Asiatic garden beetles, soybean aphids and western bean cutworms arrive in time to see fireworks
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EmailA July 4 update on Asiatic garden beetle adults, soybean aphids and western bean cutworm moths in Michigan field crops.
Asiatic garden beetle adults
Asiatic garden beetles (and Japanese and false Japanese beetles) pupated and are emerging as adults, i.e., grub damage has ended. In Asiatic garden beetle-infested fields, the main damage seems to be the stunting of plants. The stage is similar across the field, but damaged plants or areas are much shorter. Another current sign of Asiatic garden beetles is a large number of dead beetles on the soil surface.
Beetles are moving around the landscape on warm nights. Marestail, ragweed, lambsquarters and Queen Anne’s lace may show signs of adult feeding on the lower leaves, and beetles are underneath the plants during the day. We have found a few cases of spectacular congregations under weeds. See field pictures of Asiatic garden beetles.
Soybean aphids
Soybean aphid may be present in the earliest planted (late April, first week of May) fields that were not seed treated. Later-planted fields are not yet infested, having escaped aphid movement out of the overwintering sites.
Infestations often start on the windward edges of fields where aphids encounter a greenish brown interface. In Michigan, it is common for natural enemies, both predators and parasitoids, to recruit quickly from tree lines, ditch banks and maturing wheat fields to these early infestations. As long as plants grow quickly, the infestations usually disappear or remain low if beneficials can keep up.
Don’t panic or spray these early infestations. Past experience shows that spraying aphids early does not result in a yield difference at the end of the season. (The “sweet spot” for aphid spraying is typically end of July.) We very much want to avoid unnecessary sprays that trigger the formation of resistant aphid populations, as is happening in the western states.
Western bean cutworm moths
My first western bean cutworm moth of the season emerged July 1 from a bucket buried in my garden last fall. This week, look for the first catches in bucket traps in Michigan. See the Western Bean Cutworm Trapping Network.
Corn stage is extremely variable in some locations, especially in the southern counties where an early-planted pre-tassel field may be adjacent to June-planted ankle-high corn. The earliest planted fields approaching pre-tassel stage will merit scouting in a few weeks once flight picks up in the bucket traps. I am not sure if variability across the landscape is a good or bad thing as far as predicting western bean cutworm infestation. It probably depends on the scale of the variability in your area (variable fields side-by-side versus entire neighborhoods planted early or late) and the distance females can move.
On one hand, if a female can find pre-tassel corn fields for an extended period in an area, she can disperse her eggs across a bunch of fields and that could reduce the overall level in a single field. But if one field is the single pre-tassel garden spot in a neighborhood, that field may get hammered with egglaying. Further, if a female cannot find pre-tassel corn (for example, in an area with widespread rains in May), she may dump eggs onto V-stage corn. This is a dead end because larvae can’t survive on leaf tissue alone; they need tassel and silks to survive.
This all just means that trapping moths locally then scouting for egg masses once trap catch peaks is very important to determine if any fields are actually over threshold and merit a spray. If this does occur, my recommendation (aligned with that of Ontario) is to optimize spray timing for the fungicide portion of the tank-mix, giving up a little bit on insect control.
Remember, only one Bt trait, the Vip toxin, controls western bean cutworm. All other traits have little impact. That means most Bt corn acres in Michigan should be managed like non-Bt corn, for the purposes of western bean cutworm.
