Del Voight- Penn State Extension.
There are numerous articles that could be derived from a discussion of heat and the development of pests. For this week the one insect that should be tracked are alfalfa weevils as an example of the use of these models. Judging by the accumulated heat with a base of 50 with right around 250 or less as of today I double checked with my records that I keep in the Lebanon office and at this point there are 230 heat units(as of April 4, 2024) that apply to weevils this is based at 48 degree growth . I inspected a few fields in the Lebanon area and found some signs of the larvae with shot holes on south facing slopes and very small larvae so if we get some significant heat in the next week damage will be more evident. Years ago with Paul Craig we began to notice that when alfalfa weevil damage was clearly evident the colts foot and the Bradford Pears were blooming this is called a phenotypic indicator and if it holds true this year the Bradford Pears are just beginning to bloom so it is working out with our heat unit models that the weevils are becoming active. We have not seen economic levels of this pest but it is one to keep tabs on each year. As we develop more heat the weevils will grow and develop and while typically held in check by disease and other predators they could develop.
When should scouting begin?
Where should you start scouting?
How do you scout for alfalfa weevil larvae?
What do alfalfa weevil larvae look like?
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 Alfalfa weevil are small; less than 5/16-inch long. |
Are there any other insects that look like alfalfa weevil larvae?
When should alfalfa weevils be controlled?
If two or more larvae are found per stem, and 40 percent of the stems show any leaf feeding, the best option is to cut the hay within 5 days, if possible. This method of cultural control avoids the use of insecticides. If the crop is not mature enough to cut, then chemical control may be an option, depending on the economic thresholds.
What are the economic thresholds for chemical control?
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Forage value ($ per ton)
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Control cost ($ per acre) |
45
|
55
|
65
|
75
|
85
|
95
|
105
|
115
|
|
7
|
4.0
|
3.3
|
2.8
|
2.4
|
2.2
|
1.9
|
1.8
|
1.6
|
|
8
|
4.6
|
3.6
|
3.2
|
2.7
|
2.4
|
2.2
|
2.0
|
1.8
|
|
9
|
5.2
|
4.2
|
3.6
|
3.1
|
2.7
|
2.5
|
2.2
|
2.0
|
|
10
|
5.8
|
4.7
|
4.0
|
3.4
|
3.0
|
2.7
|
2.5
|
2.2
|
|
11
|
6.3
|
5.2
|
4.4
|
3.8
|
3.4
|
3.0
|
2.7
|
2.5
|
|
12
|
6.9
|
5.6
|
4.8
|
4.2
|
3.7
|
3.3
|
3.0
|
2.7
|
|
13
|
7.4
|
6.1
|
5.2
|
4.5
|
3.9
|
3.5
|
3.2
|
2.9
|
There is a lot of interest in designing fertility programs to include sulfur as a base and planning on applications. I stopped at a farm this morning that was mixing up a load of UAN and Ammonium Thiosulfate for his wheat. He put 800 gallon of UAN in a tender and another 200 gallon of thio in to get to his target. Applying 30 gallon per acre of this solution he ends up getting about 10 lbs of sulfur to his wheat crop. After talking to him more he had heard this is critical to supply this. So I did some checking and he was spot on for his crop however he uses alot of manure and I wondered whether this might not relate to increases in yields. As it turns out in this case with his liquid dairy manure applied last fall he likely will have enough sulfur without additional applications. I have assembled from several references some basic information on the removal of sulfur from soils and additionally sulfur amendments coming from either manure and or commercial sources. If one takes a look at the removal of sulfur one will note that the average across all crops is about 18 lbs of sulfur with some typical yields in the Southeastern Pa conditions. I used a 2 ton per acre goal for both grass hay and alfalfa since this is a mobile nutrient and the first cutting might benefit and pending rain events the overall yearly removal of forage might differ. What I have done here is take some known information and simply placed into an excel file and calculated some of the results for typical yields in the SE part of Pennsylvania. If you were to select out one crop such as Soybeans you will note a removal of .18lbs per bushel and so a typical 70 bu/acre crop would require about 12.6lbs of sulfur. In our studies at both Landisville and the Rock Springs research station we applied sulfur to meet the demand of soybeans however there were no yield responses across treatments to additional sulfur for the 2014 growing season. Perhaps there may be responses to sulfur additions for this crop in the future. Dr Doug Beegle has studied responses in Corn and it appears that a yield benefit to that crop is significant enough to begin looking at sulfur and managing this as a nutrient. For Corn using the table for a 250 bu/acre yield goal a removal of about 20/lbs of sulfur occurs. Since Sulphur is mobile it would be difficult to soil test and in most cases with crops a tissue test would reveal hidden deficiencies. A simple addition of several commercially available products would easily meet the demand of the crop with limited change in applied fertilizer and manure. If manure is utilize one can either use a manure test to determine applied S from manure or on average we would expect results as in the table. With a 6,000 gallon per acre of dairy manure we expect about 14lbs of sulfur supplied to the plant but this can vary extremely depending on the source of dairy manure and manure test is the best means to determine this. So for our area yield enhancements on manured soils might not be realized by purchasing off farm sources of sulfur.