White grubs of the genus Phyllophaga and related genera live in the soil and feed on roots of plants. This feeding can sever or girdle roots and cause injury or death. Root feeding of the grubs causes the principal injury to plants. Minor injury to hardwoods and pines is caused by foliage feeding of the adults, which are often called May or June beetles.

Grub damage to forest tree seed­lings in nurseries has been recognized as a problem in this country for at least 50 years. Not until the early 1940's, however, did it become evident that grubs might also threaten forest plantations. Damage to plantations in the Lake States was generally restricted to small acreages, according to early reports. Serious injury to larger areas was reported in 1953 when 8,000 acres in South Carolina were heavily attacked.

Because grub populations tend to build up on open land that has been idle for a year or more, preventive measures may be needed before these areas are planted.

Host Trees

White grubs normally feed on roots of many plants and do not especially prefer roots of trees. All coniferous and hardwood stock are readily attacked, however.

Damage

Heavy grub damage is generally first noted in the late summer and early fall when formerly healthy appearing seedlings turn color and may die. The symptoms are similar to those of drought injury. A gentle tug on such seedlings will pull them up and reveal that lateral and taproots have been chewed off or girdled (fig. 1).

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FIGURE 1.-Root injury on pine seedlings caused by feeding of white grubs. A is a healthy seedling ; the other seedlings show varying degrees of grub damage.

Survival of discolored seedlings depends on their original vigor, the nature and degree of injury, and weather conditions. Seedlings injured during dry weather are much more likely to die when dry conditions continue than when cool, wet weather follows. Even with cool, moist conditions the trees may not be able to produce new rootlets and recover before the tops dry out. Generally, coniferous stock is more severely injured and makes a slower recovery than hardwood stock. Damage is most severe on light soils.

Description

Adults of the Phyllophaga genus, which contains over 100 species, are robust, oval, brown or brownish-black June beetles (fig. 2).

Larvae are milky white. They have six prominent legs. The head is brownish and armed with a strong pair of jaws. The body is strongly curved, all the hind parts are shiny, and the body contents show through the skin. White grubs vary in length from one-eighth inch when first hatched to over an inch when full grown (fig. 3).

Pupae have the general appearance and size of adults. They vary from yellowish to brown, growing darker as transformation to adult proceeds. Pupae, inactive in the soil, do not feed.

Eggs are pearly white and oval when first laid and are about one-tenth of an inch long. Later they become cream colored and spherical.

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FIGURE 2.—Adult stage of the white grub, commonly known as a June beetle.		FIGURE 3.—A full-grown white grub.

Life History

Life cycles of the various Phyllophaga species vary from 1 to 4 years. In the South the species may have a 1-, 2-, or 3-year cycle, the 2-year cycle being most common. In the North Central States they have a 2- or 3-year cycle. Further north the cycle may be 3 or 4 years, although the 3-year cycle is most common in the Lake States. Different species in the same locality may have different cycles. Also, part of a single brood may emerge at the end of the second year and the remainder of the same brood at the end of the third year.

There is a continual overlapping of broods, with emergence much heavier in some years than others. Because of this overlapping, an overwintering population may consist of adults that have not taken flight from the soil and larvae of several sizes.

The female beetles lay their eggs at depths of 2 to 6 inches in the soil 8 to 10 days after mating. Several eggs may be laid in a single mass, with each egg in a small ball of dirt. They hatch in 2, or 3 weeks. Egg laying continues over a period of several weeks during spring and summer.

Grubs, hatched from the eggs, feed first on organic matter in the soil, but soon attack the tender roots of seedlings and other plants. After hibernating in the soil during the winter at depths determined by temperatures and frost levels, the grubs resume feeding when plant growth starts.

Adults of most Phyllophaga species emerge from the soil at dusk on warm evenings and fly to the foliage of trees. Here they mate and feed until dawn, then return to the soil.

Natural Control

White grubs are very sensitive to lack of moisture. They move up or down in the soil as moisture conditions change, in an attempt to remain in a habitat with optimum moisture. Protracted drought causes heavy larval mortality, especially among the newly hatched larvae.

Although the grubs have enemies, such as insect parasites and predators and fungus diseases, these apparently do not normally act as effective controls. Mammals and birds also eat the grubs, and skunks may greatly reduce populations in local areas. Attempts to control white grubs through the use of bacterial milky disease of the Japanese beetle have not been successful.

Preventive Control  (See pesticide caution below)

An application of insecticide before planting seed or seedlings may be needed to prevent extensive damage by white grubs. In forest nurseries 8 pounds of chlordane per acre, thoroughly worked into the soil, has given excellent control. A maintenance dosage of 1 to 1 1/2 pounds per acre is needed every 2 years. Dieldrin applied at the rate of 1 1/2 pounds per acre, followed by 1/2 pound every 2 years, may be substituted for chlordane.

Preventive control under field conditions is advisable if the grub population is one or more per 2 square feet. A grub population of 2 per square foot has caused high tree mortality. Areas with such a high rate of infestation should not be planted without control measures. Since young grubs are difficult to detect, even when they are present in considerable numbers, very careful observations must be made when sampling for damaging grub populations.

Protection of young seedlings re-quires a fast-acting chemical that will repel or kill the grubs before the roots are cut and the seedling dies. An aldrin solution of 1/2 to 1 percent, applied to the root systems of seedlings during planting, has given good results. The chemical can be applied by a sprayer attachment on planting machines or by dipping the roots of hand-planted stock.

Applied Control  (See pesticide caution below)

Once a plantation is established, control measures are extremely difficult and expensive. The insecticidal solution must penetrate several inches of soil rapidly and kill the grubs before they cause additional damage. This is seldom possible.

Insecticide dust or granules broadcast from either ground machinery or aircraft are not effective because the insecticides generally penetrate only the upper layers of soil and do not reach the root zone where the grubs are feeding.

Insecticide should always be applied before planting where damage is likely to occur, in either nursery or field.

References

WHITE GRUBS IN FOREST NURSERIES OF THE CAROLINAS. H. R. JOHNSTON and C. B. EATON. U.S. Bur. Ent. and Plant Quar. E-486, 9 pp. 1939.

WHITE GRUBS IN WISCONSIN FOREST TREE NURSERIES. R. D. SHENEFELT and H. G. SIMKOVER. Jour. Forestry 48:429-34. 1950.

PROTECTING MACHINE PLANTED TREES FROM WHITE GRUBS. R. D. SHENEFELT, H. R. LUBERG, and R. C. Doson. Tree Planters' Notes 20 :14-17. 1955.

WHITE GRUB DAMAGE TO YOUNG PINE PLANTATIONS. J. G. WATTS and JOHN B. HATCHER. Jour. Econ. Ent. 47 :710-11. 1954.

U.S. GOVERNMENT PRINTING OFFICE: 1961