The larvae of June beetles and chafers, commonly called white grubs, are formidable pests. These critters, which typically have C-shaped bodies, often cause extensive damage to lawns, gorging themselves on grass roots in the spring as well as late summer.

Consequences
Grass attacked by white grubs:

  • is less able to take up water and nutrients and its root system is severely weakened;
  • turns yellow as if parched;
  • will easily detach from the soil, and can be pulled back like a carpet;
  • can suffer further damage due to attacks by birds as well as small mammals such as raccoons and skunks, which love to feed on the larvae. 

If more than half the surface area of a lawn is affected, the entire lawn will need to be replaced—an operation that will put quite the burden on your wallet as well as the environment.

Fighting back
Can you make your lawn immune to white grubs? No. But you can make it less inviting to these pests, and there are ways to fight back quickly in case of an infestation.

A healthy lawn is more resistant to attacks by larvae of the European chafer and native June beetle (or June bug), which are the most damaging species in Quebec.

To help your lawn resist invasion:

  • Prefer a greater mowing height, ideally around 8 cm (3 in). Longer grass means a longer root system, more shade and, in turn, cool, moist soil that is less hospitable to the larvae.
  • Ensure your lawn is well fertilized. Use slow-release fertilizer with dosage adapted to the soil and type of grass. You can find out the necessary dosage in just a few clicks using the fertilizer calculator found on the website www.pelousedurable.com. The site, in French, also provides basic information on how to maintain a healthy, environmentally friendly lawn. NB : le site n’est pas traduit et nous n’avons pas pu trouver de calculatrice semblable en anglais qui est spécifique au Québec.
  • Use less exterior lighting, especially in June and July, which is egg-laying season. Beetles are attracted to light.

What about insecticides?
In prevention mode, it’s possible to curb proliferation of white grubs through reasonable application of imidacloprid, an insecticide that is authorized under the Pesticides Management Code. Use of this chemical, marketed under the brand name Merit, is reserved to companies that maintain parks and green spaces, however, and many municipalities have banned it. In addition, many specialists and ecologists are against its use, in part because it is toxic to pollinating bees as well as birds.

The Fédération interdisciplinaire de l’horticulture ornementale du Québec (FIHOC) recommends that application be limited to once a year, combined with three or four controlled spreadings of fertilizer. This group also cautions that no registered insecticide will be effective against white grubs if damage from their feeding is already apparent.

If your lawn is infested
At what point is the presence of white grubs problematic for your lawn? When more than seven European chafer larvae or more than three to five June beetle larvae can be counted within an area of one square foot (0.1 m²) of lawn.

As in any battle, one key is to “know your enemy.” The European chafer is rife in the Montreal, Laval, Lower Laurentians, Lanaudière, Montérégie and Outaouais regions. The June beetle is most common in Greater Quebec City as well as in the Centre-du-Québec (Bois-Francs, Chaudière-Appalaches, etc.) and Est-du-Québec regions.

If your lawn is already infested in the spring, the odds are against you: no weapons will be effective given that the larval development is already quite advanced despite the soil under the grass being cool.

You can attempt a vigorous counter-attack if the damage is limited to a small area. Roll up your sleeves and follow these steps:

  1. Remove all of the damaged grass, till the soil and remove the larvae by hand.
  2. Top with a good compost mix.
  3. Reseed or lay grass sod.

You can also let things take their course and rely on the natural predators mentioned about to eat the larvae. Your lawn will be fairly damaged afterward, but at least you’ll be rid of grubs by the hundreds. This option, obviously, will require you to repair fairly extensive areas of grass. 

Biological warfare
Around mid-August, a few weeks after the eggs are laid, is the best time to launch a major offensive against white grubs. You can carry out a biological strike using an army of nematodes, tiny insect-parasitic roundworms that are deadly to immature chafer larvae.

Nematode application is targeted, and is not dangerous to humans, animals or the environment. Heterorhabditis bacteriophora is recognized as one of the most effective species for controlling white grubs in Quebec. Buy them from a reputable garden centre to be sure of getting quality specimens—not to mention valuable advice.

It’s very important to know that nematodes will be effective only if you use them properly and maintain an extensive set of environmental conditions. Among the key determining factors, remember that nematodes:

  • are extremely sensitive to heat and UV radiation; and
  • need plenty of moisture to be able to work their way through the soil to the enemy’s location.

To ensure good results, therefore, you must:

  • thoroughly water the lawn;
  • mix the nematodes with water, carefully following the instructions for use, before spreading them on the lawn;
  • apply the treatment late in the day or on a cloudy or rainy day; and
  • keep the soil moist for a full week after the treatment. 

Of course, you’ll need to respect your municipality’s bylaws on watering, and you may need a permit as well. And once the job is done, you must remain vigilant, because persistence and prevention are the only truly effective weapons in dealing with these stubborn invaders!

Our thanks to agronomist Guillaume Grégoire, a technical and scientific analyst with the Fédération interdisciplinaire de l’horticulture ornementale du Québec, for his contribution to this instalment of Tips & Tricks.

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