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Irrational Fear of Alfalfa Weevils?
When SDSU set out to quantify the yield loss caused by alfalfa weevils on South Dakota alfalfa, we were absolutely sure that we will show tremendous yield loss, and that it was just a matter of how much loss in tonnage. After all the field looked terrible and everything was white and frosted in appearance from the view of our pickup truck. The field would be worthless. And so we thought.
In the year 2000, however, we are still looking for a season when we could finally show to the whole state that alfalfa weevils are indeed terrible insects that we have always perceived them to be. In other words, we have actually been looking for some justification for our massive spraying campaigns every single year.
If alfalfa weevils were such a destructive insect, how come we have not shown them to reduce tonnage conclusively? It is either we are fatally flawed in our experiment designs, or that indeed, alfalfa weevils simply do not cause significant yield losses in SD alfalfa. If the latter were true, are we then guilty of an "irrational fear of alfalfa weevils?
Below are the results of our 7 independent experiments where we attempted to implicate alfalfa weevils as destroyers of SD alfalfa. My predecessor directed all by the 1999 study. The results are surprising and unexpected.
| Tons Per Acre (dry weight) | Dollar Per acre | |||||||
| Study | Year | County | Larvae/10 sweeps of an insect Net in Untreated Plots at 7DAT | Untreated Plots | Insecticide Plots (Avg) | Yield Loss or Gain | Value of Loss or Gain @$65/ton | Value of Loss or Gain @$45/ton |
| 1 | 1993 | Pennington-Dryland | 245 | 1.52 | 1.46 | -0.06 | -3.90 | -2.70 |
| 2 | 1993 | Pennington-Irrigated | 439 | 1.74 | 1.92 | +0.18 | +11.70 | +8.10 |
| 3 | 1993 | Gregory-Dryland | 36 | 1.77 | 1.60 | -0.17 | -11.05 | -7.65 |
| 4 | 1994 | Pennington-Dryland | 192 | 0.88 | 1.10 | +0.22 | +14.30 | +9.90 |
| 5 | 1994 | Pennington-Irrigated | 53 | 1.53 | 1.46 | -0.07 | -4.55 | -3.15 |
| 6 | 1994 | Gregory-Dryland | 51 | 0.92 | 0.93 | +0.01 | +0.65 | +0.45 |
| 7 | 1999 | Gregory-Dryland | 540 | 1.71 | 1.77 | +0.06 | +3.90 | +2.70 |
| Average | 222 | 1.44 | 1.46 | 0.02 | 1.58 | 1.09 | ||
We again took some yield from our alfalfa weevil research out there in Lucas this year. I will let you know if year 2000 was the year that we finally proved that alfalfa weevils are indeed the green villains of the plains. We will also have very interesting date on the tolerance of some alfalfa varieties to feeding by the weevils. In the meantime, I pose this question: How can we recommend to farmers to spend about $10.00 per acre to spray the field when the potential return is only less than $2.00 per acre? Of course this is based on yield alone and not on quality. I will discuss the impact of alfalfa weevils on alfalfa relative feed value (RFV) and crude protein next issue or you may visit my web site at www.abs.sdstate.edu/plantsci/ext/ent
Source: South Dakota Field Facts June 13, 2000
Mike Catangui - Extension Entomologist
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