Western bean cutworm and corn rootworm populations increasing in corn

Large populations of western bean cutworm and corn rootworm have been reported in corn fields in southern and central Michigan.

My phone has been ringing with multiple reports from specific areas with large populations of western bean cutworm and corn rootworm in corn. I’m not saying populations are like this across Michigan, just giving fair warning. Also, I appreciate hearing about bad corn rootworm locations in central Michigan, especially Bt fields – contact me at .(JavaScript must be enabled to view this email address) or 517-353-5328.

Western bean cutworm in southern Michigan

At Michigan State University, I’ve trapped about 50 western bean cutworm moths last week and found 1-2 percent of plants infested with egg masses, plus newly hatched larvae. I had at least one report from southern Michigan of as many as 500 moths in a trap, and 18-25 percent of the plants with egg masses in a nearby research field. Western bean cutworm females look for late-whorl to pre-tassel stage plants for egglaying. This year, females are on easy-street because of the variability in fields; they won’t have to work too hard or fly too far to find corn in an optimal stage.

But don’t panic. As insects go, western bean cutworms are relatively easy from the scouting standpoint. Egg masses are laid on the upper surface of the top leaves, often on the flag leaf or the leaf immediately below the tassel. Placing the sun behind a row makes scouting easier because the egg masses show up as penny-sized, dark spots through the leaf. An effective tool for scouting in tall corn is a cheap, plastic face shield – it lets you look up towards the tassel without getting a corn leaf in the eyeball.  

Scouting priority is late-whorl to pre-tassel fields, in this order:

  1. Conventional corn and Bt hybrids that lack Cry1F or Vip3A, because these hybrids are fully susceptible to western bean cutworms.
  2. Bt hybrids with Cry1F Bt (Herculex 1 and Xtra, Smartstax, some Optimum products), which suppress western bean cutworms, but I wouldn’t say “control.”
  3. Bt hybrids with Vip3A toxin (Agrisure Viptera, Agrisure Duracade 5222) give good to excellent control. Shouldn’t have to treat these hybrids.

The action threshold in the Great Lakes region is lower than in the western states. Spray with a long lasting pyrethroid if 5 percent or more plants have egg masses or newly hatched larvae. Time the spray so that 70-90 percent of egg masses have hatched or are about to hatch (purple). Newly hatched larvae like to move up and feed in the tassel before it emerges, but they eventually trek back down the plant to spend the rest of their larval stage in the ear. In the process, they get exposed to insecticide residue.

For pictures of eggs on plants and management recommendations, read my bulletin, “Managing western bean cutworm in field corn.”

Rootworms in central Michigan in the Grand Ledge area

High numbers of western corn rootworms are reported in some fields in an area west of Lansing/Grand Ledge, Michigan. Multiple callers reported beetles stripping leaf tissue of late whorl stage or pre-tassel plants. This happens when there are a ton of beetles that emerge early or in late-planted fields. Hungry beetles scrape leaf tissue until something better (silks, tassels) comes along.

With so many beetles per plant, it is critical to protect silks – the threshold for silk clipping is 0.5 inch or less. You likely get one shot at spraying such a field, so I suggest that even if the leaf damage is ugly in whorl stage corn, the main focus should be protecting silks just prior to emergence and during pollination. More beetles may come out of the ground in these fields for three to four more weeks. Thus, waiting an extra week to target silks before you spray will kill beetles already in the field, plus provide seven to 14 days of residue to kill late-emerging corn rootworms and Japanese beetles. If you live in the central Michigan rootworm black hole (Clinton, Ionia and Eaton counties), now is a good time to drive by corn fields and check the corn rootworm populations.

I’d appreciate hearing about bad locations – contact me at .(JavaScript must be enabled to view this email address) or 517-353-5328. I have a handy bulletin showing corn rootworm beetle damage to corn leaves and silks, “Western corn rootworm adult feeding on corn.”

A caution: The fields described to me, with three to 10 beetles per plant, apparently were not lodged – yet. Given the huge beetle numbers, there must be root damage. The plants are short (whorl stage) now, but height and weight will increase in the next few weeks, and that’s when lodging will occur. MSU Extension strongly suggests digging and rating roots to be sure there is enough root mass – factoring in the potential for regrowth under the dry conditions – to justify an adult beetle spray. For the system to rate corn rootworm root damage, read my bulletin, “Rating corn rootworm larval damage: The 0 to 3 node-injury scale.”

Dr. DiFonzo’s work is funded in part by MSU’s AgBioResearch.

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