White grubs do severe damage when strawberries follow sod that has been infested with grubs. White grubs are the immature forms, or larvae, of May beetles or "June bugs." The grubs are large, thick bodied, and dirty white. When fully grown, they range from about 1 to l.5 inches in length. White grubs feed on the roots of a variety of plants and often completely cut off strawberry plants just below the crown. When dug from the ground, the larvae always lie in a curved position, forming the letter "C."
May beetles are dark brown and vary in length from 1/2 to 7/8 inch. They often are attracted to lights and sometimes can be seen in considerable numbers around street lights during May.
The beetles remain concealed near the soil during the day, but at dusk they emerge and fly to ornamental and forest trees to feed. Sometimes they congregate in such numbers that they completely defoliate isolated trees. They return to the soil just before dawn.
Eggs are deposited in the soil at a depth of l to several inches. They apparently are deposited most abundantly in sod that has not been disturbed for years, although they occur in almost any type of soil porous enough to permit the female beetles to crawl into it.
Eggs hatch in 3 or 4 weeks. Tiny larvae feed largely on vegetable matter in the soil during the remainder of the first season. When cold weather appears, most species of white grubs burrow down below the frost line and remain there until the following spring.
Grubs spend all of the next summer feeding on the roots of plants. During the second winter, they continue as larvae below the frost line in the soil. In the third year, they return to the plant roots and feed until late June or July, when they change to pupae in small earthen cells. The adults emerge from the pupal cases a few weeks later but remain in the soil until spring. Thus, eggs are laid every third year, and the severe damage caused by their offspring usually occurs only every third year.
Two species complete development and change to the adult stage in one year. One of these is the well-known Japanese beetle, Popillia japonica (Newman); the other is the northern masked chafer, Cyclocephala borealis (Arrow). These species attack roots of certain grasses and may become abundant enough to cause severe damage.
The most severe damage usually occurs between the time of planting and runner development. Damage is most likely to occur when strawberries are planted on newly plowed sod. The grubs feed on the roots of strawberry plants and either kill or severely weaken them.
Avoid planting strawberries on newly plowed grassland. The danger of insect damage can be reduced by rotating crops and cleanly cultivating the crop that precedes strawberries.