Preventing Bulb Pests
Look out. You’re not the only one enjoying the spring-flowering bulbs in your garden. Snails and other pests are enjoying them, too—but for all the worst reasons. They’re munching on the foliage, sucking plant juices or leaving slimy trails and other signs of damage.
Some pests even attack your bulbs in storage before you have a chance to plant them or spread viral diseases as they feed.
To guard against these pests, make sure you don’t bring any home with you. Choose healthy, top quality bulbs when you shop and avoid those with signs of injury where pests can enter. When you plant, make sure your soil drains easily since moist conditions can invite problems.
Allen R. Pyle, staff horticulturist for Jung Seed Company, says slugs and snails are often a problem in damp soils. Both are mollusks, a name that comes from a Latin word for “soft” and describes their bodies. Snails have shells while slugs do not.
Both slugs and snails hide during the day and come out at night to feed on bulb leaves. To control them, set traps by burying a cat food can, tuna fish tin or other small container in your garden, keeping the rim close to the surface of the soil. Fill the container half-full of cheap or stale beer, or use a solution made from 2 tablespoons of flour, a teaspoon of sugar, 1/2 teaspoon of baker’s yeast and 2 cups of warm water. The yeast will lure them in and they'll drown. You can also buy a commercial bait if you prefer.
You can also try placing flat boards in your garden. Check under the boards each morning and destroy the slugs you find there by dropping them into a bucket of soapy water.
Materials that interfere with snail and slug mucus can also be effective. Sprinkle diatomaceous earth, a powder made from the fossils of small aquatic organisms, around your bulbs; its sharp, microscopic edges will deter or kill these pests. Add more after it washes away in the rain or when you’re watering.
Copper barriers, which can be purchased from garden centers and nurseries, can also help. If you use them in a raised bed, be sure you've controlled the slugs and snails in the bed first or you’ll trap them inside with your plants.
Aphids are other serious bulb pests, says Pyle. Suspect them if you see black spots on your foliage. These tiny, soft-bodied insects produce droppings called honeydew as they suck plant juices. Mold that forms on the droppings causes the spots.
Distorted flowers or new growth are additional signs of aphids. Work fast to eliminate the aphids because they reproduce rapidly. If you have a small infestation, cut off the affected plant parts and trash them or crush the aphids.
For larger aphid populations, try knocking them off with a stream of water from the hose (be careful not to damage your bulbs). If they persist, use a horticultural oil or insecticidal soap.
Distorted flowers can also signal thrips, tiny bugs that feed on plant sap and make leaves and petals look speckled, silvery or stippled. Like aphids, they can damage bulbs in storage and spread viral diseases.
Thrips can be hard to see and control. You’ll probably need repeated applications of an insecticidal soap, horticultural oil or spinosad (a natural substance that comes from a soil bacterium) product. Make sure you get good coverage, applying whatever you use to both sides of the leaves and in any “nooks and crannies.”
If you're growing bearded iris, watch for problems with iris borer, a brown moth. Siberian iris, Iris reticulata and Dutch iris can also be affected but to a lesser degree.
Adult iris borer moths lay eggs that overwinter on iris foliage or debris. The eggs hatch in spring to feed on the rhizomes and make streaks and tunnels in the foliage. You may also see spots on the iris that look water-soaked.
Pyle notes that insecticides aren’t very effective on iris borer when these pests are in the larval stage and they've already bored into the rhizomes. But you can treat your plants with pyrethrin, neem or a systemic insecticide. Follow directions on the label to know how often to spray; you'll probably need multiple applications. If your damage is minor, cut off and destroy the damaged leaves and crush any larvae you find in tunnels.