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White Grubs

Ravageurs et maladies
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White Grubs.
Photo: Jardin botanique de Montréal

Onglets

Description

Summary

The larvae of June and Japanese beetles and European chafers, commonly called “white grubs”, sometimes cause significant damage to lawns by chewing on grass roots. Damaged areas turn yellow or brown and may lift up with ease. Skunks, racoons and other small mammals may dig holes in lawns to feed on the grubs.

Signs and symptoms

  • The damage caused by the larvae is visible mainly in spring and fall. Lawns turn yellow and appear parched. They remain dry even when watered. Dead grass lifts up with ease. Underneath, the white grubs are chewing the roots.
  • Small mammals (groundhogs, skunks, racoons and moles) may dig holes in the lawn in search of grubs. Flocks of birds (starlings, blackbirds, etc.) pecking at the lawn in search of food are another sign pointing to an underground infestation.

Latin name (genus)

Phyllophaga anxia, Amphimallon majalis, Popillia japonica

Host plants

White grubs eat the roots of various ornamental and food plants, especially grasses (lawns, wheat, oats, barley and corn), legumes (alfalfa and clover) and some vegetable garden plants (strawberry, carrot and potato).

Development cycle

Description and life cycle

June and Japanese beetles and European chafers belong to the order Coleoptera and the family Scarabaeidae. The adults are easy to identify, but their larvae are very similar. They undergo complete metamorphosis (holometabolic insects): before reaching adulthood, they pass through the egg, larva and pupa stages.

Eggs: May be spherical, oval or elliptical, and are white. European chafer eggs turn grey a few days after being laid.

Larvae: Whitish, C-shaped bodies, with a brown head and three pairs of spiny legs. They grow to 2 to 3 cm, depending on the species.

Pupae: They look like little mummies, with their legs, wings and antennae closely folded against their bodies. They do not feed and remain immobile. They are yellowish or brownish.

June beetles (Phyllophaga anxia), also called May bugs, May beetles or June bugs, are large dark brown, almost black insects, about 2.5 cm long. They have short antennae ending in a large club of flattened plates. There is a small tooth at the tip of the pincers at the end of their legs.

The life cycle of June beetles extends over three years. They overwinter in the soil as larvae or young adults.

In spring, between mid-May and mid-June, the adults emerge from the ground. They take flight at dusk and swarm in trees to mate and feed on leaves (ash, birch, elm, maple, oak, poplar, willow, etc.).

After mating, the females lay about 50 eggs in small earthen balls, burying them in the soil in grassy fields or lawns. The eggs hatch about a month later. During their first summer, the young larvae feed mainly on plant litter. They moult into the second instar before migrating deep into the soil for their first winter.

In the spring of their second year, the larvae (2nd instar larvae) make their way back to the surface, where they feed until late June. They then become 3rd instar larvae, the stage when they do the most damage. In fall, they again burrow deeply into the ground.

In their third year, the larvae feed until June. They then close themselves up in cases where they pupate before emerging as adults, which remain in the soil until the following spring.

European chafers (Amphimallon majalis) are similar to June beetles, only smaller, about 1.5 cm long and coppery brown. Their complete life cycle lasts one year. The insects overwinter in the soil as larvae.

In early summer (late June, mid-July), the new adults emerge from the soil. At dusk, the insects swarm into trees to mate, but do very little feeding, merely nibbling the edges of leaves. The females lay an average of 20 to 30 eggs, burying them in the soil.

A few weeks later (late July, early August), the young larvae (1st instar larvae) appear, and feed until the second or third week of August. After two moults, the larvae (3rd instar larvae) burrow deeply into the ground for the winter. They pupate (late-May) and then emerge as adults early the following summer.

Japanese beetles (Popillia japonica) are metallic green and about 1 cm long. They have bronze wing covers and tufts of white hair on either side of their abdomens. Their life cycle usually lasts one year. These insects overwinter in the soil as larvae.

The adults emerge from the ground in July to feed and mate. Unlike June beetles and European chafers, they are active in the daytime. The females lay their eggs in the soil (40 to 60 eggs) and the larvae hatch two weeks later. They feed on roots until fall. After two moults, the larvae (3rd instar larvae) burrow into the ground for the winter.

The following spring, the larvae make their way to the surface to feed. They pupate (mid-June) and then emerge as adults in early July.

Prevention and control

Favourable conditions

The females prefer to lay their eggs in sites with enough moisture and food to ensure the survival of the eggs and feed the larvae. Such sites include compost or manure piles, light soil in grassy fields and lawns. A close-cropped lawn gives them ready access to the soil for laying eggs. June beetles and European chafers are attracted to lights at night. The larvae tend to survive mild winters, which are partially responsible for the increase in populations of these insects.

Identification

Keep a close eye on your lawn, especially in spring and early fall. If you find that small mammals are digging holes in your lawn, it is a good indication that you have an infestation. Cut and lift some pieces of sod so that you can check underneath for white grubs. Dig down about 10 cm. If the soil is dry, go even deeper. It is best to wait until after it has rained for a few days, because the grubs usually stay closer to the surface when the soil is moist.

Prevention

  • Keep your lawn lush and healthy by adopting good cultural practices. Healthy lawns are better able to resist these insect pests.
  • Mow your lawn high (to a height of about 7,5 cm in the summer) to discourage egg laying.
  • Space out waterings so as not to keep the soil constantly moist (ideal for egg laying). Water deeply and only when necessary.
  • Aerate compact soil; top dress with compost; fertilize sparingly with natural fertilizer; reseed bare patches.
  • Limit outdoor lighting during the egg-laying period (June, July) since light attracts June beetles and European chafers.

What to do

  • Given good growing conditions and proper care, lawns can tolerate large numbers of grubs without showing any damage. The goal is not to totally eliminate the grubs, but to reduce their numbers if you see such damage.
  • In the event of a minor infestation, take preventive action with physical and biological control methods. Allow natural predators such as skunks and racoons into your yard to help cut down on the number of white grubs.
  • In the event of a heavy infestation (more than 50% of the lawn surface is damaged by white grubs and/or small mammals), it is best to completely renovate your lawn. If you choose this option, rake off the dead grass, turn the soil, pick off all visible white grubs and pupae and reseed or lay new sod. It is important to check your growing conditions and lawn care and make any necessary changes.

Physical control

  • Early in the morning, when the adults are less active, you can pick them off by hand or knock them off into a bucket of soapy water. Do this regularly (ideally every day) until you don't see any more of them.
  • At night, you can attract or trap nocturnal beetles and chafers by placing a light behind a white sheet hung over a bowl of soapy water. Knock the insects into the bowl.
  • Using a mechanical pointed aerator is a way to destroy some of the larvae.

Biological control

  • Attract birds to your garden because some species feed on white grubs.
  • Spread entomopathogenic nematodes on your lawn, which are parasitic on and will kill white grubs. Read the product label carefully and follow the manufacturer's recommendations.

Chemical control

The Montréal Botanical Garden does not recommend the use of pesticides to control white grubs.

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