Answer: Based on the season (autumn) and your gardening region (California), I'd venture that the little bugs you've discovered are flea beetles. Adults are black and very small (1/23 of an inch!) and are active from May-October. Eggs are laid on leaves and stems and the larvae develop on the roots of the plants. Adult beetles jump readily when disturbed, hence the name "flea beetle". While large populations of these insects can cause some damage to turfgrass, they're generally not considered pesty enough to warrant control.
If you de-thatch your lawn and rake up the debris, you'll remove the adults, eggs and larvae all in one motion.
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