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Economic Thresholds of Insects in Crops

Jul 19, 2019

Nicholas Blevins, Ag Partners Agronomist

Now is the time to ask yourself, ”what is my economic threshold when it comes to insect damage and will it pay when spraying insecticides with fungicides?”  This has been a common question since Japanese beetles, stink bugs, bean leaf beetles, and thistle caterpillars have started showing up in fields over the last few weeks. Lately, the weather in our area seems to be bringing out all the damaging insects we’ve seen in the western Corn Belt.

How do you decide when to spray (insert treatment. i.e.: fungicide, insecticide, etc)? Is it defoliation, is it number of insects on the silks, or is it the age of the insect?  It all depends on the insect and the plant they are feeding on. 

Economic Thresholds in Corn:
  • Grasshopper – If there are five to eight hoppers per square yard present from just before pollination until pollination is complete.
  • Japanese Beetle – Treat during silking if there are three or more beetles present per ear and pollination is less than 50 percent complete or silks are clipped to less than 0.5 inches. 
  • Stinkbug –In corn at pre-silk: treat if 5 percent of the plants are infested. At grain fill to hard dough: treat if 10 percent of the plants are infested.
Economic Thresholds in Soybeans:
  • Armyworms – Treat when there’s an average of 30 small worms per 30 feet of row or 30 percent defoliation pre-bloom to 20 percent bloom to pod fill.
  • Bean Leaf Beetles – Before bloom: if beetles are present and there is 50 percent defoliation. Bloom to maturity: if beetles are present and there is 20 percent defoliation, or 1-10 percent pod feeding.
  • Cloverworm – If there are 5-12 half-grown worms per foot of row or there is more than 20 percent defoliation.
  • Japanese Beetle – Treat when defoliation exceeds 30 percent pre-bloom or 20 percent during blooming through pod fill stage.
  • Thistle Caterpillar – If there are three to four worms per plant in V3 to V4 stages, 10 per row of blooming soybeans, or 4-8 ft of row, 25-30 percent pod defoliation in pre-bloom stages or 20 percent defoliation during pod stages.
  • Webworms – Approximately 10-12 percent of plants have webbing on top leaflets, or when defoliation reaches 30 percent pre-bloom or 20 percent bloom to pod fill.

Contact your Ag Partners agronomist to help you prevent or assess any damage to your crop caused by insects and help choose the correct insecticide to eliminate the pests.

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