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!