Penn State Extension


Sweet Corn Thresholds

The larvae of 3 species of moths (typically called "worms") infest sweet corn ears. The thresholds change with the growth stage of the corn.

Worm Species

The 3 "worm" species that infest ears of sweet corn are:

  • Corn earworm (CEW)
  • European corn borer (ECB)
  • Fall armyworm (FAW)

The CEW and FAW immigrate in from the south, usually later in the season. The ECB overwinters in PA.

Corn Growth Stage

Thresholds change with the corn growth stage.

Vegetative corn should be scouted, and the percent of infested plants estimated. We can tolerate 15% infestation, and up to 30% on larger, longer-season varieties, before a spray on vegetative corn is warranted. If infestation rates are low, try waiting until the "row-tassel" stage (when you can look down a row and just begin to see the tassels emerge). A single spray timed at row tassel will clean up low infestations prior to the corn moving into the reproductive stages, saving earlier sprays applied to vegetative corn.

For multiple plantings, controlling the early season populations reduces problems later in the season. For multiple planting, it is important to control ECB moving into the 1 st or 2 nd planting to prevent the 2 nd generation from spreading into later plantings.

Reproductive corn (corn that is taseling or silking) and the last vegetative stage (V12 stage, just prior to reproductive stages) attracts moths. Spray timing can be adjusted according to moth flight, which is monitored with pheromone traps. Plotting the number of moths caught over time helps determine when populations are increasing or decreasing - which is when to shorten or lengthen your spray schedule.

Remember: Control programs need to begin at row tassel.
Waiting until silking is too late for ECB and FAW.

Corn Thresholds Based on Pheromone Trap Counts

We don't have well-developed thresholds for ECB. What we have are thresholds designed for CEW, and later adapted to include ECB measured with blacklight (as opposed to pheromone) traps. These CEW thresholds were developed on older, large-stemmed, full-season corn varieties that can withstand more damage. These thresholds (adapted from Dively, Univ. MD) first ask if CEW is a problem.

Please note that the web page shows the data as an average "catch per day", regardless of how often the traps are checked. We do this to be able to represent the information from different areas that report on different time intervals.

Thresholds Based on CEW Captures

CEW catch per week

CEW catch per day

Spray frequency

almost absent


< 2

4 days to no spray depending on ECB

very low

14 to 35

3 to 5

5 to 6 day


36 to 70

6 to 10

4 to 5 day


71 to 350

11 to 50

3 to 4 day


> 350

> 50

2 to 3 day

If CEW is not a problem, then consider ECB. Trapping ECB in pheromone traps is improved by placing traps near weedy areas, in areas where the dew hangs longest in the day, and also by keeping the mouth of the trap only about 6 to 12 inches above canopy height.

ECB thresholds in the absence of CEW, estimated by dividing the thresholds from blacklight data in half to consider that pheromone traps only capture males

ECB catch per week

ECB catch per day

Spray frequency

< 18 / week

< 2

no treatment

19 to 36

3 to 5

6 day

36 to 70

6 to 10

5 day

> 70

> 10

4 day

ECB monitoring in grassy sites

(adapted from Flood et al. 1995 [1] )

Adult ECB congregate in areas - called "action sites" - where dew forms. This often is grassy/weedy areas, or alfalfa. The free water increases the potential numbers of eggs produced per female. "Kicking-up', or 'flushing' moths from action sites can be used as a rough monitoring tool. Flushing more than 100 adults in a grassy action site suggest tightening spray intervals. You can very roughly estimate egg masses per corn plant from the number of moths flushed using the equation:

Egg Masses per Plant = 0.08 + 0.007*Adults Flushed.

[1] Flood, B., R. Foster, & B. Hutchison. 1995. Sweet corn. pp. 19 - 40 IN Foster, R. & B. Flood (eds). Vegetable insect management with emphasis on the midwest. Meister Publ. Co. Willoughby, Ohio.

Fall Armyworm

Thresholds for FAW in silking corn are not well developed. Often we see CEW or ECB densities above thresholds by the time FAW is present. A threshold for whorl infestations is about 30% of the plants infested. However, FAW alone can cause serious damage when immigrating into tasseling and silking corn. You need to spray for FAW when traps are capturing significant numbers of FAW moths, especially when the trap counts are increasing.


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Authored by: Shelby Fleischer, Associate Professor

Last updated January 2001

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