You can find pictures, details and treatment options for most damaging insects at the University of California Integrated Pest Management website http://ipm.ucanr.edu/index.html . Below is a brief description of the most common insects:
Aphids
One of the most common insects comes in many colors and shapes. They deform leaves and will spread diseases. Encourage natural predators. If necessary, spray with soapy water or horticultural oil or blast of water.
Snails and slugs
One of the most destructive pests. Control by clearing areas where they can hide. Hand pick when possible. Snail baits using iron phosphate are usually safer and many are organic.
Grasshoppers
The garden provides the Nolo bait which contains natural occurring spores that causes grasshoppers to become lethargic and die. Effectiveness is questionable in our area. Hand catch is another option.
Whiteflies
Tiny and abundant sap sucking insects. Encourage natural predators. If necessary, spray with soapy water or horticultural oil or blast of water.
Sowbugs and pillbugs
Not insects. They feed on decayed plant matter and are important decomposers. They can sometimes feed on young plant stems, killing them. Keep compost away from young stems. Keep area dry around plant by watering early in the day. Keep leaves and fruit off the ground.
Flea beetles
Commonly found on eggplants. They make many tiny holes and can severely damage plants. Control with spinosad.
Root knot nematodes
Microscopic, eel-like roundworms feed on plant roots leaving galls on root ends. Plant growth slows down, leaves turn yellow and wilt under heat. Common mistake made by gardeners is to add more water and fertilizer when symptoms appear which may cause additional damage. Remove infected plants. Control by plant rotation, cultural practices. Azadirachtin may slow damage.
Tomato russet mite
Tiny insects are noticed when lower leaves and stems become bronze or russet colored. Cause extensive damage if not treated early. Remove infected leaves then spray with sulfur.
Corn earworm/Tomato fruitworm/Cabbageworm
Watch for eggs on corn silk when it first emerges. Cabbageworms create many small irregular holes in leaves. Many natural predators including parasitic wasps attack eggs. Can be suppressed by spinosad or Bacillus thuringiensis (BT) when detected early. BT is less harmful to predators but must be applied at the right time. Consider purchasing Trichogramma parasites with your garden neighbors. Covering crops with tulle fabric can also be effective.
Hornworms
These are large green worms that cause extensive damage to tomatoes and other crops. Same treatment options as other worms.
Fig beetle
Bright green beetles emerge in early to mid-summer and feast on fruit. No chemical treatments recommended but you can easily make homemade traps found on many websites. Do not purchase treatments for Japanese beetles as these have no effect on the fig beetle.
Buffalo treehopper
Green, triangle shaped, less than half inch in size they suck juice out of herbaceous or woody plants. Nymphs are reddish brown with numerous spines. Can usually be picked off by hand. Can be controlled by insecticidal soap or horticultural oil if found in great numbers.