How to Prevent Overwintering of Japanese Beetle Grubs Organically?

Preventing of Overwintering of Japanese Beetle (watch YouTube video)

Currently, beneficial entomopathogenic nematodes (Photo 1) such as Heterorhabditis bacteriophora and Steinernema carpocapsae are being used as effective biological control agents for controlling Japanese beetles, Popillia japonica. The chemical insecticides have been avoided for controlling Japanese beetles because of their detrimental effects on both human health and the environment. 

Japanese beetles are among the most destructive pests affecting turfgrasses, ornamentals, vegetables, row crops, and fruits. If left uncontrolled, they can cause severe economic damage to agricultural and horticultural industries.

For effective management of Japanese beetles, coincide timing of application of Heterorhabditis bacteriophora or Steinernema carpocapsae nematodes with the occurrence of grub and pupal stages within their life cycle. To achieve this apply nematodes in the turf grass root zone at least twice a year.

Life Cycle of Japanese Beetles

The life cycle of Japanese beetles (Photo 3) typically commences in July when metallic green adults (Photo 2a) emerge from pupae. After emerging, adults primarily feed on the leaves, flowers, and ripe fruits. Adults mate from July through mid-August. Then females lay white eggs near the root zone of turfgrass in late August.

Then eggs hatch into small cream-colored grubs within a week. Young grubs then feed on the fine roots of turfgrass from August through September. While feeding, the grubs progress through the first and second stages, and third final mature stage in September. Mature grubs are creamy white in color with distinctive “C” shape (Photo 2b).

Overwintering of Japanese Beetle Grubs

Mature grubs continue to feed on grass roots throughout September. However, as temperature cool down to around 60°F (16°C) in October, they reduce their feeding activity and begin to move deeper into the soil, typically at depths of 2 to 8 inches. When temperatures fall to 50°F (10°C) at the end of October, the grubs become inactive and enter an overwintering stage.

These overwintering grubs will remain deep in the soil to avoid harsh freezing temperatures during the winter. However, when temperatures rise above 50°F (10°C) in March, mature grubs will move upward from their overwintering sites back into the grass root zone. Grubs will resume feeding on turfgrass roots until early June and then pupate in the soil. Adult Japanese beetles then emerge from the pupae in July, completing the life cycle within a year from one generation to the next (Photo 3).

Generally, first, second, and third (mature) stage grubs are present in the turfgrass root zone during August and September months. Mature grubs also found in the root zone of turfgrass in March after returning from their overwintering sites. These grubs will then pupate in late June. 

Optimal timings for application of beneficial nematodes 

Consequently the first ideal timing for applications of Heterorhabditis bacteriophora or Steinernema carpocapsae nematodes is any time in September. The nematodes applied in September will easily infect and kill both immature and mature grubs before they could migrate deep into the soil for overwintering. This application strategy prevents the overwintering of mature grubs that in turn interrupts the Japanese beetle life cycle.

The second ideal timing for applications of beneficial nematodes to turfgrass is anytime between March through June. During this window, the nematodes can infect, and eliminate all the mature grubs that have returned from their overwintering sites to the grass root zone. They can also infect and kill existing pupae. Consequently, the beneficial nematodes will effectively eradicate both mature grubs and pupae. This will prevent the emergence of the new generation of adult Japanese beetles in July and their life cycle will be disrupted (Photo 3).

Prevention of Overwintering of Japanese beetles Organically

While the Japanese beetle life cycle comprises four distinct stages (eggs, grubs, pupae, and adults), beneficial entomopathogenic nematodes are effective against only grubs and pupae. For effective control of both grubs and pupae, apply 25,000 nematodes per square foot area of turfgrass using water cans equipped with sprinkler heads (Photo 4).

When applied to turfgrass in September or spring, beneficial nematodes will locate mature grubs in the soil. Then they will enter their body cavity through natural openings such as the mouth, anus, and breathing pores. Once inside, Heterorhabditis bacteriophora nematodes will release symbiotic bacteria, Photorhabdus luminescens into the grub’s bloodstream. Similarly Steinernema carpocapsae will release symbiotic bacteria Xenorhabdus nematophilus into the grub’s bloodstream. Then bacteria will multiply rapidly, cause septicemia and kill mature grubs within 48 hours of infection (Photo 5). This will prevent grubs from moving deep into the soil for overwintering.

Additionally, when beneficial nematodes are applied in the spring (Photo 4), they will eliminate both grubs and pupae of Japanese beetles, as explained above. This comprehensive approach prevents the emergence of adult beetles in the following July, effectively interrupting the life cycle of Japanese beetles (Photo 3).