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Biological protection from the Japanese beetle, a troublesome garden beetle

by BioGrow

The Japanese beetle (Popillia japonica) is an insect that is damaging many vegetable gardens and gardens. Just like other well-known insects, such as the Asian stink bug and the tomato leafminer, it is a foreign species that does not belong to our ecosystem. As a result, without its natural predators present, the insect finds great ease in spreading and proliferating. In Japan, its country of origin, it does not cause serious damage, as it is kept under control by its natural enemies. While, in the United States, where it was accidentally introduced in the early 20th century, it is one of the most destructive insects overall.

In Italy, it is still not clear how it arrived. The first outbreaks were reported in 2014 in Piedmont and Lombardy. In these regions, phytosanitary services have been activated to contain the infestation, preventing its spread to other areas of the country.
Let’s get to know this insect better, the damage it causes, and the techniques for biological defense to be used.
We also ask you to report the presence of popillia japonica to your regional phytosanitary service if you spot it, especially if you live in regions other than Piedmont and Lombardy.

Origin and Spread of Japanese beetle

Popillia japonica

Popillia japonica

Popillia japonica is an insect of the Coleoptera order. It belongs to the suborder Polyphaga and the family Scarabaeidae, genus Popillia. This beetle has nothing to do with other insects belonging to this order, such as the weevil or ladybugs. As the name suggests, it has clear Japanese origins, but it spread to the United States as early as 1916. It is hypothesized that larvae contained in imported iris bulbs were the cause. In the United States, numerous attempts to eradicate it have been in vain. Despite the use of insecticides and biological control agents, the beetle gradually colonized the eastern states and reached Canada. Today, popillia japonica is one of the most problematic pests present overseas. About half a billion dollars are spent each year to keep it under control. Currently, the Japanese beetle is also present in China, Russia, and Europe, where the first report was in Portugal, Azores Islands, in the 1970s. After this infestation, no further outbreaks were reported in the Old Continent, at least until 2014. In that year, it reappeared in Italy, in the Ticino Park, in the provinces of Novara and Milan. From that moment on, monitoring and fighting by regional phytosanitary services began. However, containing the insect is problematic due to its enormous capacity for proliferation and the absence of natural antagonists. There are fears that it could soon spread beyond the borders of the two regions and populate the entire country. It is worth noting that in the phytosanitary regulations 2000/29/EC, amended by directive 2002/89/EC, popillia japonica was included among the quarantine organisms whose introduction and spread in the EU territory had to be prohibited.

Identification of Popillia Japonica

Popillia japonica

Now let’s describe popillia japonica. The adults have the typical oval shape of beetles. They are up to 11 mm long and up to 7 mm wide. Males are usually smaller than females.
The upper body is metallic green, while the elytra, which cover almost the entire abdomen, are bronze-copper colored. On the sides and at the end of the abdomen, there are bands of white-gray hairs, called setae. These clearly distinguish popillia japonica from other similar species.
Adult insects also have antennae, which are usually folded but unfold into structures similar to flower petals when they sense interesting scents or pheromones from the opposite sex.

Eggs, Larvae, and Pupae of the Japanese Beetle

Larva of Popillia japonica

Larva of Japanese beetle

The eggs of the Japanese beetle are spherical, cylindrical, and almost transparent white in color. They are about 1.5 mm in size but double in volume during development. The larvae are almost transparent, covered in brown fuzz. The head, which is already distinguishable at this stage, is yellow-brown, with darker mandibles. When at rest, the larva curls up into a C shape. The larvae transform into pupae in the soil. They are generally about 14 mm long and 7 mm wide. The color of the pupa varies from cream yellow to metallic green, depending on the stage of development.

Life Cycle of Japanese beetle

It has been observed that at our latitudes, the Japanese beetle completes its life cycle in one year. Adults begin to appear in June, peaking towards the end of July. An adult lives on average 4-6 weeks. After emerging from the soil, it moves onto plants and starts feeding and mating. This is when female egg-laying begins. Preferring humid grassy meadows, they dig deep burrows up to 10 cm and can lay 3-4 eggs at a time. During their lifespan, they can lay up to 60 eggs.
Egg-laying activity continues until the month of September. The eggs, which need moist soil to complete their cycle, hatch, giving rise to first-instar larvae. These move through the soil, feeding on the roots of the plants they encounter.
The larvae grow rapidly to about 10 mm in length before undergoing the first molt.
This second instar continues trophic activity for another 3-4 weeks until they undergo another molt. The majority of the larvae reach the third larval stage when autumn is approaching.
Larval activity stops when the soil temperature reaches around 10°C. Thus, most of the larvae overwinter in the soil as third instars, at a depth of about 5–15 cm. With the arrival of spring, the overwintering larvae gradually resurface and begin to feed on roots. They transform into pupae after about 4-6 weeks.
As mentioned, at this point, the adult stage emerges, usually starting from June.

