NDSU Crop and Pest Report

Entomology


ISSUE 16  August 21, 2003

SOYBEAN APHID POPULATION INCREASES . . . WHAT SHOULD WE DO?

In general, the threshold should still be 250+ aphids on a per plant basis. Another way the threshold has been expressed is an average of 25 aphids per leaflet (though this may be difficult now as aphids move to stems and pods).

There is a good discussion from the U of Wisconsin crop newsletter that tries to address the threshold issue from a growth stage standpoint with a fluctuating scale.

This discussion can be found on the internet at:

http://ipcm.wisc.edu/wcm/pdfs/2003/03_20insect1.html

Here is a summary of U of Wisconsin:

  • Widely accepted minimum economic threshold for soybean aphid is ~250 aphids per plant with 80% of plants throughout the field infested at that level (R1 to R2 growth stage)
  • Later growth stages may tolerate higher numbers before yield loss based on trials in 2001 and 2002. Wisconsin's suggested threshold scale is: 1,000+ aphids/plant at beginning pod (R3), and 1,500+ aphids/plant at full pod (R4). (these aphid numbers are much higher than people are generally reporting in our area - P. Glogoza)
  • In Minnesota, 2002 field trials suggest impact from aphids during the R1 _ R4 stages, even when populations were below the 250 aphids per plant. Remember, last year the populations started increasing at R3, much like this year. Because of those dramatic increases last year, the response was to widen the risk window to include R3 and R4. So, the jury is still out on the thresholds and growth stage interactions.

    NOW, most of the North Dakota fields are in the R5, approaching R6 growth stage. Populations are increasing in many fields here on the eastern edge of the state. Again, We saw this last year . . . BUT the increase was short lived. Many of the aphid nymphs are beginning to develop the wing buds (see last weeks newsletter). These eventually winged aphids will be taking flight shortly, which means that aphids you see today may be gone by Monday.

    I THINK, the late growth stage combined with the high likelihood that aphids are going to be abandoning fields soon suggest that most of these population increases in fields are not likely to result in significant yield impact. I would suggest staying with the 250+ aphids per plant, 80% of the plants impacted, AND be more concerned about later planted fields that may still be in the R4 - early R5 stages.

    Treating fields in advanced growth stages that have marginal numbers of aphids based on the above threshold is not likely to produce a significant economic return. This situation may be very similar to the cereal aphids in heading wheat that we experience. Yes, there may be a

    slight yield advantage in a treated site versus an untreated, but does it protect more yield than the cost of control? In wheat, it typically is a "breakeven at best" proposition. The same may be expected in soybean.

    It may be that a good rain would do more good than an insecticide treatment in the vast majority of these fields.

    Also, we need to keep post harvest interval in mind at this late date: Asana - 21 days, Baythroid - 45 days, Furadan - 21 days, Lorsban - 28 days, Mustang - 21 days, Penncap M - 20 days, Pounce - 60 days, Warrior - 45 days.

    Currently, we have a field trial where we have been treating aphids weekly over the past four weeks. The final treatment goes out today, Wednesday, 8/20. Hopefully this will provide further insight into thresholds for next year when regional data is analyzed and we can see aphid population impact from Wisconsin-Minnesota-North Dakota with different size populations, different growth stage, etc,

    Phillip Glogoza, Extension Entomologist
    pglogoza@ndsuext.nodak.edu

     

    the following is excerpted from Wisconsin Crop Manager, prepared by Eileen Cullen, Field Crops Entomologist. This is the same article referenced earlier, but with some editing for local conditions.

    What is THE soybean aphid threshold?

    A single, sure fit, economic threshold has not been established for soybean aphid. Research is on_going throughout the north central states to better correlate aphid population densities with plant injury symptoms over the growing season and yield loss at harvest. The widely accepted minimum economic threshold for soybean aphid is ~250 aphids per plant with 80% of plants throughout the field infested at that level. Below this population level, yield loss is not expected to justify treatment cost. At this level, aphid colonies have the potential to increase to economically damaging levels. At R3_R4 (large plants) this is a conservative threshold. UW Extension Publ. A3646 pest management in Wisconsin field crops, recommends waiting until there are at least several hundred aphids per plant before treating.

    UW economic threshold and spray timing guidelines were updated earlier this year:

    http://soybean.agronomy.wisc.edu/publications/aphid2003.pdf

    These guidelines are based on review of sixteen economic threshold and spray timing studies which indicated that soybean aphid thresholds vary by crop phenology. Suggested threshold ranges are 200+ aphids/plant at full bloom (R2), 1,000+ aphids/plant at beginning pod (R3), and 1,500+ aphids/plant at full pod (R4). Results from these studies are provided as a guideline since they are limited to Wisconsin small plot trials from 2001 and 2002, and have yet to be verified across multiple seasons, under a range of growing conditions, environments, soybean varieties, aphid control tactics and in on-farm trials. However, they do provide info on the probable range of economic thresholds for this pest by soybean crop stage.

