California ground squirrels can be serious pests wherever they occur. As well as eating fruit and crops, they may damage trees by gnawing bark and girdling trunks, and burrowing around roots. Ground squirrels also gnaw on plastic sprinkler heads and irrigation lines. Burrows and mounds interfere present hazards to machinery, and cause damage structures.

California ground squirrels live in colonies that may include several dozen animals in a complex of burrows. The burrow systems may be quite extensive: 5 to 30 feet or more in length and may go 2 to 4 feet below the soil surface. Often there is more than one opening in a burrow system. The burrow openings are about 4 inches in diameter, but can vary considerably. Ground squirrels live in a wide variety of habitats. Populations may be particularly high in grazed rangelands and in areas disturbed by humans such as road or ditch banks, fence rows, around buildings, and in or bordering many crops.

Ground squirrels are only active during the day. They are most active during mid-morning and late afternoon, especially on warm, sunny days. They have a unique life cycle (Figure 1). During winter months most ground squirrels hibernate, but some young may be active at this time, especially in areas where winters are not severe. Squirrels begin to breed soon after they emerge from hibernation. They have only one litter per year, averaging seven to eight per litter. The young are nursed in the burrows and at about 6 weeks of age they are ready to venture above ground. The young grow rapidly and by 6 months of age resemble adults. During the hottest times of the year most adults go into a period of inactivity, called estivation, that may last a few days to a week or more. During these periods, the burrow appears open at the entrance but the squirrel plugs it with soil near the nest.

Ground squirrels are primarily vegetarians. Their diet changes with the seasons. After emergence from hibernation, they feed almost exclusively on green grasses and herbaceous plants. When annual plants begin to dry and produce seed, the squirrels switch to seeds, grains, and nuts, and begin to store food. Ground squirrels usually forage close to their burrows. Their home range typically is within a 75-yard radius of their burrow.

Figure 1. Life cycle of the California ground squirrel

A tree girdled by squirrels	Squirrel damage to pistachios
	Squirrel damage to strawberries

The selection of control procedures is influenced by the unique life cycle and behavior of the ground squirrel. The most effective time to control squirrels is in early spring, just after they have emerged from hibernation. Each female killed at this time means 7 or 8 fewer young to control later in the year. Trapping and fumigating burrows are control techniques that are effective any time when squirrels are active. Toxic baits are only effective after the squirrels' diet changes from green vegetation to seed (in late spring). Shooting squirrels with a .22 rifle may provide some control, but is very time-consuming.

Because of the adaptability of ground squirrels, indirect methods of control such as habitat modification, exclusion, and use of repellents generally have little value in a management program. Removal of brush piles and debris may make an area less desirable to ground squirrels, and also aid in detection of squirrels and their burrows, and improve access during control operations.