NDSU Crop and Pest Report

Entomology


ISSUE 13   July 29, 2004

WHEAT MIDGE FOLLOW-UP

As growth stage and degree day accumulation progress, most areas are moving past midge risk. Continue to scout wheat fields that are heading and have not flowered for their presence. Emergence is nearly complete at 1600 degree days, with survival and activity of adults winding down by 1800+ degree days.

 

SOYBEAN APHID ON OUR MINDS

A few more reports of aphid discoveries in soybean fields from the region, but still no reports of significant numbers. The North Dakota survey has not encountered soybean fields with aphids, though they are out there in low numbers based on the occasional report from some people who have called or written. The Minnesota crop survey has found a number of fields in the Grand Forks to Crookston to Red Lake Falls area where aphids were found. Typically these fields had a very low percent of the sampled plants with aphids (1 or 2 plants out of 30). Another report comes from out of the Fergus Falls area where numbers are very low . . . nothing similar to the previous two seasons.

Continue to watch for aphids as we move into August. The R2 to R4 growth stages will be with us through the first two weeks of August. These are the critical stages. As a reminder of how aphid populations and growth stages interact, yields from a replicated trial conducted in Prosper, ND in 2003 are summarized in along with quality in Table 1, and aphid infestation pressure (Aphid days and aphids per plant).

       


There were no significant differences in yield, plant height, pods per plant, hundred seed weight, or percent oil. Percent protein was significantly greater for one untreated check compared to the R5 treatment. The general conclusion is that the aphid infestation level was not great enough, nor did they establish early enough to cause significant impact on the variables evaluated. So keep this in mind as we enter the final couple of weeks when soybean aphid can have significant impact on yield.

For 2004, the recommended treatment threshold will be 250 aphids/plant on 80% or more of the plants when populations are actively increasing and the plant is in the R1 to R4 growth stage. Actively increasing aphid populations in the R5 stage are going to be a little more difficult to make decisions. However, if prices remain strong for soybean, even a 1 to 2 bushel advantage could provide a positive economic benefit in the R5 growth stage that in different economic times may not be realized.

Table 1. Yield Component Analysis for Soybean Aphid (Aphis glycines) Infested and Aphid_control study. Prosper, ND, 2003

Growth Stage at Treatment

Bu/Acre

Plant Height (cm)

Pods per Plant

100 seed wt (g)

Protein (%)

Oil (%)

Aphid-Days

UT check

60.1 a

105.6 b

29.7 a

24.4 a

35.1 ab

18.8 a

2868

R2

60.5 a

106.9 b

29.4 a

24.6 a

35.0 ab

18.9 a

54

R3

58.9 a

107.3 b

29.4 a

23.9 a

35.0 ab

18.9 a

249

R4

61.0 a

107.5 b

31.8 a

23.7 a

35.1 ab

18.7 a

432

R5

61.2 a

109.2 ab

34.0 a

24.3 a

34.9 b

18.8 a

1594

UT check

58.8 a

112.6 a

29.4 a

24.3 a

35.4 a

18.6 a

2428

Means followed by the same letter within a column are not significantly different (LSD, P=0.05)


Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu


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