Put the Pinch on Pests
These organic techniques stop bugs from snacking on your plants
One of my first attempts at gardening was a neat row of marigolds planted around some shrubbery. With wonderful soil and perfect weather, I was primed to become the gardener I had always intended to be. I stood back and admired my work as the freshly transplanted marigolds cheerily stood their posts. Oddly, the next morning it seemed as if there were fewer marigolds than before, but I figured it was just my imagination. But by the third morning only one marigold remained.
My marigolds were victims of the most common and destructive of garden pests—plant chewers. Whether they have true chewing mouth parts, as do beetles and caterpillars, or rasping adaptations, as do slugs and snails, the effects are the same. They leave holes behind where healthy plant tissue once thrived.
Examine the scene of the crime
Each chewer has its own way of damaging plants. Careful examination of the damage makes it easy to distinguish between the various types of chewers, and then form a plan to curtail their devilish deeds. Japanese beetles chew jagged holes in both leaves and flowers, sometimes skeletonizing their victims and leaving only the tough leaf veins intact. These garden gourmands will chomp on over 300 different types of plants, but have a weakness for rose petals and blossoms. And damage from Japanese beetles doesn’t end with the arrival of cooler weather. Their larvae destroy lawns by burrowing into the ground and feeding on grass roots during the fall into the spring.
Caterpillars are masters of disguise. Their mottled green to brown color helps them blend into their surroundings, and their common position on the undersides of leaves and leaf stems makes them difficult to spot. Caterpillars create smooth-edged holes of various sizes on a long list of trees, shrubs, and herbaceous plants. Another sign of caterpillars is the little pellets they leave behind on the chewed leaves. These voracious chewers are active day and night, but less so in cold, rainy weather.
Slugs and snails usually shear off the edges of plants and may eat through the leaves, stems, and buds until entire plants are consumed. They almost always work at night, or on cool, overcast days. Like most classic villains, they leave an unmistakable clue behind—a glistening trail of slime. Low-growing, herbaceous plants like hostas and the tender, emerging shoots of spring bulbs are especially irresistible to slugs and snails.
Manual disposal works well
The best defense against actively foraging insects is to nab them in the act. Beetles, caterpillars, slugs, and snails are easy to snatch, either barehanded or, for the faint of heart, with thin latex gloves. Large, tweezer-like forceps can also be used to pick up the beasties. Once plucked, drown your catches in a pail of soapy water, or just squish them.
Another time-honored trap really does work for slugs. I leave a pan of beer or other fermented yeast concoction, such as an aged solution of yeast and brown sugar, on the ground. Place the container—an old pie tin or a cat-food can works fine—with the opening just above ground level, and about an inch of beer inside. Slugs and snails toddle in, drink up, and expire. They also like to get cozy underneath piles of leaves and old boards. Wooden planks and orange rinds positioned as little havens in the garden make for easy pickings of these unwary interlopers early in the morning.
Physical barriers keep chewers at bay
Physical barriers prevent pests from getting to plants. I stop cutworm caterpillars from chewing through the stems of my vulnerable seedlings with collars of stiff paper or plastic. My favorite collars are 1-inch segments from plastic drinking straws slit along one side. Cutworms can’t chew through the plastic and the slit allows the segment to expand as the plant does, until it pops off.
Snails and slugs can be excluded from beds, containers, shrubs, and trees with a little electrical engineering. I tack thin sheets of copper about 3 inches wide around the edges or trunks of such targets. When moist slugs or snails touch the copper, they receive slight electrical charges, causing them to recoil and retreat. For extra protection, bend the edges of the sheeting away from the base at a right angle, but be careful, as the edges of this sheeting can be very sharp. Other barriers that discourage the lowdown and slimy are those with a rough surface or chemical irritants, from a border of crushed eggshells, ground limestone, or powdered ginger to protective plant collars fashioned from old window-screening.
Many gardeners rely on lure traps to attract and trap Japanese beetles. These traps are very effective, but in my mind, not worth the cost. It’s akin to putting up a sign advertising that you have plants ready for the munching. These traps attract many more beetles than they can capture, so the overflow ends up setting up shop in your garden.
Plants bugs won’t touch
The author recommends growing plants that aren’t prone to damage from chewing pests. Those that act as pest repellents when interplanted with damage-prone plants are denoted by an asterisk.
Bleeding hearts (Dicentra spp.)
Candytuft (Iberis sempervirens)
Catnip (Nepeta spp.)*
Clematis spp. and cvs.
Columbine (Aquilegia spp.)
Coral bells (Heuchera spp.)
Deadnettle (Lamium maculatum)*
Hardy geraniums (Geranium spp.)
Lavender (Lavandula spp.)*
Meadow rue (Thalictrum spp.)
Rosemary (Rosmarinus officinalis)*
Sedum (Sedum spp. and cvs.)
Speedwell (Veronica spp.)
Tansy (Tanacetum vulgare)*
Virginia bluebells (Mertensia virginica)
Allium spp. and cvs.*
Autumn crocus (Colchicum spp.)
Daffodils (Narcissus spp.)
Basil (Ocimum basilicum)*
Cockscomb (Celosia spp.)
Coleus (Solenostemon cvs.)
Morning glories (Ipomoea spp.)
Nasturtium (Tropaeolum majus)
Snapdragon (Antirrhinum majus)
Salvia (Salvia spp.)*
Sweet alyssum (Lobularia maritima)
Biological controls act as natural predators
Biological controls are basically diseases in a package. Bacillus thuringiensis (Bt) is a bacterium that, when applied to leaf surfaces, kills most caterpillars. It works best if spritzed on both sides of susceptible leaves early in the growing season. Caterpillars have to ingest it for it to work, so some chewing is necessary.
A disease that kills the ground-dwelling larvae of Japanese beetles, known as milky spore disease, is caused by Bacillus popilliae, which can be purchased and sprayed on lawns as the package instructions suggest. Both of these biological products are widely available by mail and at garden centers. The trick is to apply them early enough in spring before the chewers have done much damage. Bacterial controls are host specific, meaning they’re only toxic to the targeted pests, and will not harm pets or the environment.
Let your garden do the work for you
Natural controls include those already present in the garden plus those I introduce—from basil to bacteria to bats. I check plant labels and garden books for plants which are most resistant to pest damage and those that repel pests. Chewing pests, though voracious, don’t eat everything. Slugs don’t care for basil, prostrate rosemary, thyme, lavender, or wormwood. Beetles locate their hosts by scent, and are generally specific feeders, so merely mixing up my plots mixes up the beetles up as well. I jam their sensory radar by interplanting my ornamentals with snap beans, garlic, onions, catnip, dead nettle, horseradish, nasturtiums, or tansy. Onion, garlic, cosmos, and tansy are repellent to some caterpillars as well.
Mystery solved, case closed
Ever since that night, years ago, when I set out—flashlight in hand—to solve the mystery of the missing marigolds, I have been intrigued by the drama of daily life in the garden. As I waited for any sign of life, a merry band of snails, or “the three molluskateers,” silently returned to devour the last of my marigolds. They seemed to be having an awfully good time racing for that tender transplant. I plucked them up and chucked them over the neighbor’s fence. I figured it was dark, and who would know? And it’s not like the darned things could fly back. The next morning, however, the last marigold was gone. Guess I should have planted basil instead.
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