It’s perfectly normal to find some white grubs on your property. However, in large numbers, they can cause major damage to your lawn. A yellowed lawn with detached sods in spring or fall is a sign of white grub infestation.
White grubs are C-shaped larvae belonging to the family of scarabaeidae (beetles). They measure 4 mm when they hatch from the egg and can reach 4 cm by the end of the larval stage. They especially like feeding on the roots of lawn grass, horticultural species (trees and shrubs) and agricultural plants (fruit and vegetables).
European Chafer larvae and Japanese Beetle larvae are the main cause of damage to lawns, although common beetles play a minor role because they have a longer reproductive cycle. Being larger, European Chafer larvae are bigger and more destructive than Japanese Beetle larvae, but on the other hand, in the adult stage, Japanese Beetles can feed on the leaves of over 300 species of plants.
European Chafers lay their eggs throughout the month of July. After hatching, larvae feed on the surface of the soil until the end of October and then burrow deep down and hibernate throughout the winter. They rise to the surface again to feed in the spring and emerge from the ground as adults from mid-June to mid-July. The reproductive cycle of the Japanese Beetle is similar to that of European Chafer, but the emergence of adults and egg laying occur a few weeks later in July.
The first signs of the presence of white grubs are discolouration and drying out of the lawn despite adequate watering. The best indicator is the presence of dead sheets of sod that can be lifted up like carpet. Damage is greatest in spring and late October when larvae are larger. This is also when skunks, raccoons and birds make holes in the soil to feed on white grubs.
Healthy soil has greater resistance to invasive pests and a greater capacity to return to its healthy state after being disturbed and, conversely, unhealthy soil is more vulnerable and less able to regain its strength. That's why treating your soil by destroying white grubs is not enough. You must strike at the causes of the soil imbalance that made an infestation of white grubs possible in the first place.
Opt for an “Eco-Lawn”, meaning a diversified lawn with several types of grasses and legumes. For example, you can mix fine fescues, Kentucky bluegrass, perennial ryegrass and white clover.
During the egg laying period between June and July, reduce watering and keep the lawn 3 inches high.
Apply an eco-friendly repellent to repulse adults and prevent them from laying. You can mix one-third to a half-bottle (5 to 10 ml) of eucalyptus oil with water and spray the lawn with this mixture every week from mid-June to mid-July.
Practice grasscycling (leaving the cut grass on the lawn).
Allow your soil to breathe. Perforate the soil with a mechanical lawn aerator. You’ll not only aerate the soil, you may also kill some white grubs as you go.
Avoid nitrogen-rich fertilizers. Instead, use natural fertilizers better suited to plants’ overall needs.
Apply compost on your lawn as topdressing. You’ll have a healthier lawn for fighting off invaders.
Use nematodes. These are microorganisms that parasitize and kill white grubs.
Nematodes are not a miracle solution. For this strategy to be effective, you must combine it with good lawn cultivation practices.
- Application of nematodes should be conducted between mid-August and mid-September, because this is when grubs are most vulnerable.
- Nematodes are sensitive to sunlight. They should be applied early in the morning, in the evening or on a rainy day.
- Nematodes are soil-based microorganisms that move via water. Application must be preceded and followed by watering or rain to enable them to shift in the soil. The soil must remain moist for 10 days after application to prevent the nematodes from drying out.
- It is essential to follow the manufacturer's instructions.
Industrial lawn sod comprises a single kind of grass (monoculture). It therefore has low biological diversity. By comparison, an eco-lawn contains a variety of grasses, as well as legumes such as clover. Some plants are more resistant to white grubs because they have longer roots or because they are toxic to them (e.g. white clover). These resistant varieties help maintain the health of the lawn while weaker ones are in recovery. Diversification gives a lawn weapons to combat a number of pests.
Lawn diversity also attracts a variety of insects, most of which play a beneficial role in the environment. In healthy soil, the spread of insect pests is limited because predator insects attack them.
Synthetic pesticides promote the return and intensification of white grub infestations during subsequent years. Pesticides also destroy insects that play a beneficial role, such as white grub predators. They also contribute to developing resistance in the white grubs. Synthetic pesticides should be used only after all other solutions have been tried, and as a last resort. Since the use of low-impact pesticides is governed by By-law 1556, you must obtain an application permit from the Environment Division.
For more information on white grubs, please contact the Environment Section at 450-455-3371, option 1.