Pictured: Japanese beetles
Wisconsin crop fields have been largely free of major insect outbreaks so far this season, but entomologists warn that could change as heat and moisture levels rise. Krista Hamilton with the DATCP Pest Survey Program says the second half of summer brings a new set of insect threats.
Scouting in late July and early August should focus on rootworm beetles, second-generation armyworms, and corn earworm flights. Soybean growers should keep a close eye on aphids, Japanese beetles, and other defoliators, as multiple pests may begin to peak at once.
Japanese beetles, in particular, are drawn to emerging corn silks and can cause serious damage if left unchecked. Hamilton says feeding is typically concentrated in the first few rows of a field, making targeted treatment effective.
For soybeans, aphids are just beginning to colonize and multiply. Weekly scouting is essential now, especially if aphid numbers approach the treatment threshold of 250 per plant.
While armyworm activity is currently low, Hamilton advises keeping them on the radar as second-generation populations can spike later this month.
Corn Rootworm Emerges
Corn rootworm beetle emergence is also starting. Beetles of the northern species were observed on July 7 in Grant County. These insects will become increasingly common in corn this month, with peak beetle emergence occurring by mid-August. According to DATCP’s 2024 corn rootworm survey results, areas of the state most at risk of larval rootworm damage to corn are the southwestern, south-central, east-central, and northwestern crop districts, where beetle populations were highest last season. Larvae from eggs laid last summer are now feeding on corn roots and mid-July is the most effective time of year to evaluate rootworm pruning damage.
Corn Earworm Moths Arrive
Early corn earworm migrants are arriving in Wisconsin. The pheromone traps at monitoring sites in Dane, Dodge, and Rock counties registered large flights of 123-227 moths this week. The two-week total catch of 988 moths in nine traps indicates the migration is off to a strong start and will likely produce earlier-than-normal larval infestations.
For fresh market sweet corn, the standard treatment threshold is five to 10 adult earworm moths per night for three consecutive nights (when green silks are present). The treatment threshold for tomatoes is seven moths per pheromone trap per week.
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