A continued menace across much of the United States, the Japanese beetle population density is currently lower than in recent years. The decline, in large part, is due to the cyclical nature of the insect, as well as cooler spring temperatures.
Reporting from Iowa State University estimates that beetle emergence from the soil in the 2019 growing season was between 7-10 days later than previous years, a commonality amongst many insect species this summer. The report cites cooler spring and early summer conditions and a resulting fewer growing degree days.
For full maturation, Japanese beetles need approximately 1030 growing degree days and will continue emergence until 2150 growing degree days. Fortunately, lower population densities combined with somewhat-high threshold tolerances in both corn and soybeans, should provide for minimal economic losses from the Japanese beetle this growing season.
Extension professionals throughout the Midwest warn, however, that later planting dates will create pest vulnerability from other insect pests as crops maturity varies.
As crops reach maturity later in the growing season than normal pest concentrations will build both above and below the soil surface, creating issues for this growing season as well as 2020.
Crop maturity should be a top-tier consideration growers keep in mind when prioritizing scouting this season and the 2020 growing season as pest hot-spots will most certainly exist in later planted fields as noted in a recent University of Nebraska, Lincoln grower update:
“Later in August and September, we will likely see a concentration of insects in later maturing corn and soybean fields. Bean leaf beetles, rootworm beetles, various caterpillars, soybean aphids, and other insects may concentrate in later maturing soybean fields as more mature fields have leaves that are yellowing and beginning to drop. Similarly in corn, later silking fields are a “bug magnet” for many insects, particularly corn rootworm beetles. If growers plan to plant corn in these later maturing fields in 2020, there is a greater risk for injury from corn rootworm larvae, because high numbers of adults may move into these fields from surrounding fields and lay eggs there,” UNL Extension Entomologist, Robert Wright reports in the July 2019 update.