Updated: July 22, 2021
By Paula Shrewsbury

So many Japanese beetle adults! Do natural enemies attack them?

In some years, often multiple years in a row, there are areas in Maryland that have high densities of Japanese beetle adults. Defoliation damage on a variety of plants goes along with these high beetle densities.  There are many natural enemies of Japanese beetle adults.  First, it is important to point out that in general most pest insects are cyclic in their population densities. When conditions are favorable (ex. abundant food resources, weather) herbivore populations will increase.  In general, many natural enemies respond to increasing prey (or food) abundance and increase in numbers. It takes time, however, for the natural enemies to “catch up” to the herbivore populations and actually start to reduce their densities. This is referred to as lag time.  As natural enemies do catch up to herbivores and reduce pest abundance (their food resources) over time, natural enemy abundance also goes down, then allowing pest populations to eventually increase again. This is partly why over time we see high densities of pests, then they “disappear”, and then increase again (cyclic populations). Secondly, we observe more natural enemies attacking the larval (white grub) stage of Japanese beetles than the adult stage.

One of the more common natural enemies attacking Japanese beetle adults is a group of parasitoids referred to as tachinid flies. Tachinid flies are true flies (Diptera) in the family Tachinidae. There are over 1,500 known species of tachinid flies and they can vary in size (3-14 mm) and color (black, grey, and orange). In general, most tachinid flies are robust and have stout hairs on their abdomen. At first glance, many look similar to the common housefly but they are very different animals. Tachinid flies are one of the most important families of parasitic flies providing biological control of numerous insects that are pests in ornamental, turfgrass, and agricultural systems. Tachinids are parasitoids of many caterpillars, sawfly larvae, beetle adults and larvae, earwigs, grasshoppers, and some true bugs. Most importantly for this conversation we frequently see tachinid flies attacking Japanese beetle adults!

A tachinid fly is on an aster flower.

A white tachinid fly egg on a stink bug nymph.

Adult Japanese beetles have caused extensive damage to a littleleaf linden.

Another predator, this wheel bug is feeding on an adult Japanese beetle.

Tachinid flies have interesting and variable egg laying strategies. In some species, eggs are laid on foliage near a host insect, the eggs hatch and the maggots are consumed by the host insect when it feeds on the foliage, then the maggots feed on and develop in the host insect – of course killing the insect. In other species, tachinid females have long ovipositors that they use to pierce the skin of the host insect and insert their eggs. In yet other species, the adult tachinid glues her eggs somewhere on the outside body of the host, eggs hatch, and the maggots penetrate into the host’s body. This is the most common strategy we see for tachinids that attack Japanese beetle adults. Look for the white eggs attached to the outside of the body of Japanese beetle adults. Regardless of the egg laying strategy, all tachinid flies are internal parasitoids of their hosts as larvae and they exit the host body to pupate. Tachinids can have one to multiple generations a year. Adult tachinid flies also feed on liquid such as nectar from flowers and honeydew from aphids and soft scales. In our studies on using conservation strips of flowering plants to conserve beneficial arthropods, we frequently observe tachinid fly adult activity.  

Other natural enemies observed attacking Japanese beetle adults include many generalist predators such as spiders, assassin bugs, predatory stink bugs, and birds. Given all of these predators, it seems natural enemies have a hard time suppressing Japanese beetle adults. This may relate to the fact that Japanese beetles are exotic insects, native to Asia. Therefore, their natural enemy complex is likely more limited in the U.S. than in its native range. Keep working toward conserving natural enemies to help their populations “catch up” to and suppress Japanese beetle densities. Also remember there are natural enemies that attack the white grub stage of the beetles which add to the complex of enemies of the Japanese beetle!