UVM Extension: White grubs, friend or foe?

In larval and adult stages, these beetles are major pests. The larvae feed on roots of turf grass and other plants, which impacts the plants’ ability to access water and nutrients, and causes plants to wilt, yellow and stop growing. The adult beetles feed on plant foliage and fruit.

While many beetles (such as the familiar ladybugs and ground beetles) are considered beneficial because they consume aphids, slugs, caterpillars and many other insects, the larvae of beneficial beetles do not look like white grubs.

In late May to early June, June bugs emerge. They are typically 1-inch long, reddish-brown beetles often recognized by a loud buzz followed by a thud on a window screen on a summer night. June bug larvae take three years to mature.

European chafers are golden brown beetles appearing in mid June to early July. Two weeks later, Japanese beetles, easily recognized by their metallic green head and copper wings, emerge. Both European chafers and Japanese beetles are ½-inch long and take one year to mature.

Adult beetles lay eggs in late summer and fall. Once the eggs hatch in the fall, the larvae feast until temperatures drop then burrow deeper in the soil to overwinter. In spring, as the soil warms up, the larvae creep upward to resume feeding on roots. The most damage occurs as the larvae mature.

Controlling these pests depends on the stage of their life cycle and their host preference. Integrated pest management consists of monitoring and adopting cultural and biological methods before using chemicals as a last resort.

Check your grass for spongy spots or brown patches. Dig a little or lift a patch of grass to look for grubs. A handful of white grubs is not concerning, but a dozen or more will create visible lawn damage. In addition, the presence of predators like raccoons, skunks, starlings or black birds suggests a grub problem.

Since beetles tend to lay eggs in short-cropped lawns, mow your lawn higher than 2 inches. Use a mulch lawnmower to allow the lawn clippings to feed the soil. Build deep roots by watering deeply no more than once a week.

If you notice adult beetles on your plants’ foliage, handpick them, preferably in the morning when they are lethargic. Drop them into a container of soapy water.

Choose tolerant grass varieties such as fescues and ryegrass. They contain a fungus that produces toxic compounds called alkaloids that repel beetles. Similarly, larkspur and geranium produce alkaloids and can be used as companion plantings in the garden.

Lastly and only if necessary, to be effective, a pesticide should be applied before the beetles lay their eggs. Always consider that pesticide applications may negatively affect beneficial insects.

For more information, visit the University of New Hampshire Cooperative Extension factsheet on white grubs at go.uvm.edu/white-grubs online.

Nadie VanZandt is a UVM Extension master gardener from Panton.