When to Treat for Grubs in St. Augustine Grass?

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St. Augustine grass is among the most widely used grasses along the Gulf Coast regions of the United States. The grass has a superior tolerance for heat and high adaptability to limestone shores and sandy soil. Like all grasses, St. Augustine grass has disease and pest related problems. This includes white grubs that are likely to cause serious damage if not treated at the right time.


Pest Identification

White grubs are the young or larval stage of a number of beetles including masked chafer, Japanese beetles, May and June beetle, and scarab beetle. The insects are the most destructive pest of turf grasses, including St. Augustine grass, cites the Utah State University Extension. Though the adult beetles differ significantly in habits and appearance, the white grubs are almost similar in appearance and cause the same level and type of damage to grass.


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The larva of all beetles is dirty white to gray in color and anywhere from 3/8 to 2 inches in length. The pests lie in a distinct C-shape in the soil. The head capsule is dark brown in color. The only distinguishing feature between the different species of white grubs is the arrangement of hairs and spines on the posterior body. The adult beetles are not harmful to turf grasses. Some beetles occasionally feed on the foliage of certain landscape trees but this does not cause any heavy damage.


Curative Insecticide Timing

The best time to apply curative insecticides to any infested turf grass is during late summer and fall, cites the University of Rhode Island Extension. Avoid treating in spring as the mature grubs have buried deep in soil and hard to reach with chemicals. Suggested treatment time is between August 1 and September 15. Insecticides are most effective when applied at the second or third instar or larvae development stage. Recommended insecticides include trichlorfon. St. Augustine grass produces heavy thatch that often prevents the penetration of insecticide into the roots. Makes sure grass is mowed low and dethatched prior to treatment.


Preventative Insecticide Timing

As the name suggests, preventative insecticides are used prior to egg hatching and larva maturation in order to control pest population. Best time to apply preventative insecticides is early in the season a month before the eggs hatch and when larvae are very young and feeding near the soil surface. Suggested treatment time is between April 1 and August 15. Recommended insecticides include imidacloprid or halofenozide.



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