Life cycle of the Japanese Beetle

A Beetle's Life Cycle

controlling grubs

Bulletin #TP1075

Horticultural Observations for Healthier Landscapes

JAN-FEB	Grub in winter cell.
MAR-APR	Grub comes up near surface to feed.
MAY	Grub forms cell and prepares to pupate.
JUNE	Grub changes to pupa and then to adult, which emerges from ground.
JULY	Beetle lays eggs in ground.
AUG	Eggs hatch. Young feed on living roots of plants.
SEP-OCT	Grubs continue to feed and grow rapidly. Visible injury to roots of plants is most common at this time.
NOV-DEC	Grubs are mostly full grown and go to depths 4 to 8 inches below surface to pass winter in earthen cell.

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It is generally best to verify that grubs are actually present before applying pesticides. Often however with grubs, they may be too deep within your soil to discover.
For turf grass: Use a heavy-duty knife or a spade to cut a 1-square-foot flap of sod and roll it back. Examine the soil and roots in the top 3 or 4 inches. Repeat this process in several locations. If you find an average of five or more grubs per square foot, a pesticide application is justified. The condition of the turf, its value, and its uses (for example, whether it is a home lawn or a golf green), and the amount of damage done by animals searching for the grubs may affect your decision on whether to apply a pesticide.

For most other deep rooted plants the grubs can destroy the plants root system before you find them (Agave are especially subject to this as they may not display any stress).
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Treatment & Control

If you have soft rooted plants, as with most succulents, and these get water more often than is generally recommended for drought tolerant succulents, they will be at risk for grubs due to the moisture in the soil. Preventative treatment is recommended as you may not know of their presence in time for curative treatment.

The timing of the insecticide application is critical if control is to be effective. There are two approaches, preventative and curative. Imidicloprid (Tree & Shrub) is most effective as a preventative, and when applied prior to when the eggs are laid. Applications made to lawns in July and early August will have the best control because the insects will be immature. For shrubs applications can be effective between March and August.

The curative approach is used when an existing infestation is detected. The best time to apply curative insecticides is when the grubs are actively feeding near the soil surface. Pesticides applied any other time will be ineffective. As indicated in the chart above, this feeding occurs from August through October, and again in April through early May. Curative treatments applied in late summer or fall are usually more effective than spring applications because the grubs are small. Carbaryl or Bifenthrin are generally an effective treatment for actively feeding grubs. As usual; an internet search for these insecticides will provide current brand names that use these chemicals.

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