    This brings us to the question of which threshold to use. Should you follow the 250 aphid/plant threshold throughout the season from bloom through pod set? Do we have enough data to wait until 1,000+ aphids/plant are reached at R3 and 1,500+ aphids/plant at R4? Risk management invariably comes into play as you consider the rapid population growth potential of aphids, role of natural enemies, fungal pathogens, weather conditions, aphid dispersal flights, plant stress, and a likely population decline upcoming in August.

    Given the range of suggested guidelines and lacking a universally validated threshold at R3-R4, use 250 aphids/plant as a minimum economic threshold for treatment, or a starting point for more intensive monitoring. If at 250 aphids/plant you are observing a large number (~50%) of winged aphids and alatoid nymphs (with wing pads), high predator activity, and/or diseased aphids, treatment delay is recommended. Scout these same fields again within a few days to note increasing or decreasing populations. At this point in the season, with soybean aphid populations peaking, if population densities increase to several hundred aphids/plant between field checks, it is likely they will continue to build toward 1,000 or more – further warranting treatment. Declining aphid densities between field checks may indicate the expected August population collapse, so it will be important to factor this into your treatment decision.

    Source: Eileen Cullen. 2003. Still Critical to Monitor for Increasing Soybean Aphid Numbers. Wisconsin Crop Manager. Vol 10, No. 20.

     

    INSECTICIDE EFFICACY AGAINST ADULT GRASSHOPPERS ON FLAX, 2003

    Registered and experimental insecticides were field tested to determine their potential for controlling adult grasshoppers on flax in North Dakota. A heavily-infested field was used near Blaisdell, North Dakota. The majority of the adult grasshoppers were twostriped grasshoppers. Plots were 10 ft wide by 30 ft long and arranged in a RCB with four replicates. Insecticides were applied at the green boll stage on 31 July 2003 using a handheld CO2 boom sprayer with XR 8001 nozzles (5 nozzles total), 40 PSI, and 10 GPA. Grasshoppers were at damaging levels, 12 adults / yd2, at the time of application. Two counts per plot were conducted on the number of adult grasshoppers / yd2 and the number of clipped bolls on the ground / ft2 on 1, 4, and 7 days after treatment (DAT). Data were analyzed by ANOVA and Fisher’s PLDS at the 5% significance level.

    The untreated check and low and high rate of Sevin XLR had significantly higher numbers of adult grasshoppers / yd2 than the remaining insecticide treatments on 1 DAT. At 4 and 7 DAT, all of the insecticide treatments had significantly lower numbers of adult grasshoppers compared to the untreated check. For the number of clipped bolls on the ground at 1 DAT, the untreated check and high rate of Sevin XLR also had significantly higher numbers of clipped bolls on the ground / ft2 than low and high rates of Mustang Max, Proaxis, and Baythroid. The low rate of Sevin XLR was not significantly different from any treatments at 1 DAT. Again, all of the insecticides were effective in reducing the number of clipped bolls at 4 and 7 DAT. This suggests that Sevin XLR was slower acting than the pyrethroid insecticides at 1 DAT, but it effectively controlled adult grasshoppers at 4 and 7 DAT on flax. At 7 DAT, a higher number of adult grasshoppers / yd2 and clipped bolls / ft2 were observed. This is probably due to the reduced insecticide residue at 7 DAT compared to 1 and 4 DAT. Temperatures were also high during the evaluation period, average high of 81oF and maximum high of 93oF, which could reduce insecticide residue.

    Efficacy of insecticides to control grasshopper and their impact on boll clipping in flax. Blaisdell, North Dakota. 2003.

    Treatment/formulation

    Rate fl oz./ acre

    grasshoppers / yard2

    bolls on ground / ft2

    1 DAT

    4 DAT

    7 DAT

    1 DAT

    4 DAT

    7 DAT

    Untreated check

    7.5a

    10.8a

    14.3a

    7.0a

    7.5a

    13.9a

    Sevin XLR 1 SC

    16

    5.6a

    6.1b

    8.5b

    5.0ab

    5.3b

    6.9b

    Sevin XLR 1 SC

    32

    6.8a

    5.5bc

    7.8b

    6.1a

    4.9b

    7.0b

    Mustang Max 0.8 EC

    2.8

    2.1b

    5.0bc

    8.3b

    2.6b

    2.8b

    6.0b

    Mustang Max 0.8 EC

    4.0

    2.0b

    3.5bc

    8.3b

    3.3b

    4.2b

    7.5b

    Proaxix 0.49 CS

    3.8

    1.6b

    3.0c

    9.1b

    2.9b

    3.3b

    8.0b

    Baythroid 2 EC

    2.8

    1.6b

    4.1bc

    8.9b

    2.8b

    4.1b

    6.1b

    Means followed by the same letter are not significantly different (ANOVA, Fisher’s LSD, P<0.05)
    DAT-Days after treatment

    Janet J. Knodel
    Area Extension Specialist Crop Protection
    North Central Research and Extension Center
    Minot, ND


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