Green Valley Gardeners: Preparing for grubs in the garden

This cluster of bright Trichocereus Cactus flowers originated from a grub-attacked plant that was restored by trimming away the damage, allowing it time to recuperate, and then replanting.

Did your large columnar cactus turn a bit yellow? Next, did its skin crinkle? Finally, did it break off and fall over? What could have happened? Careful inspection shows that most of the roots have disappeared, the center has rotted away, and a couple of fat, white worms are lounging in the soil where the cactus once stood.

White grubs are the larval stage of May and June Beetles. They feast on and destroy the roots of many otherwise hardy desert plants. Visible damage is most severe in late summer.

Usually found curled into a ‘C’ shape, these grubs are generally ½ inch to 1-1/2 inch long, milky white with a brown head. Certain birds, such as Desert Wrens and Curved-bill Thrashers, consider them a tasty delicacy.

Hard-shelled brown adult beetles emerge from the soil during May and June. They mate and lay eggs in the ground. Grubs next hatch and begin feeding on nearby roots. They prefer soil containing organic matter and supplemental water, which make it easier to move around. The same landscape areas may be re-infested year after year, as long as the needed food is present.

The larva feed during warm weather and move deeper into the ground during winter as the soil cools. When soil again warms, they return to the root zone and voraciously feed. This is when most of the plant damage occurs. After completing their development, the grubs stop feeding and turn into adult beetles which then emerge the next season to reproduce and begin the cycle again.

White grubs are among the most difficult soil pests to eliminate. Their numbers are easiest to control when they are young. Larger grubs are nearly resistant to insecticides, plus it can be a problem to get effective amounts of the chemicals to the root zone. Commercially available are soluble sprays and granular formulas, but these are often less than 75% successful.

Follow the insecticide package labels carefully. Watering immediately prior to applying the treatment encourages the grubs to move closer to the soil surface.

Insecticides need to be ingested; therefore to be most effective, the application should take place when grubs are feeding. When finished distributing the insecticide, wash any remaining residue from the plant leaves.

‘Milky Spore’ is an environmentally safe product that has been successfully used for grub management in the eastern United States; however, it is not quite as effective in desert soils. There are also parasitic nematodes commercially available as controls, but these too are only marginally successful.

To counter the root destruction by grubs, timing is important. Insecticides should be applied during the months of May through August. Waiting for plant damage to show is usually too late!

Mary Kidnocker is a University of Arizona Master Gardener who lives in the Green Valley area. Her articles are featured weekly.

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