Japanese Beetles – E Letter

OK, the annual Japanese beetle questions have started coming in.  I saw my first one early this week.  (I think someone removed the head from that one.)

What you can do about Japanese beetles is ...  not much.  But here are some thoughts:

1.  Observe which plants are most seriously damaged and consider replacing them with plants from the list available at http://www.treelink.org/joa/2004/nov/held11-04.pdf  It will make your life in June and July much easier in later years.  This link starts off with 2 pages of scientific method followed by 7 columns of plants rated for resistance to feeding by Japanese beetles.  (Don't bother looking for resistant roses.)

2.  Where practical consider covering plants with light netting.  This won’t make the plant more attractive but it gives the insects a challenge.

3.  You can use insecticides such as carbaryl (Sevin), malathion, and imidacloprid (Merit).  There are some others such as the pyrethroids that may last longer (cyfluthrin, bifenthrin).  Spinosad and Neem based products seem to have little effect.  Insecticides are seldom effective for controlling damage to flowers and never recommended.  You can spray the outside of a bud this morning and find the beetles feeding on the inside of the open flower this afternoon.  And flowers attract so many beneficial insects that the net environmental impact may be negative.  Insecticides probably provide more satisfaction for gardeners than actual reduction of damage.  But you can realize some results if you’re willing to spray about once a week for the next few weeks.

4.  You can hand pick.  Some people take perverse pleasure in torturing the little fellers.  Remember that when disturbed Japanese beetles tend to drop.  Hold an open container of soapy water beneath for them to drop into.  Do this several times a day for best results.

5.  Japanese beetle traps can catch a lot of Japanese beetles and may even reduce the population by as much as 30 to 40 percent if enough traps are used (1 to 10 per acre).  That means that for every 100 in the area on a given day, the ones roaming free can be down to 60 or 70 with good trapping habits.  The traps must be emptied every day or two.  Apparently the smell of dead beetles is a less effective attractant when they get close enough to notice.  If you attract them from ½ mile away only to repel them with a full trap, they’ll visit your hibiscus and grapes instead.  Traps alone are seldom an effective control.

6.  The grubs or larvae can be controlled with soil-applied insecticides in late summer.  Since the adult Japanese beetles fly great distances when they emerge from the soil, grub control is seldom an effective control for the adults.

7.  #1 above seems to me to have the greatest long-term promise.  But if no one grows roses, cherries, and hibiscus anymore and everyone plants Atlantic white cedar instead, will they learn to feed on something new?

8.  Watch for me to wax more optimistic on another topic soon.

alcooke