Sometimes we can look at our gardens and know that something’s eating our plants. If there are big holes in the leaves – or a whole plant is missing! – it’s pretty obvious. Other times we just see leaves turning yellow, or plants staying small, and we have a hard time knowing if it’s having its lifeblood sucked out by insects and mites, or if it’s diseased, or needs more fertilizer and water. There are times when we might need an expert, like someone in OSU Extension to help us identify the source of the problem, but often we can figure out enough on our own to treat the problem. Here are some basic guidelines for identifying and controlling pests. Keep in mind there’s a lot more to know than what I have room and time to say here.
The Chewers
Caterpillars, beetles and slugs are pests that have jaws for chewing, and are generally the culprits when you see leaves with holes chewed or sections removed.
Caterpillars, the larvae of moths and butterflies, are expert hiders, good at camouflage, but even if you never see them, you may see their pellets of frass, aka “poop”, that look sort of like seeds. They can leave holes in the centers of leaves since their clinging feet and flexible bodies can take them all over the plants. Leaves of Zinnias, Delphiniums, and many more, and the centers of Coneflowers, fall victim to caterpillars. Budworm-type caterpillars will eat unopened flower buds, most commonly of geraniums and petunias, leaving you wondering why your flowers have stopped blooming. They haven’t stopped producing flowers, they’re just being eaten before they can develop.
Beetles are hard-shelled flying insects. They eat holes in leaves and flowers, and leave a “frassy” mess on flowers. The spotted yellow Cucumber beetles are a common problem. Root Weevils, some of the most damaging beetles, hide near the soil surface and come up to eat at night. They chew around the edges of leaves, making a distinctive notched pattern, especially on Rhododendrons and Azaleas. Their larvae live in the soil and chew on roots and crowns of shrubs and perennials, making this pest doubly damaging.
Slugs need no introduction, unless you just moved here from the desert Southwest; we’re all familiar with their shiny slime trails. But sometimes they’ll be around even when you don’t see slime. If it’s cool moist spring weather, and you see leaves being stripped to the veins, young perennial shoots being chewed off as fast as they come up, and marigolds eaten to the bare main trunk, put out the slug bait, whether you see slime or not, because you know it’s too early for caterpillars or beetles to be around.
Now here’s one to really fool you: Shot-hole disease on cherry trees. It looks like something’s eating holes all over your flowering or fruiting cherry, but it’s actually a fungus disease, also called Coryneum Blight, and must be treated with a fungicide.
The Suckers
Aphids, whiteflies, thrips and mites are like fleas on our pets, like tiny vampires – nasty little things.
Aphids, though small, are big enough to see, and are usually visible clinging thickly to leaves and stems, though sometimes you have to turn over a leaf to find them. They weaken plants, and cause leaves and new growth to twist.
Thrips feed inside flower and growth buds, and distort and stunt them. Leaves of badly infested plants will have a sort of silvery spotting to them. They are very small, and you usually have to shake a flower over a sheet of paper to knock them out in order to see if they’re there. Thrips spread virus diseases, too, a bad problem in itself.
Whiteflies are easily recognizable as “miniature seagulls”, white flocks of them flying away when you move your Fuchsia basket or shake your rose bush. In a bad infestation you’ll see the tiny bumps of their pupae on the undersides of leaves – they’ll look tiny eggs crusted on. They cause overall weakening and yellowing of plants.
Mites are the smallest, and thus can escape detection the longest. But the pale, stippled look of infested leaves is distinctive. In bad infestations, mites will form very fine webbing across the undersides of leaves and between close branches. Don’t confuse mites, even the type called spider mites, with real spiders. Spiders are good, they don’t eat plants – they eat insects that eat plants. Any creature that lives by eating insects is a friend of mine. I just don’t want them on my bed with me, though.
Here’s another sneaky look-alike: Septoria leaf spot, a common fungus disease on tomatoes, among other plants, causes yellowing and some down-curling of leaves. Its damage looks a lot like that of sucking insects and mites. But insect all summer won’t make a dent in it; you have to spray with a copper fungicide.
What to do about it
Always start with the least-toxic method you can. Always read the label of any product you use, and follow the directions carefully. Keep in mind that full elimination of insect pests is a fleeting, unattainable dream. At some point you have to live with a certain amount creatures sharing your garden.
Caterpillars – There’s a safe, effective spray control, “Bt”, the toxin produced by the bacteria Bacillus thuringiensis. We carry it by the name Safer brand Caterpillar Killer. Caterpillars are around from late May into fall. Spraying earlier than that to get a jump on them won’t help. You’ll probably need to spray several times through the summer, since new eggs are laid, and new caterpillars hatch out and eat all season.
Beetles – They’re hard to stop because they’re big (by insect standards) and sturdy, and can just fly away from whatever you try to spray them with. The most common ones, Cucumber beetles, are nearly always on flowers, so you can’t spray them with anything without harming bees. So, do as I do – squish them when you can and curse them when you can’t. There isn’t much else we can do.
Root Weevils – The adult stage emerges and starts eating in early summer, and can be controlled with “hard” chemicals, like Orthene (acephate). Spray it on whatever plants you see being damaged, in the evening so the spray is fresh when they come up to feed that night. There is no biological or organic control that I know of. The larvae are in the soil from fall through early spring. There’s no good treatment for that stage of their life cycle. The bio-control nematodes that are promoted for that purpose frequently are unable to survive in the soil temperature or moisture conditions at the times of year they’re needed. Spring soil is too cold for them, fall soil is too dry for them.
Slugs – Fortunately, most baits work fine – you just have to get it out and use it. For the non-squeamish there are other methods, too. The classic pan of cheap beer works well, you just have to be able to face handling a pan of bloated, rotting corpses of drowned inebriated slugs. My grandma used to make the rounds with a knife early every morning, impaling any slug she saw. I shy away from the old salt-shaker method, because if you have a lot of slugs you could actually damage your soil that way. Sadists like watching them squirm, though.
The Suckers – All the suckers can be fought off the same way (notice I did not say “controlled”) by coating and suffocating them with either an insecticidal soap spray or a horticultural oil spray. Neem oil has a combined action of oil-coating plus plant-derived insecticidal action. Suckers will require more than one treatment, since there’s always more than one stage of the insects’/mites’ life cycle on the plant at the same time, and you have to catch them in a vulnerable stage. We carry both insecticidal soap and neem oil sprays.