Japanese Beetles: Strategies for Prevention and Control

Japanese beetles (Popillia japonica) are an invasive species which was introduced into the United States in 1916. Since that time, the beetles have spread across the country rapidly and have proven to be major agricultural and horticultural pests wherever they are found.

Beetle larvae are small, white grubs which can be identified by the c-shaped curve to their bodies and the characteristic orange/brown patch on their heads. The larvae, which are found beneath the soil, munch on turf roots which causes the turf to turn brown and eventually die, often creating unsightly patches of stressed grass. In its lifetime, each adult beetle produces over 50 grubs.

Pictured: Japanese Beetle Grub, Adult Japanese Beetle

Once the larvae mature into adult beetles, they emerge from the ground ready to wreak havoc on ornamental and food crops. Beetles feed on leaves, fruit, and flowers of more than 300 different plant species, the most common of which are:

  • Linden trees

  • Fruit trees

  • Birch trees

  • Grapes

  • Beans

  • Soybeans

  • Corn silk

  • Hardy hibiscus

  • Japanese maple

  • Roses

  • Zinnias

  • Marigolds

Japanese beetles most often consume the soft tissue between leaf veins, which leads to a skeletonized appearance. Affected leaves may turn brown and die. Most mature plants will tolerate japanese beetle damage without suffering serious harm as the beetle damage is mainly cosmetic, but young plants are susceptible to stunted growth or life-threatening injury.

Beetle Control Options

1. Early intervention is best! Watch for young beetles to begin emerging from the ground in late June and early July. They will feed on plants until mid- to late August; for the most efficacy, treatment strategies must be employed for the entire feeding time period. There are several options to choose from:

Physical removal - beetle infestations in small yards can be managed by removing beetles from plants by hand each day and drowning them in a bucket of soapy water. Since the beetles emit a strong pheromone which attracts more beetles, it is important to remove and dipose of them as soon as they are sighted. Beetles may be easier to capture at dawn or dusk when they are slower-moving.

2. Curative insecticide - these insecticides are contact sprays which are effective for managing beetles after the adults have emerged and damage has been noted. Look for products containing pyrethrin (such as Bonide’s Japanese Beetle Killer spray) or lambda-cyhalothrin (such as Bonide’s Eight). Neem oil is also an effective solution. For up to 14 days of protection, try Bayer’s rainproof Rose and Flower Insect Control spray. All of these products are available at Village Green.

3..Biological control - An organic product called Milky Spore contains naturally-occuring bacteria, Bacillus popullae, which is host-specific to Japanese beetle grubs. The grubs swallow the spores while feeding on turf roots and the spores reproduce inside the grubs. Within 20 days, the grubs are killed and their decomposing bodies release millions of new spores back into the soil, which spreads the bacteria across the entire beetle grub population.

Milky spore, which is sold at Village Green, is available in powdered and granular forms and must be applied for 2 years consecutively before the entire area is inoculated. Once inoculation is achieved, however, the bacteria remain active in the soil for up to 15 years. Milky spore can be applied at any time when the soil temperature is above 65℉. For best results, apply milky spore in August and September when new grubs are nearest the soil surface and actively feeding. Milky spore is specific to Japanese beetle grubs and will not harm humans, pets, or other animals and insects.

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