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Watch for cutworms in emerging corn
By Jason Kolln
Pioneer Hi-Bred agronomist
As corn emerges this spring, a variety of cutworms may attack. Black cutworms are the main threat to emerged corn, but there are others. With the exception of migrating black and variegated cutworms, these "native" cutworms are capable of overwintering in Iowa. Because these "native" cutworms commonly overwinter in an egg or larval stage, they accumulate heat units before the migrants arrive to lay their eggs. This head start in development means an earlier onset of damage. For example, reports of dingy and sandhill cutworm damage usually occur one to two weeks before black cutworm infestations are noticed. Black cutworms will feed on leaves and leave irregular holes prior to actually cutting plants. Fields will need to be monitored as the cutworms grow to cutting size. Thresholds for treatment of black cutworm are 2 percent to 3 percent cutting when cutworms are 1 inch or less and 5 percent for larger cutworms. If the field has a reduced stand, treatment should occur earlier. Rescue treatments are very effective.
Monitor early growth: Every year there are some cornfields with less than optimum stands. This may be due to planter problems, insects, seedling blight or poor soil conditions. Now is the time to determine if insects are reducing your stand or making plants appear unthrifty. Seed corn maggots will directly attack the seed. Some seeds may germinate, yet not do much more. Wireworms can attack the seed or the mesocotyl (shoot area right above the seed) and can either kill the seedling or make it unthrifty. Annual and true white grubs plus grape colaspis generally feed on the roots and can cause plant death or a lack of vigor. If seed attacking insects are present, rescue treatments are not an option. If stands are reduced significantly, replanting may be necessary.
When should I start cutting alfalfa? Alfalfa should be harvested at the start of bud stage for highest quality forage. For higher yield and persistence, growers may wait until first flower, but further delays are detrimental. At first bud, alfalfa hay analysis will be approximately 20 percent crude protein, 30 percent acid detergent fiber (ADF) and 40 percent neutral detergent fiber (NDF). Cutting at this stage also allows more cuttings per year and increases total tonnage for the year. The following chart adapted from Penn State University illustrates the advantages of early harvest of alfalfa.
Stage CP* ADF**
Mid bud 19 30
Late bud 18 31
First flower 17 33
1/10 bloom 15 35
Later 1/10 12 39
*Crude Protein
** Acid Detergent Fiber, a measure of digestibility Apply one-half of the annual phosphorus and potash requirements after removing the first cutting of alfalfa. With each ton of forage harvested, 60 pounds of K2O and 12 pounds of P2O5 are removed.
Editor's note: Crop Talk is brought to you by agronomists serving your community to keep you informed about current growing conditions and crop management issues affecting the crops in your area. Jason Kolln, Webster City, Iowa, is an agronomist with Pioneer Hi-Bred.
Date: 5/18/04
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