Damage to Crops

Popillia japonica crop damage

Popillia japonica crop damage

Over more than a century of presence in the United States, the habits of the Japanese beetle have been studied. It has been observed that adult specimens cause damage to over 300 types of different plant species, including fruit trees, ornamental plants, and vegetable crops.
Among the most common species affected are: tomatoes, vines, roses, peppers, plums, pears, peaches, blackberries, corn, peas, and blueberries.
The damage caused by popillia japonica consists of intensive erosion of leaves, flowers, and fruits. This beetle has a marked gregarious behavior, so it is possible to find hundreds of insects on a single plant. Naturally, this causes irreversible damage in a very short period of time. While a single beetle cannot cause significant damage, dozens of specimens can do so quickly. Starting from the top and working downwards, leaves are completely eroded, leaving only the veins of the leaf intact. Another noteworthy point is that the Japanese beetle is active during the hottest hours of the day, preferring plants that are well exposed to the sun.
The larvae, on the other hand, by feeding on grass root systems, pose a serious problem for grassy areas like golf courses, regular gardens, lawns, and pastures. The damage manifests as extensive areas of vegetation that first turn yellow and then dry out. Furthermore, the larvae attract birds and moles, which, by digging into the soil to catch them, cause indirect damage.

Biological Defense Strategies against Popillia Japonica

The characteristics of this beetle make infestation control difficult. Additionally, different strategies must be used for adult specimens and larvae. Given that the presence of the insect must be reported by law to the phytosanitary authorities, let’s see what are the most commonly used biological techniques to combat popillia japonica.

Physical Removal

The simplest remedy for combating adult specimens is manual removal. The gregarious behavior often results in many insects being clustered in one spot. Thus, it is easy to remove them. Shaking the infested plant during the hottest hours of the day, letting the insects fall into a bucket of water and Marseille soap, can effectively eliminate them. Beetles are not good swimmers and will drown.

Insect Nets

If you cultivate fruit trees and high-value commercial plants, using insect nets can be an excellent solution. This technique has been previously discussed when dealing with Asian stink bugs and is certainly very effective.
These types of nets are readily available on the market and can be purchased here.

Monitoring and Mass Capture Traps

Trap for Popillia japonica (Japanese beetle)

Trap for Popillia japonica (Japanese beetle)

In the long history of biological combat against the Japanese beetle in the United States, one of the most effective methods is the use of monitoring and mass capture traps for adult insects. These traps contain a dual attractant consisting of a food lure and a specific pheromone for the species.
These traps must be acquired from specialized stores. Their use represents an easy and economical way to reduce the population of popillia japonica. However, they must be used correctly. Paradoxically, positioning the traps too close to the plant you want to defend could worsen the situation by attracting more specimens. Therefore, it is better to place the traps away from the field, as on average each trap can capture only about 75% of the beetles present in the area.

Control of Larvae with Bacillus Thuringiensis

To eliminate larvae from the soil, other remedies must be employed. One that has already been tested is Bacillus thuringiensis var. japonensis, a specific strain that is only available in specialized stores.
We’ve previously discussed the characteristics of Bacillus thuringiensis. This “japonensis” strain must be directly sprayed onto the soil to reach the larvae, preferably during their early stages. The watering should be abundant, so that the solution penetrates deeply. This ensures that the larvae of the Japanese beetle will consume it.

Entomopathogenic Nematodes

Another bio-defense technique against the Japanese beetle is the use of entomopathogenic nematodes, as previously discussed in relation to the black vine weevil.
These microscopic organisms enter the beetle larvae and inoculate them with bacteria. These bacteria, in turn, reproduce by feeding on the larvae themselves, causing their death. The next generation then reproduces at the expense of the larvae, thereby fueling a cycle. This type of biological agent is harmless to humans, animals, and other crops.

Predatory Insects and Animals

In nature, there are some insects and animals that feed on the larvae of the Japanese beetle. We’ve already mentioned moles and birds, which, however, are not viable solutions as they cause damage themselves. The same applies to ants, which are voracious consumers of beetle eggs.
There is also a particular family of beetles that love to feed on the larvae of their “cousin”, the Japanese beetle. These are the Carabidae, a family that counts 40,000 species worldwide, with more than 1,300 in Italy.
Finally, in the United States, the use of other parasitic insects against beetle larvae has been successfully tested, all of which are of Asian origin. These include the tiphia vernalis and tiphia popilliavora, which are hymenopterans of the Tiphiidae family, and the istocheta aldrichi, a dipteran from the Tachinidae family.

Further Reading

  1. NCBI Insects – “Evaluation of Indigenous Entomopathogenic Nematodes as Potential Biocontrol Agents against Popillia japonica (Coleoptera: Scarabaeidae) in Northern Italy” – This study evaluates the potential of indigenous entomopathogenic nematodes as biocontrol agents against Popillia japonica in Northern Italy.
  2. NCBI EFSA Journal – “Pest categorisation of Popillia japonica” – An article that categorizes Popillia japonica as a pest and discusses its impact.
  3. NCBI Insects – “Mulches Used in Highbush Blueberry and Entomopathogenic Nematodes Affect Mortality Rates of Third-Instar Popillia japonica” – This research investigates how different mulches used in blueberry cultivation can affect the mortality rates of Popillia japonica larvae.
  4. TandFOnline Biocontrol Science and Technology – “Differential susceptibility of Popillia japonica 3rd instars to Heterorhabditis bacteriophora” – This article discusses the varying susceptibility levels of Popillia japonica larvae to a specific nematode strain.

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