Illinois Field Crop Applicators Practice Test

What is the economic threshold for corn rootworm beetles in corn when scouting during the season?

1. Five beetles per plant when silks are dry

2. When silks are green, 5 or more beetles per plant if silks are clipped within 1/2 inch of the ear tip

3. One beetle per plant in all stages

4. Two beetles per foot of row

The correct answer is: When silks are green, 5 or more beetles per plant if silks are clipped within 1/2 inch of the ear tip

The economic threshold for corn rootworm beetles is particularly critical when assessing potential crop damage and deciding whether control measures are warranted. The threshold of five or more beetles per plant is significant primarily when the corn silks are green, as this indicates a reproductive stage where the plants are more vulnerable. The observation that silks are clipped within 1/2 inch of the ear tip underscores the urgency; this damage can lead to decreased pollination and ultimately lower yields. Monitoring the presence of beetles during this critical period can help farmers make informed decisions regarding insecticide applications. Recognizing the timing of the silking stage is essential because it coincides with the period when plants are most susceptible to damage from pests. Thus, when scouting reveals this level of beetle activity under the described conditions, it's a direct indicator that economic losses could occur, making it necessary to consider management strategies.