New England gardeners have waged war on plant-devouring Japanese beetles for generations now — especially those who grow roses and grapes! But the extensive damage these beetles cause could become a thing of the past, thanks to a tiny specialized fly called a Tachinid fly (Istocheta aldrichi). A parasite of adult Japanese beetles, the female fly glues her eggs onto young adult Japanese beetles (see the white dot on the beetle at the right?). Over the course of about a week, the eggs hatch into fly larvae, which then burrow into the beetle’s body to eventually consume and kill it.
Thanks to this beneficial insect, I’ve observed a noticeable reduction in Japanese beetle populations over the past ten years here in central MA. This host-specific tachinid fly species was initially introduced in the US as a control method in the 20th century, but it has taken many decades to become widely established. You can help speed up the process locally, though. Here’s how:
Attract tachinid flies to your property by planting plenty of flowering nectar plants (flower nectar and aphid honeydew are food for adult tachinid flies), and avoiding any pest control method that involves killing all the adult beetles present (including pheromone traps!). You don’t want to kill any beetles that are infected with the fly eggs — this will terminate the fly’s life cycle. You want the baby flies to live through to maturity and expand their local populations in your yard.
To really speed up the process of building up tachinid fly populations, work on selective culling of beetles NOT infected with the fly eggs. Do this when you see adult Japanese beetles present (usually the month of July in northern climates). This involves an occasional walk through your gardens with a bowl of water, using your hands to sweep adult beetles into the water, where they will float on the surface but cannot fly away. Look carefully for any beetles that are infected with fly cocoons (the white dots), and flick those beetles out so they can live a bit longer (wear gloves if you find it icky to touch them). The infected beetles’ death from the fly larvae will happen very soon, and they won’t damage your plants for much longer.
If you have chickens, they LOVE to eat Japanese beetles, so feed them your unparasitized beetles. Otherwise, just leave the uninfected beetles to drown in the bowl.
My hens follow me around during my beetle hunts clucking for their bowl of beetle treats!
Use the bowl method as often as possible during the month or so that the beetles are active. Over time, you will notice that more and more beetles are infected with the parasitic fly, and overall beetle numbers will begin to go down within a few years. Yes, this is a long-term approach, but one that does not require toxic insecticides that also kill beneficial species
The first year I started this method of Japanese beetle control (when they defoliated my pink and white Virginia roses, it was out and out WAR), I found perhaps 1 in 15 or 20 beetles infected with the fly larvae. Every year since, I’ve found and released higher numbers of infected beetles, and this year three-quarters (75%) of Japanese beetles were sporting the white dots. For the first time ever, my roses and my New England Asters (some of their favorite plants!) show very little damage from the dreaded beetles.
It just goes to show…with a little help from determined gardeners and the avoidance of pesticide use, some of our worst imported garden pests may just go away on their own, thanks to the natural balances provided by Mother Nature.
I DO feel badly for my hens, though. They’re wondering what happened to their daily beetle treats!
Controlling Japanese beetle grubs: Japanese beetles also cause plant damage at other times in their life cycle — their underground grubs (beetle larvae) love to eat grass roots, and large populations of these white grubs can destroy lawns especially in areas with zero habitat for tachinid flies (neighborhoods with mostly lawns, few flowering plants, and pesticide usage). Avoid using any insecticides containing Imidicloprid, a neo-nicotinoid insecticide that is highly toxic to bees and other insects. (Imidicloprid is sold in stores under such trade names as Bayer Merit). Instead, reduce Japanese beetle grub populations using a natural insecticide called Milky Spore, which contains bacteria that specifically kill Japanese beetle grubs and not other insects. Sprinkle Milky Spore granules (spores) into your lawn when grubs are actively feeding (fall and spring) — the grubs need to ingest the spores to become infected and die. In northern climates, spread of the spores in our cold soils can be slow, so expect to see results after about 3-5 years.