Second in a series for new gardeners on managing herbivores in the garden, this week’s column focuses on the Japanese beetle, a non-native leaf-munching insect that is here to stay.
Some gardening seasons, such as the one just passed, will be worse than others in terms of sheer numbers of these beetles, and each season will bring an extension of the Japanese beetle’s range.
Just a few years ago, we had none in our Ellsworth garden. Last year we fought a constant battle. I needed a ladder to reach some of the Japanese beetles on our grape vine, struggling to position the pail of soapy water under a high leaf just before the beetles slipped their hold and tumbled to their end. My summer days were punctuated with beetle predation.
One piece of good news is that gardeners are not the only creatures preying on Japanese beetles. 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 millimeter) 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 it before it has a chance to reproduce.
So keep a watchful eye this coming garden season. 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 that 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, aphid secretions on 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 hand-picking 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 lesson.
This past year, I found Japanese beetles on native birches surrounding our garden for the first time, more evidence that they have become firmly entrenched in our area. This is very disturbing, since birches are ranked second only to oaks as larval hosts for native caterpillars that are a primary food for garden birds. I know of no bird that eats adult Japanese beetles, so their presence can only reduce the food source for important native herbivores and, ultimately, for our garden birds.
Finally, gardeners should consider eliminating Japanese beetle magnets in their garden, plants such as rugosa rose and Norway maple. Both of these plants are invasive species, both displacing native shrub and tree species in invaded natural areas. Eliminating them from the garden would benefit both the garden and surrounding natural areas.
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