PRODUCERS QUESTION: If I Treat Alfalfa for Blister Beetles Today, Will Residues Left on the Plant Continue to Kill the Insects Throughout the Required Waiting Period?

For years this has been a common question at alfalfa meetings, and for years a simple shrug of the shoulders was the response. In the summer of 1995 we took an initial look at the problem.  

The most commonly used insecticide for blister beetle control is Sevin^® at the 1 lb. a.i./A rate. Sevin^® does an excellent job of controlling blister beetles; however, it has a 7-day waiting period before harvest. Because of the highly mobile nature of blister beetles, alfalfa producers were concerned about immigrating swarms, following application of insecticide. The solution for some became premature (illegal) harvest of the hay before the waiting period expired.  

Approximately 3,000 striped blister beetles were collected from the field to conduct the initial phases of studies on controlling these insects. Trials were conducted under shaded greenhouse conditions.  

Alfalfa plants for this experiment were grown in the greenhouse in six inch plastic pots. Plants were established and cut back approximately one month before use. The plants used were 8-10 inches in height.  

Treatments consisted of treated (1 lb. a.i./A of Sevin^® XLR) and untreated alfalfa. Treatments were replicated three times. All treated plants were sprayed at the aforementioned dosage using a simulated aerial applied rate of two gallons per acre. Therefore, 21 of 42 plants were treated.  

Beginning one hour after application, and continuing once per day for the next six days, six plants (3 treated and 3 untreated) were infested with 50 blister beetles. Cages were placed over the plants using a 12 inch acrylic cylinder with a mesh top and two mesh side portals. Cages were used to keep beetles on the plants and to simulate a swarm. Cages were held in shaded greenhouse conditions with ambient air temperatures. After each 24 hour period, plant cages were removed and percent mortality of beetles recorded.  

Table 1 depicts average mortality for 3 replications of treated and untreated alfalfa over the 6-day period. Although mortality of untreated beetles increased over time, this was probably due to long-term storage. Regardless, the level of control obtained on treated plants remained high throughout the trial. Therefore, under these conditions, good to excellent control can be obtained for up to six days after application of Sevin^® insecticide. Obviously, these conditions do not approach the intense sunlight and possible rainfall that the chemical would be exposed to in the field, however, it represents a start at better management of this insect problem for alfalfa producers.  
  
Table 1. Percent mortality of blister beetles exposed to treated (1 lb a.i./A Sevin^®)  
and untreated alfalfa. Mean of three replications.  
 

  
For other information on blister beetles in alfalfa, see Alfalfa Harvest Management Discussion with Cost-Benefit Analysis. OSU Extension Circular E-943. HTML Format (faster) or PDF Format (Requires Adobe Acrobat Reader) and OSU Extension Facts 2072 Blister Beetles and Alfalfa.