Which Insecticide is Best for Grasshopper Control (07/15/21)
All stages of grasshoppers (1st instars to adults) can be observed in the field crops now. Some species of grasshoppers emerge earlier than other species, so different life stages are common in early summer. Due to the heat pushing degree days, an increasing number of adult grasshoppers are present, which represents a greater risk to field crops. Adults are more mobile (wings to fly) and fly to find greener fields for feeding, and have a larger appetite. Continue to frequently scout for grasshopper infestations.
In 2020, low and high labeled rates of pyrethroid and premix insecticides were tested for control of adult grasshoppers in late growth stage soybeans at the NDSU Agronomy Farm, Casselton, ND. Insecticide products, active ingredients, and application rates are listed in Table 1.
Insecticide applications were made on August 20 when soybean plots were at the R5 growth stage. Applications were made using a CO2 sprayer equipped with TeeJet 11002 flat-fan air induction nozzles at 40 psi and using a carrier volume of 20 GPA. The center two rows of each plot were harvested on October 6.
Grasshopper counts averaged 4 grasshoppers/yd2 and percent defoliation averaged 14.7% across all treatments, determined immediately prior to insecticide application. There were no significant differences among treatments for pre-spray grasshopper counts or percent defoliation, indicating that grasshoppers were evenly distributed across the trial and were at a population density great enough to threaten economic yield loss.
At 7 Days After Treatment (DAT), the untreated checks had significantly more grasshoppers/yd2 and greater defoliation than all insecticide treatments (Table 2). All insecticide treatments had higher grain yield compared to the untreated checks, and there were no significant differences among insecticide treatments (Table 2).
Our results indicate that all low and high labeled rates of all insecticides tested provided control of grasshoppers, and prevented economic yield loss. Percent defoliation increased in the untreated checks, while remaining steady in the insecticide treatments. Additionally, substantial pod feeding was noticed in the untreated checks compared to the insecticide treatments. Yield loss in the untreated checks was likely due to a combination of defoliation and pod feeding during the critical pod-filling period between the R5 and R6 growth stages.
Remember to consider the big picture of insect pests present in fields before selecting an insecticide to use. For example, soybean and dry bean fields may also have spider mites or soybean aphids present as well as grasshoppers. For control of spider mites, avoid using pyrethroid, Group 3A, insecticides, which flare mite populations. The only pyrethroid that will control spider mites is bifenthrin (consult product labels for spider mite rates). Other modes of action for control of spider mites, soybean aphids and grasshoppers are organophosphates (Group 1B), such as chlorpyrifos (Lorsban and generics). Another alternative is premixes (2 or more A.I.) of an organophosphate and a pyrethroid, such as Match-up or Tundra Supreme (chlorpyrifos + bifenthrin), or Stallion (chlorpyrifos + zeta-cypermethrin), which will control spider mites, soybean aphids and grasshoppers.
Disclaimer: Mention of any insecticide products do not imply endorsement of one product versus another nor discrimination against any product not mentioned by the authors or NDSU.
Research Specialist, Extension Entomology