Lawn and Garden Pests
What Is Eating Your Plants?
By the Clark Pest Entomology Team
Last updated: October 28, 2026
Quick answer: Check the underside of the leaves. Small, pear-shaped bugs in green, yellow, brown, red or black are aphids. Tiny insects with yellowish bodies and whitish wings are whiteflies. Bumps on stems or leaves with no obvious head or legs are scale insects. Aphids, whiteflies and soft scales also leave sticky honeydew.
Which Plant Pest Do You Have?
Clark covers three sap-sucking pests that turn up on garden plants, shrubs and trees across California and Northwestern Nevada. Tap the one that sounds closest.
AphidsSmall, soft-bodied and pear-shaped, in green, yellow, brown, red or black. Large groups curl and yellow the leaves.
WhitefliesTiny insects with yellowish bodies and whitish wings. Large colonies sit on the undersides of leaves.
Scale InsectsStill bumps on bark, leaves or fruit with no obvious head or legs. Many look tan to brown and some look like tiny oyster shells.
Scale insect photo: Whitney Cranshaw, Colorado State University, Bugwood.org, via Wikimedia Commons, licensed CC BY 3.0 US.
Aphid, Whitefly or Scale? Quick Tests
These three look nothing alike up close, but they leave the same sticky mess. Use these clues to tell them apart.
| Clue | Aphid | Whitefly | Scale insect |
|---|
| Looks like | Small and pear-shaped with long legs and antennae | Tiny, with a yellowish body and four whitish wings | A still bump with no obvious head or legs. Coverings run about 1/25 to 1/4 inch |
|---|
| Where you find it | On plants, especially under the leaves | In large colonies on the undersides of leaves | On bark, stems, leaves or fruit |
|---|
| Makes honeydew? | Yes | Yes | Soft scales do. Armored scales do not |
|---|
| How fast it grows | Newborn to adult in 7 to 8 days in warm weather for many species | Develops rapidly in warm weather | Young crawlers move, then settle and stay put |
|---|
| Ants around it? | Often, especially on trees and shrubs | Ants come for the honeydew | Ants feed on the honeydew and guard the scale |
|---|
Signs Something Is Sucking Your Plants
Sticky leaves and black mold
Aphids, whiteflies and soft scales all leave honeydew. It is a sticky liquid that coats leaves and can end up on pavement and cars below. Black sooty mold can grow on it.
Ants climbing the trunk
Large numbers of ants moving up a tree trunk are a clue. Look on the limbs and leaves above for aphids or other insects that make honeydew.
Curled, yellow or stunted growth
Big aphid groups can curl and yellow leaves and stunt shoots. Whiteflies can turn leaves yellow or kill them.
Why Ants and Natural Enemies Matter
Ants protect the insects that feed them. UC IPM says ants protect aphids from natural enemies. It says ants interfere with the natural enemies of whiteflies and attack the predators of scales. That lets the sap-suckers build up.
Look for lady beetles, lacewings or syrphid fly larvae on the plant. Aphids that look like round, discolored shells are called mummies. Parasitic wasps made them. Both signs can mean nature is already working on the problem.
What to Do First
Start with water and pruning. A strong stream of water can knock aphids off sturdy plants, and the aphids that fall usually do not get back on. Prune off badly infested leaves and stems. The UC IPM aphid guide covers more home garden steps.
For whiteflies, UC IPM lists hosing plants and shiny reflective mulch, which can repel them. Yellow sticky traps help you monitor the adults. See the UC IPM whitefly guide.
Be slow to reach for a spray. According to UC IPM, insecticides often are not needed for scales. It also says to avoid broad-spectrum insecticides that harm natural enemies. Soaps and oils are gentler choices if you need a spray. Clark's scale guide adds that a waxy covering protects adult female scales, so spraying often fails and can backfire if it kills their predators. The UC IPM scale guide explains the types.
Cut off the ants too. Clark and UC IPM both point to a sticky barrier around the trunk and ant baits. Controlling the ants lets the natural enemies do their job.
Clark's Expert Insight
Check the trunk before you spray. Ants marching up a tree can signal sap-feeding insects overhead. Stop the ants and lady beetles and lacewings can get to work. A spray alone does not stop the ants that protect the pests.
Garden Pest Questions Homeowners Ask
How can I tell aphids from whiteflies?
Look at the body. Aphids are small and pear-shaped and can be green, yellow, brown, red or black. Whiteflies are tiny with yellowish bodies and whitish wings. Both gather under the leaves.
What is the sticky stuff on my plant leaves?
It is honeydew, a sticky liquid that aphids, whiteflies and soft scales leave behind. Black sooty mold can grow on it. Control the insects and the mold has nothing to grow on.
Why are ants climbing my tree?
They may be feeding on honeydew. Look on the limbs and leaves above for aphids or scale insects. Ants guard these insects and chase off their predators.
What attracts whiteflies?
They feed on a wide range of plants, including many vegetables, ornamentals and weeds. In many parts of California they breed all year and move to new plants as crops are harvested or dry up.
Are scale insects really insects?
Yes. Adult females sit still under a waxy covering, so many look like bumps on a stem. They belong to the same order as aphids.
Do I need a pesticide for aphids?
Often not. UC IPM lists water, pruning and natural enemies as steps to try before spraying. If you do spray, soaps and oils are gentler on the helpful insects.
When should I call a professional?
Call when the pests keep coming back, when a tree or large shrub is covered or when you cannot tell what you have. A technician can name the pest and recommend a plan.