Lebanon Crop Management Video

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29 March 2016

Keeping tabs on heat units assists on maximizing scouting times particulary for Alfalfa Weevil

Del Voight- Penn State Extension.
There is a tremendous amount of time and effort put into determining key times for insect activity and determining soil temperatures and other weather related factors that dictate agriculture and crop growth activities.  One source of this information is the Pa PIPE modeling system.  If one were to navigate to this web site a screen shot would appear with various colorations to indicate the activity of either soil temperature, accumulated growing degree days and or pest and crop stage of growth.  Right now Penn State has dozens of locations with traps out to monitor for the Eastern Black Cutworm which is utilized to determine a peak flight date and then that information is fed into the PAPIPE for growers and scouts to refer to estimate the peak cutting time for this pest in corn.

  Below is an example of the screen shot of the activity of flea beetle based on the heat development and know activity of the pest.   From this one could derive that at the date of this writing the flea beetle in the area of Lancaster is now in the larvae to pupa form and will be transforming into the adult. One will note that there are numerous tabs on the right hand column that allow one to toggle between heat unit, soil temperature and other known pests.














Here is another screen shot detailing soil temperatures as of today.  One can easily determine that much of the state currently is below 50 degrees in soil temperature and it clearly outlines the climatic differences that exist within Pennsylvania.
 One key area that crop managers utilize is growing degree days.  There are different thresholds of temperature used to determine the heat development each day.  For the most part 50 degrees is the base however there are acceptions like alfalfa weevil which is a base of 48 and corn rootworms which is a base of 53.  Here is a map of base 50 development. The pest tabs on the side of the column would be expected activity by different pests based on research completed on their specific development.  If one assesses this map as of today there are less than 250 heat units to date.  However this is base 50.


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 170 heat units 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?

Larvae begin hatching at approximately 200 degree days in fields south of I-80, and 250 degree days in fields north of this highway. Typically in Pennsylvania this accumulation occurs early in the month of April. Time scouting from 3-500 heat units. This sometimes correlates to just before alfalfa enters the bud stage or around 640 heat units(base 41), however in explosion years we have seen this much earlier.

Where should you start scouting?

Begin on south-facing hillsides. Larvae will hatch here first because these areas warm up quicker than northern hillsides.

How do you scout for alfalfa weevil larvae?

Save some time by using a sweep net to quickly and easily determine if larvae have hatched in your field. If larvae are found in the net, then collect 30 stems and look for larvae in the upper leaves. When collecting stems, do not break them too hard or you will knock off larvae still on the plant. The best way to collect the most larvae is to grab the tip of the plant with one hand and break the base of the stem with the other hand, or cut it with a knife. Place stems inside a white, 5-gallon bucket and beat them against the side. Large larvae will dislodge and can be counted easily, but newly developing leaves must be pulled apart to find very small, newly hatched larvae hidden in the plant tip.

What do alfalfa weevil larvae look like?

They have a very dark head, almost black, and are pale green with a white stripe down the back. They are about 1/16 of an inch in length when they hatch and may be light yellow. After feeding for several days, they turn green. They are 5/16 inch in length when full grown.
Alfalfa weevil are small; less than 5/16-inch long.


Are there any other insects that look like alfalfa weevil larvae?

Yes. Larvae of the clover leaf weevil look very similar, but are larger, have a light brown head, and often have the white stripe edged with pink. Clover leaf weevil larvae usually hide around the base of the plant during the day and feed mostly in lower leaves at night. They rarely cause economic yield losses and should not be counted as part of the alfalfa weevil sample.

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?

New economic thresholds have been developed by University of Nebraska entomologists. These thresholds are for alfalfa at the early bud stage, when third- and fourth-stage larvae do 90 percent of the damage. To use the economic threshold chart, first determine the control costs in dollars per acre, then estimate the forage value in dollars per ton. Where these two values intersect (Table 1) is the average number of alfalfa weevil larvae per stem needed to justify chemical control. For example, if control cost is $10 per acre and forage value is $75 dollars per ton, then an average of 3.4 larvae per stem would be needed to justify chemical management (Table 1).
 Table 1. Economic thresholds for alfalfa weevil larvae in early bud stage alfalfa (average number of larvae per stem). Current high prices of forages will impact this chart.  Entomologists will need to adjust to this current market pricing. For now here is a relative economic chart.

Forage value ($ per ton)
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

Resample the field in 3 to 5 days. Chemical management may be needed then, or possibly the crop may have reached a stage where it can be cut.

What if the weevil count is below the economic threshold?

Several days after it would be important to recheck to determine if thresholds are reached otherwise there is a point of no action.

What chemicals are labeled for alfalfa weevils?

Several chemicals that can be used for alfalfa weevil control are shown in Table 2. Read and follow all label directions before using any insecticide. The Agronomy Guide has more specifics.
Table 2. Insecticides labeled for alfalfa weevil.

Insecticide
Rate per acre

at low and high rates
Harvest

interval

(days)
Cost/Acre Jan 2014 Pricing. At low and high rate.
Ambush 2E
6.4-12.8 ounces
0-14
2.94-5.87
Baythroid XL
1.6 - 2.8 ounces
7
1.68-4.71
cylfuthrin 
various

cyhalotharin
various

Lorsban 4E
1-2 pints
14-21
3.34-6.68
gamachalothrin
various

Pounce 3.2EC
4-8 ounces
0-14
1.84-3.67
Sevin XLR+
3 pints
7
9.56
Warrior 1T/Proaxis
2.56-3.84 ounces
7
3.78-4.34
Mustang Max
2.24 to 4.0 ounces
3
????
Cobalt
19-38 ounce
7-14
6.67-13.35

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