Estimating the economic injury level for headworms is complicated because the potential yield loss varies with the size of the larvae. That is why it is necessary to record the number of small (up to 1⁄4 inch), medium-size (¼ to ½ inch long) and large (1/2 inch long or longer) headworms. Small larvae consume very little grain (about 10 percent of the total) and about 80 percent of them die in this stage. Therefore, small larvae should not be considered in determining the economic injury level. If most headworms are this size, sample the field again in 3 to 4 days. About 19 percent of medium-size larvae survive beyond this stage. Thus, the potential grain loss from medium-size larvae is only 19 percent of the potential loss from large larvae. Most corn earworm larvae larger than ½ inch will survive to complete development, and these large larvae are most damaging; they consume 83 percent of the total grain consumed during larval development. If most of the larvae are larger than ¼ inch, determine which size (medium size or large) is most common and use the corresponding threshold to make treatment decisions.
The beat-bucket technique is the best way to estimate the number of headworms in sorghum. Shake sorghum grain heads vigorously into a 2.5- to 5-gallon plastic bucket (a small white office trash can works well), then count the caterpillars in the bucket. For easy math I like to work with sets of 10; where I shake ten random heads as I walk down the row then I count and evaluate the size of the larvae. If more heads are sampled in a set there may be too much “trash” in the bucket to efficiently make counts. Record the number of small, medium and large headworms found in the samples. Then use the threshold appropriate for the size of the majority of the headworms.
|Control cost $/acre|
|Grain value in $/cwt|
|Medium larvae/head (1/4 to 1/2 inch)||0|
|Large larvae/head ( > 1/2 inch)||0|
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