Don’t Let White Grubs Gobble Up Your Vegetable Crop
True white grubs are the larvae of May beetles in the genus Phyllophaga, of which there are dozens of species distributed throughout the U.S. and Canada.
May beetles are about ½ inch to 1 inch long. The adults often are yellow to brown to black, robust, oblong, shining beetles. Some, such as the green June beetle, are more brightly colored.
Larvae are white with a C-shape body, brown head, and three pairs of legs. Two parallel rows of spines on the underside of the last abdominal segment distinguish true white grubs from similar-looking larvae. Length varies from 1 to 2 inches. Primary damage comes from feeding activity by larva on the roots of crop plants.
Although white grubs can be a problem every year, the most serious damage occurs in regular three-year cycles with greatest damage to crops the year after the appearance of the adults.
Survival And Spread
The life cycle of the more destructive and abundant of these beetles extends over three years, but varies somewhat between species.
The adults mate in the evening. Females deposit 15 to 20 eggs 1 to 8 inches deep in the soil. Since the adults are attracted to trees to feed, they tend to lay more eggs in sod near wooded areas. After three weeks, the eggs hatch into larvae that feed upon roots and decaying vegetation throughout the summer. In the fall, they migrate downward and remain inactive until the following spring.
The greatest amount of damage occurs as the larvae return near the soil surface to feed on the roots of the plants. The next autumn, the larvae again migrate deep into the soil to overwinter, returning near the soil surface the following spring (the third spring) to feed on plant roots until they are fully grown in late spring. These grubs then form earthen cells and pupate. The adult beetles emerge from the pupal stage a few weeks later, but they do not leave the ground. The beetles emerge in May or June when feeding, mating, and egg-laying take place.
During the years of heavy May beetle infestation, deep-rooted legumes should be planted. If corn or small grains like sorghum are present, fields should be kept free of grass and weed growth as this will reduce the number of eggs laid. The year following heavy flights of May beetles, planting corn or potatoes should be avoided in fields that were previously under sod or grass.
Late spring or early autumn plowing destroys many larvae, pupae, and adults in the soil and also exposes the insects to predators such as birds. For this practice to be effective, plowing must occur before the grubs migrate below the plow depth.
Natural enemies that control these white grubs include a number of parasitic wasps and flies. An enthomopathogenic fungus also infects the grubs.
Soil-applied insecticides may be effective. For insecticide recommendations for white grubs, see UF/IFAS recommendations for currently labeled insecticides in Florida vegetables.