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ISSUE 14  August 27, 2009

INCREASED RISK OF SOYBEAN APHIDS ON K-DEFICIENT SOYBEANS

In a recent NDSU Ag News Release, Dr. R. Jay Goos of NDSU reported an increasing number of calls on potassium (K) deficiency symptoms in soybeans, especially on fields with sandier textures and a longer history of soybean production. Dr. Goos mentioned that the large increase in soybean acreage is partially responsible for this problem. A 40-bushel per acre soybean crop removes almost 60 pounds per acre of K2O equivalent where as a 120-bushel per acre corn crop removes only 20 pounds per acre of K2O equivalent. Symptoms of potassium deficient soybeans appears as a yellowing (chlorosis) or death (necrosis) along the edge of the leaves on older leaves (Fig. 1). For the complete article, see the following weblink:

http://www.ag.ndsu.edu/news/


Figure 1.
Symptoms of potassium
deficient soybeans
(J. Goos, NDSU)

The following report on soybean aphid and potassium deficient soybean fields was written by Dr. D. Hogg and published in the Wisconsin Crop Manager Vol. 16 No. 24, and is being reproduced here with his permission.

Soybean fields with suboptimal potassium levels are at greater risk of soybean aphid population increase and yield loss.

Following the 2000 discovery of the soybean aphid in Wisconsin, entomologists and agronomists noticed that soybean aphid infestations seemed to be more sever in K deficient soybeans. In Figure 2, the yellow beans on the left were literally dripping with soybean aphids and were presumed to be K deficient, whereas the healthy beans on the right had few aphids and were thought to have adequate K. The demarcation line follows the field contour.


Figure 2.
Soybean aphid infestation in
K-deficient soybeans
(John Wedberg, Univ. WI)

Subsequent research has proved this observation to be correct, plus we now have a better understanding of why this occurs. What happens is that low K actually makes the soybeans more nutritious for soybean aphids, promoting higher aphid reproduction and leading to more rapid aphid populations increase. To give an idea of how this might work, under field conditions in a K deficient field an aphid infestation can increase from 10 per plant to 230 per plant in 10 days; in a field with adequate K, that same population would increase from 10 to 150 aphids per plant. Further research suggests that K deficient beans have a greater percentage of asparagine in the plant phloem where the aphids are feeding. Asparagine is known to be an important amino acid for aphid nutrition.

Finally, we think the yellowing of K deficient soybean leaves may preferentially attract migrating soybean aphids, placing K deficient field at a further disadvantage. The color yellow has been shown to be highly attractive to a number of aphids.

Bottom line, maintaining adequate K levels in soybeans goes a long way toward managing soybean aphid.

David Hogg, University of Wisconsin

 

SOYBEAN APHID POPULATIONS BELOW ECONOMIC THRESHOLD LEVELS

With the cool weather, soybean aphid populations have remained steady and most fields remain below economic threshold levels (an average of 250 aphids per plant and 80% of plants infested in a field). Soybean fields are maturing and in the R5 (beginning seed) to R6 (full seed) stage of development where they are less susceptible to yield loss from soybean aphids. If treatment is necessary, early R5 stage soybeans are more likely to have a positive yield increase than soybeans in the late R5 stage. Treatments for aphids are not recommended after the R6 (full seed). If producers need to spray a mature soybean field, remember to check the Pre-Harvest Intervals (PHI) on insecticide label.

 

SWATH GRAIN WITH HEAVY INFESTATIONS OF WHEAT STEM SAWFLY

Due to the large number of complaints of lodged wheat from wheat stem sawfly in southwestern and west central North Dakota this year, I’m re-printing this article that was published in the July 23rd Issue 11 of Crop & Pest Report to help producers with decision-making for swathing grain infested with sawfly.

Producers should sample wheat crops and determine the percent of plants infested by sawflies before harvest. The presence of wheat stem sawfly can be verified by splitting stems and looking for the S-shaped larvae about ½ to ¾ inch long and cream colored with dark head inside the stems (Fig. 3).


Figure 3.
Larva and sawdust-like frass
of wheat stem sawfly inside stem

(W. Morrill, Montana State Univ.)

Another symptom of sawfly feeding is the presence of sawdust-like frass inside the wheat stem. Infested wheat stems often have a darkened area on the stem just below the nodes as a result of the internal feeding from sawfly. This can be used to detect a sawfly infestation without splitting the stems. However, it is best to split stems to confirm sawfly infested stems. If more than 15% of stems are infested by sawflies, producers should swath the wheat crop. Producers should swath sawfly-infested wheat as soon as kernel moisture drops below 40% to save infested stems before they lodge. If producers decide to swath grain, use a high swathing height to conserve the parasitoids that attack wheat stem sawfly. Research from Montana State University (source: Dr. David Weaver) has shown that taller residue (at least the lower of the plant)is better for conserving the parasitoids.

If 10 to 15% of the crop was cut by sawfly in 2009, a solid-stemmed variety of wheat is recommended for 2010 wheat planting. The 2009 NDSU hard red spring wheat release is named ‘Mott’ with the solid-stem characteristic that has high resistance to wheat stem sawfly.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu

 

BIOLOGICAL CONTROL OF CANADA THISTLE

Canada thistle (Cirsium arvense) is an invasive weed that currently infests over one million acres in North Dakota. Current management programs often rely on herbicides, mowing, tillage, and crop rotation, although more effective integrated pest management methods are needed. The North Dakota Department of Agriculture (NDDA) is one organization interested in controlling this noxious weed, and in 2004, they released a biological control agent, Ceutorhynchus (Hadroplontus) litura across North Dakota. Ceutorhynchus litura is a weevil (a type of beetle) (Fig. 4) , and the larval stages of this insect feed within Canada thistle stems (Fig. 5).


Figure 4.
Adult Ceutorhynchus litura
(N. Rees, USDA Ag Res. Serv, Bugwood.org)


Figure 5.
Larvae of Ceutorhynchus litura
(N. Rees, USDA Ag Res. Serv., Bugwood.org)

Although C. litura is generally considered the most effective biological control agent of Canada thistle, how much it reduces thistle populations is unclear, as these weeds can often compensate for weevil damage after larvae stop feeding and pupate in mid-summer. The general consensus by researchers in the U.S. and Canada is that larval feeding can make plants more susceptible to other stresses, such as diseases and adverse environmental conditions, and a combination of factors will likely result in thistle suppression.

Recently, the NDDA partnered with entomologists (Prischmann-Voldseth, Harris) and weed scientists at NDSU (Gramig, Lym) to begin investigating C. litura biology and the ability of this insect to control Canada thistle. Current project objectives include evaluating the success of the 2004 NDDA weevil releases, establishing new release sites, exploring effective sampling methods for larvae and adults, and attempting to maintain C. litura on both Canada and Flodman’s thistle (a native thistle species).

Deirdre A. Prischmann-Voldseth
NDSU Entomologist
Deirdre.Prischmann@ndsu.edu


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