Updates from Dr. Chris DiFonzo, Michigan State University
Western bean cutworm flight is increasing. Unconfirmed reports of hundreds in traps in the Montcalm County area (a historic hot spot with favorable sandy soils). In a bucket on campus, 6 moths last week & 60 this week. Egg masses, fresh and hatching, also were reported. The corn is further ahead of normal, and many fields in southern and central Michigan are already past having a fresh tassel. These fields will rapidly become less attractive for egg laying. That means that moths will seek out younger corn, and perhaps may accumulate in later-planted fields in your neighborhood. The threshold in corn is 5% of the plant with an egg mass. Remember that this threshold is CUMULATIVE. i.e add the % infestation from last week to the next week’s %. A tip sheet for WBC in corn is attached to this email.
If the corn isn’t attractive anymore, females will switch over to dry beans in areas where they are grown. Its impossible to find egg masses and small larvae in beans without making it your career. Instead, use a total trap catch over 120-150 as a cue that populations are high in your area. I also suggest walking nearby corn for egg masses. Then walk dry beans fields to look for pod feeding. The optimum timing for a spray in dry beans tends to be the first week of August (not too early or late). Here is a tip sheet for WBC in dry beans: https://www.canr.msu.edu/fieldcropsent/uploads/files/45-WBC-Dry-bean-QuickRecSheet.pdf
Other caterpillars in corn - Saw both true armyworm and fall armyworm in corn fields on campus. Larvae were still small (3rd or 4th instar), not very numerous, and in a weedy section of a field. But non-Bt and organic fields do merit a walk-thru to check for these species plus western bean cutworm and corn borer.
Western corn rootworm: adults have emerged. They will feed on (scrape) leaf surfaces until the tastier tassels or silks emerge.
Several people asked about a tiny green bug that is in great numbers in some corn fields. It is the rice leaf bug, Trigonotylus caelestialium, a handsome and striking bug with a bright green body and pink antennae. This species prefers grasses, but I have seen it on hemp too. Nothing to worry about, its simply doing well this summer.
Grasshoppers: This is a banner year for hoppers. In corn and soybeans, hoppers are messy feeders and create ragged leaf damage. Hopper poo looks like small brown dirt pellets (they often use the corn whorl as a litter box). The infestations that I’ve seen were in rows along the field edge, where the hoppers moved in from the ditch or roadside. The feeding petered out further into the field so spraying isn’t necessary.
Japanese beetle: Adult are feeding in corn and soy. Like grasshoppers, the beetles often congregate on edges near lawns, grassy roadsides and ditches, where the grubs probably developed. Groups of mating beetles will defoliate a section of beans or clip the silks of a few corn plants, but feeding usually drops quickly further into the field.
Stink bugs: Several people sent me pictures of mystery egg masses, sometimes with tiny insects gathered around. These are stink bugs. The eggs are distinctly barrel-shaped, and some types have a spiky crown. Hatch is synchronized; the colorful little bugs stay by the egg mass for a bit before dispersing over the plant. Most of the plant-feeding stink bugs that I’ve collected in corn and beans are the brown ‘one-spotted’ stink bug. One-spotted adults move out of wheat as it dries down, into corn and soy where they lay eggs. If you see a mass of stink eggs which are extremely spikey, this could be Podisus maculiventris, the spined soldier bug. This is a voracious predatory stink bug that attacks other insects, including pest caterpillars in corn.
Other defoliators in soy: I’ve seen many other defoliators in soybean besides hoppers and Japanese beetles. Bean leaf beetles are plentiful in some fields, chewing the small round holes. Thistle caterpillars and leafrollers are both present, feeding from webbed shelters on leaves. The threshold for defoliation in beans is quite high, 20% of the canopy. Typical defoliation levels in Michigan soybean fields run less the 1%.
Leafhoppers: I can’t stress enough the importance of sampling alfalfa and dry beans for potato leafhopper. Don’t wait until you see yellowing! This is one instance where damage has already occurred, and spraying will probably pay. My last Fonz facts, sent July 6, discussed leafhopper sampling and thresholds.