2017 Populations of Alfalfa Weevils Active Back »

Written collaboratively by Adam Varenhorst, Ruth Beck, Karla Hernandez, Patrick Wagner, and Amanda Bachmann.


This week we received a report of insecticides being applied to manage alfalfa weevils in South Dakota. In 2016, we believed that the reports that we received during the same time period in May were early. Now, it appears that populations may simply be present earlier in alfalfa than previously reported for the state.

Alfalfa weevils can be a pest of alfalfa throughout the growing season and their feeding has been associated with the stunting of later cuttings. The alfalfa weevil is an introduced pest and, as a result, it has very few natural enemies in South Dakota. This allows for populations to build to levels that can cause severe feeding injury to alfalfa crops. Although scouting for alfalfa weevils can be time consuming, early detection can prevent serious yield losses down the road.

Description

The larvae of alfalfa weevils have three pairs of legs and range in size from 1/16” when they first hatch to 5/16” when fully grown. During development, the larvae vary in color depending on their age. Shortly after hatching the larvae are light yellow-green (Figure 1), but will turn a darker green after feeding on plant material (Figure 2). The distinguishing characteristics of the alfalfa weevil are its dark brown-black head capsule and a prominent white stripe on the back that runs the length of the body. The adult alfalfa weevils are approximately ¼ inch long. They are light brown in color with some mottling and have a darker brown stripe that runs down their midline. The adults have an elongated snout that extends and curves down from the head. Their antennae originate on the elongated snout (Figure 3).


Figure 1
. Alfalfa weevil larva. Notice the distinct black head.
Courtesy: A. Bachmann
 


Figure 2. Alfalfa weevil larva. Notice the distinct white stripe in the middle of the back. Courtesy: A. Varenhorst
 


Figure 3. Alfalfa weevil adult. Courtesy: A. Bachmann
 

Scouting

Although both the adult and larvae of the alfalfa weevil are capable of causing defoliation injury, larval feeding is the most severe. The presence of large larvae populations can reduce the tonnage and quality of the alfalfa crop. Fields that have extensive defoliation occurring from alfalfa weevils will have a “frosted” appearance (Figure 4). Scouting for the alfalfa weevil requires a sweep net and a five-gallon bucket. The sweep net is used to determine if alfalfa weevils are present or not. If alfalfa weevil larvae are observed in the sweep net, the next step is to walk in a “W” or “Z” shaped line and randomly select 30 plants along the path. Each selected plant will first be measured to determine height, and then gently broken or cut at the soil surface. Any unnecessary force may dislodge larvae that are present. The next step is to shake the plant against the inside of a five-gallon bucket and record the number of alfalfa weevil larvae that are present on the bottom and/or sides of the bucket. Repeat this process for all thirty plants and add up the total number of larvae observed in the field. Economic thresholds for alfalfa that is 0 to 12 inches tall are in Table 1, 12 to 18 inches in Table 2, 18 to 24 inches in Table 3, and 24 to 30 inches in Table 4.


Figure 4. Alfalfa weevil feeding injury to alfalfa leaves. Courtesy: A. Varenhorst
 

Management

Although the alfalfa weevil is considered a sporadic pest, it is commonly reported as a pest in South Dakota. However, the areas that are affected vary, which makes scouting very important. If thresholds are exceeded and plants are less than 16 inches tall, there are several insecticides available for management. Please refer to the current version of the South Dakota Pest Management Guide: Alfalfa for labeled insecticides. The alternative management option for alfalfa weevils with a severe infestation is to cut the alfalfa when it reaches 16 inches in height. This option will conserve natural enemies that may be feeding on other pests present in the alfalfa. 

Table 1. (Alfalfa 0 to 12 inches tall) Economic thresholds for alfalfa weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample. 

  Insecticide application cost per acre
Value of Hay Per Ton $8 $12 $16 $20
  Total alfalfa weevil per 30 stems
$60 84 127 169 211
$80 63 95 127 158
$100 51 76 101 127
$120 42 63 84 105
$140 36 54 72 90
$160 32 47 63 79
$180 28 42 56 70
$200 25 38 51 63
$220 23 35 46 58


 

Table 2. (Alfalfa 12 to 18 inches tall) Economic thresholds for alfalfa weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample. 

  Insecticide application cost per acre
Value of Hay Per Ton $8 $12 $16 $20
  Total alfalfa weevil per 30 stems
$60 91 137 183 225
$80 68 102 136 171
$100 54 81 95 137
$120 45 68 91 114
$140 39 59 77 99
$160 34 51 68 86
$180 30 45 60 77
$200 27 41 54 69
$220 25 37 49 63


 

Table 3. (Alfalfa 18 to 24 inches tall) Economic thresholds for alfalfa weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample. 

  Insecticide application cost per acre
Value of Hay Per Ton $8 $12 $16 $20
  Total alfalfa weevil per 30 stems
$60 99 149 199 240
$80 75 113 150 189
$100 62 90 120 149
$120 50 75 100 124
$140 43 64 86 107
$160 37 56 75 93
$180 32 50 67 84
$200 29 45 60 76
$220 26 41 55 69


 

Table 4. (Alfalfa 24 to 30 inches tall) Economic thresholds for alfalfa weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample. 

  Insecticide application cost per acre
Value of Hay Per Ton $8 $12 $16 $20
  Total alfalfa weevil per 30 stems
$60 104 156 182 260
$80 78 117 137 195
$100 63 94 110 156
$120 52 78 105 130
$140 45 67 90 112
$160 39 58 79 98
$180 35 52 70 87
$200 31 47 63 79
$220 28 42 57 72

References: 

  • Gesell, S. G., N. L. Hartwig, A. A. Hower Jr., K. T. Leath, and W. C. Stringer. A pest management program for alfalfa in Pennsylvania. Penn State Extension.
  • Peterson, R. K. D., S. D. Danielson, and L. G. Higley. 1993. Yield responses of alfalfa to simulated alfalfa weevil injury and development of economic injury levels. Agron. J. 85: 595-601. 
  • Tooker, J. 2013. Alfalfa weevil. Entomological Notes. Penn State Extension.
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