Early October is the best month to kill Japanese beetle grubs with beneficial nematodes / by Ganpati Jagdale

Damage caused by Japanese beetles

Japanese beetles (Popillia japonica) are one of the most damaging insect pests of many agricultural and horticultural crops, ornamental plants and turfgrasses. Larvae of Japanese beetles are also called as grubs that mainly damage plant roots whereas their adults damage only aboveground plant parts like flowers, fruits, leaves and twigs.  Symptoms of feeding damage caused by Japanese beetle grubs to turfgrass is obvious, as their early feeding on turf roots causes localized patches of dead turf grass in the yards or golf course greens. As the feeding activity of grub progresses, these small patches of dead turf join together and form the large patches. Due to severe root damage, dead turf becomes loose and can be easily picked up with hand like a piece of carpet. The most important sign of presence of Japanese beetle grubs in the lawn is that the infested areas of lawn is destroyed by small animals like raccoons and skunks or by birds that are looking for grubs to feed on them. The adult Japanese beetles cause a severe feeding damage to leaves, flowers and fruits of different plant species. Heavy infestation of Japanese beetles can completely skeletonize all the leaves and eventually defoliate the whole plants. 

Identification of Japanese beetles

Japanese beetles are very easy to identify, as their adults are oval shaped, about 1.5-inch long and shiny metallic-green colored beetles (Photo 1). Females lay white colored and oval shaped eggs that hatch into whitish in colored larvae with yellowish-brown head capsule and three pairs of thoracic legs. If disturbed, larvae generally form C- shape (Photo 2). Pupae of Japanese beetles are pale creamy to greenish in color.

 Photo 1. Adults of Japanese beetles.

Photo 1. Adults of Japanese beetles.

 Photo 2. "C" Shaped grubs of Japanese beetles.

Photo 2. "C" Shaped grubs of Japanese beetles.

Life cycle of Japanese beetles

Japanese beetles usually take one year to complete their egg-to-egg life cycle. Generally, life cycle begins when adults of Japanese beetles emerge from pupae in the late June through July and start feeding on leaves, flowers and fruits of different plants. After mating, females lay eggs about 1-2 inches deep in the soil near to the grass roots. These eggs hatch within 2 weeks (i.e. early in August) into first instar grubs that immediately start feeding on grass roots. While feeding, grubs develop into second and third instars from August through October. When temperature starts cooling down during late September through October, these third instar grubs start moving deep into soil for locating overwintering sites. When they find suitable overwintering site deep in the soil, the third instar young grubs will become immobile and spend whole winter (November through March) without feeding until temperature begin warming up early in the spring. Once these grubs sense that temperature is warm, they will move back into the turf root-zone where they resume feeding on turf roots until they become mature. These matured grubs will then pupate in the soil early in the June. Then emerge as adults from pupae by the end of June, females will lay eggs and life cycle continues.

Biological control of Japanese beetles with beneficial nematodes

Because third instar grubs are constantly on the move to find overwintering sites in the soil during September through October, they become easy targets for beneficial entomopathogenic nematodes like Heterorhabditis bacteriophora (Photo 3.) and Steinernema carpocapsae (Photo 4.)It has been also demonstrated that the young grubs are more susceptible to beneficial nematodes than the mature grubs (last instar) of Japanese beetles. When individually applied, both of these beneficial nematodes can be very effective in killing grubs but their effectiveness can be maximized if they are applied together. This is because each nematode uses different strategy to find its host. For example, S. carpocapsae uses an "ambush” whereas H. bacteriophora uses a “cruise foraging” host finding strategies to seek their insect hosts. In an “ambush” strategy, the infective juveniles of S. carpocapsae will “sit-and-wait” and attack passing by and highly mobile Japanese beetle larvae/ grubs that are searching for overwintering sites. In a “cruise foraging” strategy the infective juveniles of H. bacteriophora will move actively in search of immobile insect stages like overwintering Japanese beetle grubs or pupae of Japanese beetles in the soil

 Photo 3.  Heterorhabditis bacteriophora  nematodes

Photo 3. Heterorhabditis bacteriophora nematodes

 Photo 3. Steinernema carpocapsae nematodes.

Photo 3. Steinernema carpocapsae nematodes.

How do beneficial entomopathogenic nematodes kill Japanese beetle grubs or pupae?

When the infective juveniles of beneficial entomopathogenic nematodes such as S. carpocapsae and H. bacteriophora are applied individually or together to the soil surface or thatch layer, they will move in the soil and seek Japanese beetle grubs and pupae. Once they find a grub and/or pupa, they will enter into their body cavities via natural openings such as mouth, anus and spiracles. In the insect body cavity, S. carpocapsae will release symbiotic bacteria called Xenorhabdus nematophila or H. bacteriophora will release symbiotic bacteria called Photorhabdus luminescens from their gut in the insect blood. Nematodes use these symbiotic bacteria as weapon to kill Japanese beetle grubs or pupae and as food. Once in the blood, bacteria will multiply rapidly, cause septicemia and kill Japanese beetle grubs or pupae usually within 48 h after infection. Inside the grub or pupa cadavers, nematodes mainly feed on multiplying bacteria, mature into adults, reproduce and then emerge as infective juveniles from the cadaver to seek new Japanese beetle grubs, pupae or other insect hosts that may be present in the soil.

How many beneficial nematodes should be applied?

For the effective control of Japanese beetle grubs and/or pupae, beneficial entomopathogenic nematodes like H. bacteriophora and S. carpocapsae should be applied at the rate 23000 nematodes per square foot area to the soil surface or thatch layer using water cans or traditional knapsack sprayers.

When beneficial nematodes should be applied?

Since beneficial nematodes are susceptible to UV radiation, it is recommended to apply them in the evenings. Also, nematodes need a film water for their easy movement in the soil, apply them when there is enough moisture present in the soil. It also recommended applying irrigation after application of nematodes so that irrigation water will wash them from grass blades into soil.