Identify Before You Treat: Ten Common Garden Pests and the First Thing to Try

A spray bought for the wrong pest does nothing for your plants and can kill the insects that were already helping you. The quickest route to fewer holes in your leaves is to name the culprit first, then start with the least drastic thing that works on it.

The Garden Pest Identifier covers ten pests that turn up in most vegetable plots and flower beds. Pick the one that matches what you are seeing and it shows the identifying signs, the organic treatment options and the plants it usually attacks, along with a note that organic sprays are pesticides too and must be used by the label. This guide goes through all ten, with the one sign that best separates each from look-alikes and the step to try before anything comes out of a bottle.

First, look properly

Most misidentifications come from looking once, at midday, from standing height. Before you pick a pest:

• Turn leaves over. Aphids, whiteflies, spider mites and squash bug eggs all live on the undersides.
• Go out at dusk or after rain. Slugs, snails and cutworms feed at night and hide by day.
• Look at the soil line. Stems cut through at ground level point somewhere very different from holes in leaves.
• Note the pattern of damage. Ragged holes, tiny shot-holes, skeletonized leaves, speckling and sticky residue each point to different pests.
• Take a clear photo. It helps you compare with the identifier and with anyone you ask for a second opinion.

The ten pests and what to try first

1. Aphids

The giveaway: clusters of tiny green or black insects on new growth, sticky honeydew, curled leaves. Try first: knock them off with a firm jet of water every few days. Ladybugs, lacewing larvae and hoverfly larvae eat aphids, so a small colony is often cleared without you. Usually on: most vegetables, roses.

2. Slugs and snails

The giveaway: irregular holes in leaves and silvery slime trails, worst after rain. Try first: hand-pick by flashlight in the evening, and set beer traps sunk so the rim sits a little above the soil (so ground beetles, which eat slugs, do not fall in). Copper tape around raised beds and pots is another barrier. Usually on: lettuce, hostas, seedlings.

3. Tomato hornworm

The giveaway: large green caterpillars, stripped leaves and stems, dark droppings on the leaves below. The droppings are often easier to spot than the caterpillar. Try first: hand-pick. If a hornworm carries small white cocoons on its back, leave it: those are parasitic wasps that will hatch and hunt the next generation. Usually on: tomato, pepper, eggplant.

4. Cabbage worms

The giveaway: velvety green caterpillars, ragged holes in brassica leaves, small white butterflies (the cabbage white) visiting the bed. Try first: floating row covers from the day you plant, sealed at the edges, so the adults cannot lay eggs; hand-pick any caterpillars you find. Usually on: cabbage, broccoli, kale.

5. Spider mites

The giveaway: fine webbing and stippled, yellow-speckled leaves, worst in hot, dry weather. Tap a leaf over white paper: moving specks are mites. Try first: spray water on leaf undersides and raise humidity around the plant. Water-stressed plants suffer most, so check your watering too. Usually on: beans, cucumbers, tomatoes.

6. Whiteflies

The giveaway: tiny white flying insects that rise in a cloud when you brush the plant; yellowing leaves and honeydew. Try first: yellow sticky traps hung just above the plants, which also tell you whether numbers are rising or falling. Usually on: tomato, pepper, brassicas.

7. Japanese beetles

The giveaway: metallic copper and green beetles and skeletonized leaves, with only the veins left. Try first: in the cool of the morning, hold a jar of soapy water under the beetle and tap the leaf; they drop when disturbed. Many extension services advise against scented bag traps in small gardens because they can draw in more beetles than they catch. Usually on: roses, beans, grapes.

8. Squash bugs

The giveaway: brown or gray shield-shaped bugs, clusters of bronze eggs under leaves, wilting vines. Try first: scrape egg clusters off every few days, and lay boards on the soil overnight; the bugs shelter under them and can be collected in the morning. Row covers until the plants flower keep them off early. Usually on: squash, pumpkin, cucumber.

9. Flea beetles

The giveaway: tiny black beetles that jump when disturbed and a shot-hole pattern of small round holes. Try first: row covers on young plants; established plants usually outgrow the damage. Usually on: eggplant, radish, brassicas.

10. Cutworms

The giveaway: seedlings cut off at the soil line overnight, with gray or brown caterpillars curled into a C shape just under the surface nearby. Try first: a collar around each transplant stem (a cardboard tube pushed an inch into the soil works), and hand-picking at night. Usually on: seedlings, tomatoes, brassicas.

Our guides to aphids, whiteflies and spider mites and to holes in seedlings go further into the pests most often confused with each other.

When you do use a product: the label rules

Several treatments the identifier lists are pesticides, even though they are organic: insecticidal soap, neem oil, Bt (Bacillus thuringiensis), diatomaceous earth and milky spore. "Organic" describes where they come from, not that they are harmless. The identifier shows this note beside every pest, and it is worth following to the letter:

• Use only a product whose label lists your plant and this pest. The label is the legal instruction for that product.
• Follow the label exactly: the rate, the protective gear it asks for, how long to keep people and pets off the treated area, and how many days to wait before harvesting a food crop.
• Never mix products together, and never make your own sprays from household chemicals. Home-made brews have no tested rate and can burn plants, harm you, or poison wildlife.
• Keep children, pets and waterways clear, and store products locked away in their original containers.
• Protect pollinators. Do not spray open flowers, and spray in the early morning or evening when bees are not flying. Soap and neem can harm beneficial insects they land on.
• Treat only what is affected. Bt kills caterpillars generally, including those of butterflies you may want, so use it only on plants where the pest is.
• Diatomaceous earth is a fine dust: avoid breathing it in, wear the mask the label asks for, and expect it to stop working once wet.

What the identifier does not do

• It does not see your plant. You choose from ten pests; it cannot tell you that you chose the wrong one. Identification by description is a best guess.
• It covers ten pests only. Thrips, leafminers, scale, earwigs, vine borers and many others are not in it.
• It does not cover diseases. Blight, mildew, wilts, viruses and nutrient deficiencies can all look like pest damage. Yellowing or speckled leaves with no insects present may be something else entirely.
• It gives no rates. For any product, the rate comes from the label, not from us.

If you are not sure, or the damage is spreading fast, confirm before you treat. In the US your local cooperative extension service runs plant diagnostic help and can tell you which products are allowed where you live; in the UK the RHS offers advice to members; many garden centers and botanic gardens run plant clinics. Bring a sample in a sealed bag or a sharp photo of both sides of the leaf.

Prevention beats every spray

• Rotate crop families between beds each year so soil-dwelling pests do not find their host waiting.
• Use row covers on brassicas and squash from planting.
• Space plants so air moves between them; the Plant Spacing Calculator gives a starting distance for twenty common crops.
• Grow flowers such as dill, alyssum and marigold among the vegetables to feed the predators that eat pests.
• Water and feed for steady growth: a stressed plant attracts and suffers from pests more.
• Walk the garden a few times a week. A pest found early needs a hand and a jar; found late, it may need much more.