A cool start with some flashes of extreme heat with consistent rain — the summer of 2019 has been a little off-kilter, which has been affecting the insects of Iowa, in turn.

Two types of insects, Japanese beetles and thistle caterpillars, have been especially abundant and havoc-wreaking throughout the month of July due to separate anomalous weather events.

Japanese beetles generally break out for six to eight weeks in early June, but due to the cool weather at that time, the adult beetles arrived in en masse around the beginning of July.

These beetles can cause a great deal of damage when it comes to linden and crabapple trees, fruit-bearing trees and grapevines (which can pose issues for vineyards) along with other foliate flora.

So if a beloved tree or bush has suddenly turned brown partly through the summer, these beetles are likely to blame. It's likely too late to treat for them, but luckily the damaged has passed and it's not permanent.

"For deciduous trees, Japanese beetles feeding on the leaves is disfiguring and stressful," said Donald Lewis, an entomologist at Iowa State University, "but not fatal."

The thistle caterpillars have been hatching in unusual droves across Iowa soybean fields. Thistle caterpillars are not considered a pest species and, according to Robin Pruisner, Iowa's head entomologist, haven't posed a serious threat to crops at any time in her memory.

This specific type of caterpillar is propagated by the painted lady butterfly, which migrates to Iowa in the summer from southern parts of the nation. Large amounts of these butterflies were reported in the spring, blown north in greater quantities than usual by the many spring storms or an unexplained aberration in amount migrating or surviving the winter.

Thistle caterpillars are noticed the most on soybeans, a legume that painted lady butterflies prefer and can be found in greater numbers along the edges of soybean fields.

The high number of thistle caterpillars noticed this year could be problematic for soybean plants, though no one is sounding the alarm quite yet.

"The defoliation of soybeans and can reduce yield," Lewis said. "But soybeans have a good ability to recover and compensate and since thistle caterpillars are concentrated at the edges of soybean fields, the overall impact and minimal and treatment is rarely needed."

"Populations rarely build up to treatable levels in these other crops," Erin Hodgson, also of Iowa State University, has written. "Treatments recommendations would be warranted if fields reach 30 percent defoliation."

Though outbreaks of Japanese beetles and thistle caterpillars may appear concerning, the general cycle of nature plays out and neither species is capable of causing long-lasting to the common tree, garden or soybean field.

Aaron Calvin covers trending news for the Register. Reach him at acalvin@registermedia.com or 515-556-9097.

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