Careful site selection, soil preparation, adequate and proper irrigation and mulching are some ways to help plants resist pests.
Some insect examples are:
Aphids – many colors, pear shaped, winged, wingless. Leaves, stems and buds are distorted and sticky, also all females born pregnant.
Beetle, Asiatic – long, brown, velvety. Leaves have irregular holes in edges.
Beetle, Japanese – long metallic blue or green with coppery wing covers. Leaves and flowers have holes and may be skeletonized.
Borers – moths or beetles. Stems exude sawdust-like material and break, leaves wilt. Iris borers cause irregular tunnels in leaves as well as damaged or rotted rhizomes.
Tree Bug – usually shield-shaped, brown, black, green or brilliantly colored and patterned. Buds and leaves become deformed or dwarfed.
Cutworms – gray or brownish moths. Seedlings or young plants are cut off at soil level.
Leafminers – long, wasp-like with yellow-striped black bodies and clear wings. Leaves have tan or brown blotches or serpentine tunnels.
Scales – long, with grayish, brownish orange, reddish brown, or cottony white shells. Males have wings and females do not.
Slugs & Snails – long, gray, tan, green, black, yellow or spotted, eyes at the tips of small tentacles. Snails have a single spiral shell. Slugs are shell-less. Leaves – large, ragged holes. Product: Slug/Snail specific.
Spider Mites – long, reddish brown or pale spider-like mites with 8 legs, wingless. Leaves become stippled, reddish to yellow, and have fine webbing.
Thrips – long, with yellow, brown, or blackish bodies and 2 pairs of fringed wings. Flower buds die, petals become distorted and growth is stunted.
Whiteflies – long, white, moth-like insects. Leaves turn yellow, and plant is weakened.
To control most pests in the garden you can use: Eight, Nolo Bait, Spinosad, Diatomaceous Earth, Neem Oil.