June and July, 2017 will be remembered as one of the worst years in recent memory for Japanese beetles in Iowa. Japanese beetles came back in abundance after the absence of two or three years and achieved very high numbers in isolated locations around the state. Emergence of the first adults of the season was reported on June 24-25 and the population exploded quickly (more quickly than normal!) the last week of June. By the week of the Fourth of July, populations in the areas of emergence were at their peak and defoliated trees were already turning brown due to the combination of beetle skeletonization feeding and hot, dry winds.
Beetle emergence continued through the first week of July and by the second week of the month it seemed as if populations had started to recede. Trees and flowers that were defoliated will continue to look unsightly for the remainder of the season even after the beetles disappear for the year. Japanese beetles usually emerge over a 6 week period from late June to early August, though some stragglers are still present in September in some years. It appears the beetles came out more rapidly than usual and they may not persist as long into the summer as usual. We'll see.
Here are points to consider now that the worst of the attack is over in most areas.
- Healthy trees and shrubs are not killed by defoliation. Trees will survive the setback, though they may be weakened, so provide what “T-L-C” that you can. That means “water” during dry periods, especially for newly planted trees and shrubs.
- Spraying trees that are already brown is not useful, and will not make the tree green again (at least not right away). Defoliated trees may releaf weakly if moisture is available.
- Reports indicate most recommended tree and shrub insecticides killed beetles in infested trees and shrubs but that repeated applications were needed to keep up with beetle emergence.
- Though some homeowners reported that insecticidal soap stunned or killed beetles on contact, we have no evidence that insecticidal soap, extracts of garlic, hot pepper, or orange peels will be effective when sprayed on infested plants.
- Despite continued warnings that Japanese beetle traps do not alter the overall population, that traps will not protect landscape and garden plants, and that traps attract more beetles than they catch, use of traps persists, apparently because it is emotionally satisfying to see and smell thousands of dead beetles. If you do put out a trap, put it as far from your crop as possible.
- Large numbers of Japanese beetle or masked chafer adults DO NOT predict the number of white grubs or amount of turfgrass damage you will see in turfgrass this fall. To say your lawn will be ruined by white grubs (Japanese beetle and masked chafer larvae) without insecticide treatment is fraud intended to sell more pesticide.
- Spraying Japanese beetle adults on ornamental plants in July does not prevent white grub damage in the lawn in September.
- Similarly, controlling white grubs in the turfgrass in August does not reduce the number of adult beetles on ornamental plants the following year.
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