Complete Guide To The Animals That Eat Japanese Beetles
Japanese beetles are major pests in the United States. They cause huge losses every year by damaging plants upon which they feed. Many people struggle to protect their plants from these abundant and voracious insects and wonder what creatures eat them. If you wonder this too, this post can help.
Ants, raccoons, skunks, opossums, moles, and shrews are the primary predators of Japanese beetle eggs and larvae in the United States. Birds like crows, starlings, and grackles are the primary predators of adult beetles, with some contribution from orb-weaving spiders and predatory insects.
At first glance, such a lengthy list of predators would imply that Japanese beetles are easy to control. Unfortunately, this is not the case, at least in the United States. These insects continue to expand their range and cause widespread damage. So why is this? Read on to learn more about Japanese beetles and their natural predators.
What eats Japanese beetle larvae?
A large variety of predators eat adult Japanese beetles and their subterranean-dwelling larvae. These include insects, mammals, and especially insectivorous birds.
Mammals
Mammals such as opossums, raccoons, shrews, and moles eat Japanese Beetles, especially the larvae (called “grubs”).
Moles are mammals that live underground and have keen senses of hearing and touch. They also sense vibrations through the soil with their whiskers. Even though beetle grubs are small, moles can zero in on their locations. The grubs move slightly as they wriggle into position next to plant roots and chew the root tissue. Moles follow the vibrations and tunnel their way through the soil to the grubs.
Other predators such as raccoons, opossums and skunks hunt the larvae from above. These nocturnal predators dig the grubs out of the ground from above.
However, while they undoubtedly consume large number of grubs, these predators can cause problems of their own.
Moles cause damage of their own by disturbing the soil from below. Their subsurface tunneling humps the soil into long tracks. Deep tunnels make cone-shaped molehills so these mammals are unwelcome pests in their own right.
Raccoons, opossums, and skunks dig holes and damage plants themselves as they forage for food.
All this burrowing and digging disrupts the smooth grassy lawns that homeowners and golf courses strive for. So, these predators aren’t welcome solutions the problem of controlling Japanese beetles.
Birds
Ground-feeding birds like crows, starlings, grackles and American robins also eat Japanese beetle larvae and don’t disturb the grass or soil to the extent that the previous predators do. They probe or scratch the soil to find the grubs instead of digging down or tunneling below the surface.
Insects
Parasitic Natural Predators
Additionally, two species of parasitoid wasps were introduced to the United States to control Japanese beetles and attack the grubs.
Scientists have also introduced natural predators to the continental United States deliberately, in attempts to control the spread of these beetles.
Between 1920 and 1933 entomologists with the United States Department of Agriculture introduced 49 species of natural Japanese beetle enemies to the northeastern part of the U.S. (Potter and Held, 2002)
Unfortunately, only three out of the 49 species established itself in any significant way here in the United States (Potter and Held, 2002). Tiphia vernalis is a species of parasitoid wasp that parasitizes over-wintering grubs from the previous season. Tiphia popilliavora attacks larvae in the late summer (Potter and Held, 2002). The third species is a tachinid fly called Istocheta aldrichi that parasitizes the adults (Resh and Carde, 2009).
Nematodes are tiny, unsegmented, worm-like creatures one millimeter to two millimeters (0.04 to 0.08 inches) long on average. They are incredibly abundant in soil and many of them are parasites of invertebrates like Japanese beetle grubs.
What eats adult Japanese beetles?
Although many of the same predators consume the adults as the larvae, their tactics change from digging and searching for it on the ground to capturing the beetles in flight or plucking them off plants.
Japanese Beetles don’t spend very much time on the ground after they pupate from larvae into their adult form.
They are strong fliers but they are large insects. This makes them easy to spot and catch, whether in mid-air or while they are feeding or mating on plants.
Birds and mammals
Insect eating birds, like crows, starlings, grackles and blue jays eat adult Japanese beetles during the day, while skunks, raccoons and opossums hunt them at night.
Spiders
Both hunting and orb-weaving spiders prey on Japanese Beetles.
Active hunting spider species include ground hunters like wolf spiders and jumping spiders, which are more likely to hunt in raised vegetation. Adult Japanese Beetles tend to feed of leaves above the ground, so jumping spiders are major threats.
Orb-weaving spider species build large, elaborate webs with silk strong enough to snag and hold adult Japanese beetles. Once trapped in the sticky strands, the beetles are powerless against the webs’ large, predatory owners.
However, orb-weavers do not actively hunt Japanese beetles; they wait patiently until one happens to fly into their web.
