Each year, I hear the statement that grasshoppers must be sprayed when they are small because adult grasshoppers are notoriously hard to kill. I was not certain that this statement was true, so last year I sprayed adult differential grasshoppers in a laboratory experiment. Ten adult grasshoppers were singly caged in paper cups and covered with nylon mesh for each treatment. Cups containing the grasshoppers were then sprayed in a mechanical spray chamber with an insecticide at a rate of 19.6 gallons of water per acre and 25 psi by using an 80005-E nozzle. The results (Table 1) strongly indicate that adult grasshoppers can be killed with several insecticides, although it took more than 24 hours to achieve maximum mortality. Therefore, small grasshoppers do not have to be sprayed now if they are not causing significant injury to either corn or soybean. You can wait until injury reaches the economic threshold before applying an insecticide. Economic thresholds for grasshoppers in corn and soybean are printed in the June 23 Integrated Crop Management newsletter, page 98. A number of insecticides are labeled for grasshoppers in corn and soybean (Table 2).
|Large populations of young grasshoppers moving into soybean.|
|Adult differential grasshoppers killed with an insecticide.|
Table 1. Mortality of adult differential grasshoppers with selected insecticides (Iowa State University, 2002).
|Treatment||Rate/Acre||24 hours||48 hours|
|Asana XL||5.8 oz||60||80|
|Asana XL||9.6 oz||80||100|
|Furadan 4F||8.0 oz||90||100|
Table 2. Insecticides and product rate per acre for grasshopper control in field corn and soybean. Read and follow all label directions.
|Product||Rate/Acre||Preharvest Interval (days)|
|Asana XL*||5.8-9.6 ounces||21 (corn) 21 (soybean)|
|Capture 2EC*||2.1-6.4 ounces (corn only)||30 (corn)|
|Dimethoate 4EC*||1 pint||14 (corn) 21 (soybean)|
|Furadan 4F*||0.25-0.5 pint||30 (corn) 21 (soybean)|
|Lorsban 4E*||0.5-1 pint||35 (corn) 28 (soybean)|
|Mustang Max||2.9-4.3 ounces (corn)||30 (corn)|
|3.4-4.3 ounces (soybean)||21 (soybean)|
|Penncap-M*||2-3 pints||12 (corn) 20 (soybean)|
|Sevin XLR Plus||1-3 pints||48 (corn) 21 (soybean)|
|Warrior*||2.56-3.84 ounces (corn)||21 (corn)|
|3.20-3.84 ounces (soybean)||45 (soybean)|
This article originally appeared on page 127 of the IC-490(17) -- July 21, 2003 issue.