June 25, 2018
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Japanese beetles are here to stay, but…

Image courtesy of the Montana Department of Agriculture
Image courtesy of the Montana Department of Agriculture
Introduced to the U.S. in 1916 at a nursery near Riverton, New Jersey, the Japanese beetle is now established in every state east of the Mississippi River. It is here to stay.
By Reeser Manley

This garden season may well be remembered as the beetle summer. Cucumber beetles in

historic numbers caroused on the blooms and leaves of squash, pumpkin and, of course, cucumbers, while Japanese beetles ran rampant among the roses, raspberries, grapes and a host of other garden plants.

Looking back to last winter, a thick blanket of snow protected the overwintering cucumber beetle adults as they snuggled beneath the forest floor litter, while Japanese beetle larvae hibernated peacefully deep underground. Fewer winter mortalities translated into more summer beetles to keep gardeners on their toes.

I needed a ladder to reach some of the Japanese beetles on our grapevine, to position the pail of soapy water under the leaf just before they slipped their hold and tumbled to their end. My days have been punctuated with beetle predation. The bad news is that the Japanese beetle is here to stay. While annual fluctuations in summer populations will occur, nothing we do will eliminate this beetle from our gardens. Get used to it.

One piece of good news is that we are not the only creatures preying on Japanese beetles in our gardens. A small tachinid fly (Istocheta aldrichi) imported from Asia in 1922 and released into the wild to combat agriculturally-important herbivores, including the Japanese beetle, is our ally (with assurance from the USDA that it is host specific).

The larva of this fly is an internal parasite of the adult Japanese beetle. The adult female fly produces up to 100 tiny (about 1 mm) eggs over a two-week period, attaching a few to the thorax of any adult beetle it can find. Upon hatching, the maggots bore into the beetle body, killing the beetle before it has a chance to reproduce.

So, if you find a Japanese beetle with three or more small whitish dots just behind its head, don’t destroy it! Let it live out its short doomed existence so the tachinid eggs will hatch and the larvae will consume the beetle on their way to becoming adult flies.

Reports on the effectiveness of tachinid fly parasitization of Japanese beetles in northern New England range from 20 to 40 percent. Sounds encouraging, but hold on, there are some interesting wrinkles preventing the fly from becoming the be-all and end-all.

First, the presence of the tachinid fly depends on the presence of the adult fly’s major food source, the aphid secretions of Japanese knotweed, a non-native invasive plant that many of

us are trying hard to eliminate. So, to combat the Japanese beetle we need to encourage an Asian fly which depends for its existence on an invasive plant species from Japan.

Also, the tachinid fly’s life cycle is not well synchronized with that of the beetle. The flies emerge several weeks before the beetles and thus only lay eggs on the first emerging beetles, then disappear before the peak of beetle emergence.

Another ally in our fight against the Japanese beetle is an insect-eating nematode that seeks out beetle grubs in the soil during the fall. There are two such species of nematodes (microscopic round worms), but only one that is commercially available, Heterorhabditis bacteriophora.

This nematode forms a symbiotic relationship with a species of bacteria. The nematode penetrates the beetle grub and inoculates the grub with the bacteria. The bacteria

reproduce rapidly as they feed on the grub tissue. The nematode feeds on the bacteria.

You can purchase the nematodes from lawn and garden shops or through biological mail-

order catalogs. They are applied to the soil (typically lawns) with an ordinary sprayer. For best

results, they should be applied now, during the last three weeks of August. Encourage your

neighbors to treat their lawns as well.

Pheromone traps are controversial, largely because of their misuse. They are intended for monitoring beetle populations, not for primary control. If placed within the garden, they actually attract Japanese beetles that will feed on your garden plants before finally ending up in the bottom of the trap. However, if you place the trap at least 50 feet from your garden plants, beetles will leave them alone and go for the trap.

Consistent handpicking should remain a weapon in your arsenal, but check for white dots behind the head before dispatching a beetle. Also, susceptible vegetable crops can be covered with lightweight row covers to exclude beetles.

Don’t even think about toxic chemical controls, even those recommended for Japanese beetle control. The beetle will develop resistance while the chemical destroys beneficial insects, including pollinators, and the life in your soil. Surely, by now, gardeners have learned this


There are two garden plants that are particularly susceptible to Japanese beetles, rugosa rose and Norway maple. Why not eliminate both of these Japanese beetle magnets from your garden, eliminating two non-native invasive plant species in the bargain?

Meanwhile, the ladder stands next to the grapevine, a pail of soapy water between its legs.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.


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