Japanese Beetles fly well but they tend to remain stationary, preferring to crawl around on their chosen food plant rather than fly off in search of different leaves. Therefore, even large orb-weaving spiders such as Golden Silk Orbweavers (Nephila clavipes) or Yellow and Black Garden Spiders (Argiope aurantia) do not impact the overall number of Japanese beetles at any given time.
Insects
Most insects that attack Japanese beetles attack the grubs rather than the adults. Still, the insect predators that do attack adult beetles do so with great success when they put their minds to it.
Insects that prey on adult Japanese Beetles include:
- Assassin Bugs (family Reduviidae) such as Wheel Bug (Arilus cristatus)
- Praying Mantids (family Mantidae)
- Predatory Stink Bugs (family Pentatomidae)
- Dragonflies (order Odonata)
Dragonflies hunt Japanese beetles easily. Japanese beetles may strong fliers but dragonflies are much, much faster and far more agile in the air.
But like orb-weaver spiders, dragonflies fail to impact the overall Japanese beetle population very much. Some of this is because the two animals prefer different landscapes. Japanese beetles inhabit primarily grassy and cultivated areas while dragonflies hunt mostly in wild areas around open water.
These insects are more than capable of meting death to Japanese beetles.
A tachinid fly called Istocheta aldrichi attacks and parasatizes newly emerged adults (Potter and Held, 2022) (Resh and Carde, 2009). It is one of only three predator species deliberately introduced to the United States for Japanese Beetle control to successfully establish themselves (Potter and Held, 2022).
Why are Japanese beetles still so abundant?
Japanese beetles remain abundant because conditions support their reproductive needs, they reproduce quickly, the grubs gain protection by remaining underground, and introduced predators failed to thrive here in the United States.
Reproductive success
Like many insects, Popillia japonica reproduces quickly, especially in supportive habitats. A female can lay 40-60 eggs in a reproductive season and the sex ratio of larvae is about 1:1 (Regniere et al, 1981), which ensures a high numbers of males and females who will mate and create fertilized eggs. With the possible exception of the insect predators, all the predators that attack Japanese beetles reproduce slower than the beetles themselves.
Japanese beetle larvae gain a lot of protection by remaining underground until adulthood. Even though there are some predators that attack the grubs as discussed above, the impact is lower on the larval population than if the larvae were above ground. This increases the number of larvae that turn into adult beetles capable of reproducing.
No native predators
Japanese beetles gain another advantage in that they are non-native invaders of the continental United States. Native predator and prey species evolve together over time in complex relationships that result in keeping the populations of both species in check.
But when the Japanese beetle species arrived in the United States, it left its natural predators behind in its native Japan.
Predators native to the United States have learned to prey on them over time. But since none evolved to eat Japanese beetles specifically, the beetles form only a portion of the native predators’ overall diet. That leaves the Japanese beetles free to out-reproduce their predators.
Finally, most of the natural enemies introduced into the United States from the beetle’s native Japan failed to establish themselves here.
Only the two parasitoid wasps species and the tachinid fly lasted longer than a few years after release into the wild. Even after many years, these natural predators are distributed only sporadically across the northeastern United States. Japanese beetles are widespread from Maine through South Carolina. So their control over the overall beetle population is poor (Potter and Held, 2002).
The combination of all of these factors mean that the pressure exerted by even this lengthy predator list isn’t sufficient to control the beetles’ spread.
Conclusion
In conclusion, Japanese Beetles remain an abundant pest, despite predation and concerted control efforts.
They were first noticed in the U.S. state of New Jersey in 1916 and have become one of the most successful biological invaders in our history. Even today—100 plus years later and counting—the United States spends millions of dollars trying to control these insects with only moderate success.
Despite all this expense and control effort, Popillia japonica continues to thrive, quietly munching away both above and below our feet.
Where to go from here
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References
Potter, Daniel A. and David W. Held. 2002. “Biology and Management of the Japanese Bettle.” Annual Review of Entomology 47: 175. https://www.annualreviews.org/doi/abs/10.1146/annurev.ento.47.091201.145153
Regniere J, Rabb RL, Stinner RE. 1981. Popillia japonica: simulation of temperature-dependent development of the immatures, and prediction of adult emergence. Environ. Entomol. 10:29096 https://academic.oup.com/ee/article/10/3/290/2392528?login=true
Resh, Vincent H., and Cardé, Ring T., eds. 2009. Encyclopedia of Insects. San Diego: Elsevier Science & Technology.
