Japanese beetles have begun emerging in the Quad-City area, making holes in the leaves and flowers of some of our favorite plants. They will continue to be present through most of the summer, generally ending around late August. They peak for about six weeks.
Japanese beetles eat almost 300 different kinds of plants and are particularly fond of linden and birch tree leaves, roses, raspberries and the fruits of trees such as peaches and plums. They don't kill plants, but they skeletonize the leaves.
What to do
Entomologists from university extension services say that gardeners' first line of defense should be to regularly scout their plants and hand-pick any beetles they see, dumping them into a bucket of sudsy water or rubbing alcohol, where they drown.
Hold the jar under a beetle, poke it, and the beetle will fall into the jar. Late afternoon is the best time as they tend to drop when disturbed; they are more likely to fly earlier in the day.
Experience suggests that the presence of one beetle invites the presence of more, so if gardeners can keep up with the population, the numbers can be kept in check.
Chemical insecticides — including sprays and powders — are effective, but they also are indiscriminate, killing numerous insects. This includes pollinators, such as bees, that are getting so much attention these days because their numbers are declining.
Gardeners concerned about not harming pollinators need to read the fine-print labels of products on the garden center shelves to determine the active ingredients.
About linden trees
One of the key concerns is linden trees. In previous summers in the Quad-Cities, the leaves of some of these trees had been skeletonized to the point that the trees appeared brown. It's easy to see why homeowners would want to protect them.
And of course hand-picking is not an option with trees.
But university extension entomologists recommend against treating lindens with a family of chemicals called neonicotinoids.
These include imidicloprid (found in Bayer Tree and Shrub Insect Control and Merit), clothianidin (Arena) and thiamethoxam (Meridian) because these chemicals are incorporated into the plant itself (systemic control), which means that it becomes part of the leaves, flowers and pollen.
This could harm visiting honey bees and other pollinators.
"Until we know more about translocation into pollen in various plants, it is prudent to avoid these insecticides in applications to plants attractive to pollinators," Phil Nixon, a University of Illinois Extension entomologist, has said.
A Japanese beetle eating the linden leaves will die, but honey bees also will take tainted pollen back to their hives and feed it to their larvae.
"I don't know of anything that will control adult Japanese beetles (on lindens) that will not have at least some effect on honey bees and other pollinators," Nixon said. "On the other hand, Japanese beetles can essentially destroy the aesthetic usefulness of a linden tree after early July."
So, the decision is up to the homeowner.
For smaller plants, a popular product called Sevin contains the active ingredient carbaryl which, while not a neonicotinoid or systemic, can also be taken back to a honey bee hive and fed to larvae because the honey bees confuse the dusty particle remaining after the spray dries, with pollen.
The caveat is that the bees have to find the Sevin first. If it is applied only to leaves or plants without flowers, "chances are excellent the bees won't notice it," Nixon said.
A family of chemicals called pyrethroids (found in Tempo) "is toxic to pollinators including honey bees" but the insecticide is not taken back to the hive, Nixon said.
Ditto for the organic chemical spinosad (found in Captain Jack's Dead Bug Brew) - it is toxic to pollinators, but is not carried back to hives.
A key with the application of any chemicals is to avoid the plant's flowers, because that is what the pollinators are after, Nixon said.
Although traps that attract beetles and kill them are not recommended by university horticulturists, many gardeners are strong advocates of their use.
Extension horticulturists say the traps simply attract beetles to your yard. Some gardeners say the more dead beetles, the better.