We can always use good news when it comes to dealing with problem insects in our landscapes. Anyone who has battled Japanese beetles to prevent plant carnage knows the amount of work and dollars it takes to manage them every summer.

Where the Japanese beetle is native, in northern Japan, there are natural insect enemies that keep the populations manageable, and they are not a major problem as they have become along the Front Range and many other states. When the beetles were first detected in New Jersey in 1916, their natural insect enemies weren’t present. Subsequently, Japanese beetle numbers grew and spread unchecked. They had a literal free-for-all buffet table set for them with millions of acres of irrigated lawns and more than 300 favored plant species.

Natural insect enemies and specific pathogens are collectively called biocontrols, which can cause injury or death to pest insects. A common example of natural pest enemies that gardeners know well are ladybugs, which rid ornamental plants of plant-sucking aphids. Another example is a natural pathogen product that many gardeners use for mosquito larvae in water features that are sold as mosquito dunks, or bits containing the active ingredient Bacillus thuringiensis (Bti) and only toxic to mosquitos and fungus gnats.

This is truly good nature at work.

Over the decades, East Coast and Midwestern states have paved the way for successfully introducing biocontrols to reduce Japanese beetle numbers. Professionals in Colorado followed suit and started the releases in 2015.

Spread this good news far and wide: This is real hope and help for many plants we love to grow here.

The three biocontrols that have been released have been strictly approved, coordinated and regulated by state and federal authorities so they don’t cause harm or have any other unintended consequences to our soil and insect populations, which include pollinators. All three biocontols have been reviewed and permitted for release in Colorado to specifically target Japanese beetle. However, they are not for purchase or allowed for distribution by the general public.

The professionals who began the releases are Whitney Cranshaw, emeritus professor of entomology from Colorado State University, and biological control specialists with the Colorado Department of Agriculture. The long-term goal of the biocontrol releases, according to Cranshaw, is that “one or more of the organisms will establish and reproduce at the release site. The organisms will then spread on their own over time to cover a wide area. As the organisms establish and increase, populations of Japanese beetle will decrease – and these reductions will be permanent.”

A successful outcome from the releases might be that “in ten to fifteen years, there would be three Japanese beetles on your roses instead of ten,” Cranshaw said.

The three biocontrols that have been released include:

  •  A fungal spore pathogen that harms both Japanese beetle adults and their offspring white grubs (larvae). The spore is called Ovavesicula popilliae, and when applied to the soil where JB white grubs are residing, it will become infected by the spore pathogen which then hinders the white grub’s health and longevity. Disease-affected Japanese beetles become more susceptible to winter death, shortened survival as an adult and reduced ability to reproduce. Ovavesicula popilliae has become established in areas where it has been released, including Boulder, Denver, west Arapahoe County and Pueblo.
  • A parasitic fly — also referred to as a tachinid fly and commonly called the winsome fly (botanically named Istocheta aldrichi) — targets adult Japanese beetles. They look very similar to a common house fly. So far, they have been found in Arapahoe County in the Littleton area.
  • A parasitic wasp called Tiphia vernalis targets overwintering Japanese white grubs in the soil under grass turf. The wasp, often called the spring tiphia, are not established at the time of this writing.

There are also plans for future biocontrol releases in various areas along the Front Range.

What is a parasitic insect?

A ladybug will eat pest aphids. In nature (and some horror movies) there are other ways in which good insects kill or consume bad insects. Two of the biocontrols for Japanese beetles are parasitic type insects that eat the pest but in a different life stage of its reproduction.

Parasitic insects, including the Tiphia vernalis and Istocheta aldrichi, lay eggs on the target Japanese beetle. Once the egg grows to the larva stage, the larva uses the Japanese beetle as a food source as it burrows and grows into the host pest.

Once the egg is laid on an adult Japanese beetle, it becomes the food source for the larva of the winsome fly or tachinid wasp. Over time, the larva will pupate and spend its life inside the decaying host pest insect. When it has developed into its adult stage, it will begin the cycle all again looking for a host Japanese beetle.

Thanks to Mother Nature and the whole coordination of predator needing the correct prey at the correct time, the winsome fly has a one-year life cycle just like Japanese beetles (same for Istocheta aldrichi, the spring tiphia wasp).

The predator-prey process may seem a bit cringe-worthy, but this is part of nature’s plan and biological cycle of life and death.

Detection or parasitization of a Japanese beetle by a winsome fly is easy to spot. A white egg or possibly more eggs will be visible on the upper portion of the beetle. If an egg is seen, resist squishing the beetle, using any type of spray, or flicking it to a soapy death because instead of us doing the killing, the winsome fly needs the Japanese beetle to grow and develop, and then will then destroy the beetle.

Gardeners and homeowners cannot technically know if a Japanese beetle has been infected with the Ovavesicula popilliae fungal spore pathogen. However, as the pathogen spreads over time, gardeners will begin seeing fewer beetles on plants.

How do we encourage and support beneficial parasitic insects? They need food sources, too.

Instead of “build it and they will come,” we should “plant it and they will come and stay” — the good biocontrol insects that is.

The two natural enemy insects of Japanese beetles are present for a short period of time over the summer, so try to plant plants that coincide with their presence. The spring tiphia wasp is most active in late May while the winsome fly is present in late June and early July.

Many of the plants listed below are also beneficial to butterflies, pollinators and other natural enemy insects which will be around longer through the summer.

Food sources for the spring tiphia wasp come from honeydew, plant pollen and nectar. Honeydew is a byproduct of aphid poo, the sticky stuff that gets on car windows when parked under trees. We would rather not have honeydew around, but some parasitic insects feed off it, especially the spring tiphia wasp. They also dine on the sugary substance (extrafloral nectar) that peonies emit before they bloom. You’ll often see ants on the buds too. That’s OK, since tiphia wasps and ants can share. Yes, plant more peonies.

Great plant food choices (nectar and pollen) for biocontrol insects are plants from the Apiaceae or carrot family, the Compositae or aster family  and Brassicaceae or mustard family. The winsome fly especially likes white flowers in the carrot family. The blooms on carrot family plants look large in their entirety, but the individual flowers are quite tiny and shallow, just the right size for beneficial insects to access. Plants in this group include annual Bishop’s flower also called ammi, dill, fennel, cilantro, parsley, lovage, chervil and angelica.

Aster family plants include asters, cosmos, coneflowers, blanket flower, yarrow, daisies and sunflowers. Mustard family plants include radish, mustard, arugula, broccoli, cauliflower, kale, (allow these annual vegetables to flower all summer), sweet alyssum, yellow stardust draba, rockcress, rabbitbrush, nasturtium, wallflower, candy tuft and annual stock.

Always respect natural enemy insects by avoiding broad-spectrum insecticides including organic ones that can adversely affect these good insects we want to encourage in our gardens.

This fall is a great planting time to get a jump on many of these plants for next spring.


BugGuide Species Istocheta aldrichi

Colorado Department of Agriculture Biocontrol

Developing Japanese Beetle Biological Control Programs

Japanese Beetle Fact Sheet CSU

Japanese Management Colorado

Japanese Beetle Plant Damage Lists

Natural Enemies of Pests

Plant Flowers to Encourage Beneficial Insects

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