, No. 4, May 1st, 2008

Last season was a banner year for worms and beetles, but where were the soybean aphids? Chris DiFonzo, the self-proclaimed soybean aphid queen (that's me) predicted an aphid outbreak for 2007. Where was the outbreak? Will the prediction for 2008 be equally as accurate? Let's recap the 2007 season. In the winter of that year, entomologists in the Midwest predicted a soybean outbreak. That prediction was based on two lines of evidence: suction traps and egg numbers.

The Northcentral Aphid Suction Trap network consists of over 40 traps in 10 states, running in the summer and fall. Fall catches of soybean aphid are important because they consist of winged males and females leaving soybean for buckthorn, the overwintering host. The females land on buckthorn and have daughters; the daughters mate with the males (in human terms, mating with their uncles), and lay eggs. The egg stage allows soybean aphids to survive the cold northern winters. In general, a large fall flight predicts an aphid outbreak the following year, in this way:

Large fall flight of soybean aphid leads to…
More eggs on buckthorn in the winter, which means…
Higher spring aphid populations on buckthorn in May, so there is…
A heavier flight to the soybeans in June, and finally…
An aphid outbreak in July and August.

At the end of 2006, dozens to hundreds of winged soybean aphids were captured in the suction trap network (Table 1), The numbers may not seem high, but remember the trap is a vertical PVC pipe drawing in air 25-30 feet above ground. It is sampling a small amount of air overall, so the trap catch is a tiny fraction of the aphids actually flying from soybean to buckthorn.

In the winter of 2006-07, entomologists sampled buckthorn for soybean eggs. Eggs were found at numerous locations, hundreds on branches at some locations. Again, this may not seem very impressive, but aphid eggs are very small and generally difficult to find. Finding eggs at all, much less multiple eggs on a bud or branch, is significant. Because of the fall suction trap counts and egg numbers, we predicted an outbreak.

A spring freeze in April damaged buckthorn buds across the Midwest. In central Michigan, buckthorn shrubs growing along the edge of wood lots or the road were severely hit, and the damaged branches did not leaf out quickly, and did not support soybean aphids. However, many buckthorn shrubs further back from the edge were not damaged, and leafed out normally. Thus the aphid population was probably "dinged" a bit, but not knocked out.

In Michigan, aphids colonized soybean fields as normal, beginning in early June. By mid-June, fields in the Frankenmuth area were over threshold, reminiscent of the early aphid infestation in southwest Michigan in 2005 - our last big outbreak year. Only fields planted early (in April) went over threshold; May-planted fields emerged later and missed the aphid flight that colonized the April fields. Early-planted fields in Ontario also went over threshold. The over-threshold fields were treated, and predators and parasitic wasps attacked the survivors.

Aphid numbers increased slowly until early July. Then unseasonably hot weather and minimal canopy resulted in very hot temperatures at the soil surface. In research plots, the ground was too hot to sit on. That same week, the building aphid numbers took a dive. Soybean aphids can survive hot weather when they are under a cool plant canopy, but they likely cannot tolerate 90-degree days when plants are still small. The population growth rate appeared to slow, and perhaps predators had a chance to catch up. Fields were essentially aphid-free until August, until winged aphids blew in from surrounding states. Some fields went over threshold, but for most fields it was too late to build aphid populations.

Surrounding states, northern Indiana and the Toledo, Ohio area, had a similar experience to Michigan. An outbreak was predicted, but did not materialize. In contrast, many other states did have their predicted "aphid year." From Quebec to the Dakotas, millions of acres were treated for soybean aphid. Michigan didn't have an outbreak, but many other states did. We were the hole in the outbreak donut, so to speak.

What about 2008? Table 1 shows that the suction trap catches in the fall of 2007 were considerably lower than at the same locations in 2006, with one exception, Michigan. A couple of traps did catch aphid moving out of soybean to buckthorn. That makes some sense, because 2007 was essentially a non-outbreak year for Michigan, and low years tend to be followed by outbreak years. However, I have not found any eggs this winter on buckthorn. Across the Midwest, a low aphid year is expected in 2008, but Michigan is more of a mixed bag. We may see some pockets of infestation if certain locations have high egg numbers. The key is to scout fields to catch these potential infestations before they go over threshold.

Table 1. Soybean aphid suction trap catch in 2006 and 2007

S-B-A and K….what's the link?
Christina DiFonzo, Entomology
Source: MSU Field Crop Advisory Team Alert newsletter, Vol. 23, No. 4, May 1st, 2008

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.)

Table 2. Hypotheses - Soybean Aphid Populations increase on K-Deficient Plants

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).

Table 3. Soybean Aphids reproduce faster on K-Deficient plants

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