What Are Those Little Black Slug/worms In My Garden

Slugs and snails affect many ornamental and vegetable plants. (Photo by Joanne Young)

As I readied my fork to eat a homegrown salad with edible flowers, a few aphids paraded into sight. Less worried about eating them than I was about my gardener-colleagues knowing I had an aphid infestation, I discreetly picked off aphids as I ate. And I made a mental note: aphids like nasturtiums.

Because my flower garden is not a display garden and I'm not a commercial market gardener, I'm not too hard on myself when I have a few bugs in the garden—most of the ones I see are benign. I'm more interested in balance and ease of care than total bug annihilation.

One way to keep infestations small is to monitor plants to detect problems before they reach epidemic proportions. I don't patrol, notebook in hand, tallying pest levels, but when I'm picking flowers or vegetables, I look for anything out of the ordinary, and when I notice more than a few, I begin the appropriate treatment.

In the world of bugs, there is most often an alter ego: a younger or older stage. For example, caterpillars turn into moths; beetles have larval stages. Some we never see. It's important to know that many do no harm, and many help by eating pests. But a few cause gardeners heartache. Here are 10 to watch out for, along with advice from experts on how to reduce their numbers in your garden.

Best practices for pests

Aphids

Aphids are small, pear-shaped, sap-sucking insects with a "stylet," straw-like piercing mouthparts. They often cause leaf curling and distorted growth at shoot tips. There are many species (including red-, green- and black-coloured aphids — having winged and wingless stages), affecting a wide variety of plants. They all reproduce quickly.

Paul Zammit, former Nancy Eaton director of horticulture at Toronto Botanical Garden, uses strong jets of water to dislodge aphids, coupled with rubbing his hands along leaves and stems to crush them. Victoria Bick, historic garden coordinator at Dundurn National Historic Site in Hamilton, Ontario — a restored kitchen garden managed according to 19th-century principles — says, "Historically, when aphids became out of control, the gardeners would be deployed with hand-pumped water sprayers to shoot them off the plants. We make use of similar techniques, although with modern plastic water bottles and a bit of soap."

For larger areas, trial garden manager Rodger Tschanz at the University of Guelph in Ontario recommends carefully spraying with insecticidal soap.

Natural predators, such as the aphid midge (Aphidoletes aphidimyza) — which has an orange larval stage that voraciously gobbles up aphids — often arrive at aphid colonies and take care of the problem for you. Donna Balzer, a garden writer and horticulturist in Alberta, says she was late checking for aphids this year. "By the time I saw my first aphids, there were already three different predators at work," she says. "Aphid mummies, lady beetles and aphid midges."

Slugs & snails

The trail of glistening mucous in the morning sun is a sign that slugs and snails have glided into your plantings overnight; along with the trail, look for large, ragged holes in leaves. You might catch them in action on rainy days or when there is dense foliage nearby that keeps conditions cool, dark and damp.

Slugs and snails affect many ornamental and vegetable plants. Because they like moisture, dry weather often results in less slug and snail damage. An open garden, with a less dense, protective canopy, helps too.

Slugs can be trapped and drowned by putting a shallow basin (such as a jar lid) filled with an attractant, such as beer, in the garden. Sue Clarke, instructor and curator at Niagara Parks Botanical Gardens and School of Horticulture in Ontario, says, "For a home gardener, beer really does work, but even better is planting resistant species or cultivars where slugs are a problem."

"Slugs and snails are my primary garden pest," says Niki Jabbour, who gardens in Nova Scotia and is the author of The Year-Round Vegetable Gardener. She hand-picks as soon as she notices them in spring. "Every morning for a week or so, I go out and gather up all the slugs I can find, dropping them in a bucket of salty water." She keeps at it all season, but says the early hand-picking greatly reduces the overall population.

Japanese beetles

These metallic-green beetles feast on a wide range of flowers and foliage; leaves are often skeletonized, with only the veins remaining, while flowers are thoroughly chomped. The larval stage of this beetle is one of the white grubs that feast on turf roots.

In my garden, Japanese beetles head straight for the roses. They like the grape vines and bean plants, too, and Zammit says that in the Toronto Botanical Garden the adult stages feed on the leaves and flowers of perennial hibiscus and the foliage of persicaria.

Zammit has used pheromone traps in the past with mixed results, but now relies on hand collection and crushing. The change in tactics also solved the problem of raccoons discovering the traps and making a mess of them trying to snack on the trapped beetles. Tschanz recommends knocking the beetles into a pail of soapy water.

Stacey Hickman, entomologist at Natural Insect Control in Stevensville, Ontario, recommends using nematodes to control the grub stage on ornamentals and lawns.

Japanese beetles feast on a wide range of flowers and foliage. (Photo by Joanne Young)

Spider mites

Like the aphid, spider mites have piercing, sucking mouthparts with which they feed on the underside of leaves. While the insects are small and difficult to see, their feeding causes a recognizable speckling on foliage. There are many types of mites affecting many kinds of plants, but the one most often seen in ornamental and vegetable gardens is the two-spotted spider mite.

Clarke says spider mite infestations are usually the result of environmental conditions. "Hot and dry, we have spider mites; hot and moist, we don't," she says. Clarke adds that proper irrigation and soil fertility helps. "A healthy plant is more able to resist insect attack," she explains.

For bad infestations, spray with insecticidal soap, but test plants first as some are susceptible to burning, Clarke cautions. Tschanz recommends a high-pressure water spray to dislodge the mites from leaves and stems, taking care to set the water pressure so that it doesn't damage plant tissue.

Cabbageworms

"On a sunny day in the Dundurn garden, visitors may see white butterflies flitting from plant to plant," says Bick. And while fluttering butterflies may sound idyllic, she notes they are actually the adult stage of the imported cabbageworm. This green worm, along with the cabbage looper, feeds on members of the cabbage family. They "focus their attention on cabbage, kale, cauliflower, broccoli and Brussels sprouts," says Bick.

The worms are well camouflaged. I once unwittingly served them steamed (along with the kale leaves on which they had been feeding) to my wife. Alas, she has had less enthusiasm for my homegrown kale ever since.

Jabbour uses lightweight row covers to keep butterflies from laying eggs on foliage. "These cabbage family vegetables don't require pollination to produce a crop, so I leave the covers on until harvest," she says.

I grow broccoli varieties with multiple small heads, making it easier to hand-pick and cut out worms. If you're alert enough to spot the eggs on the undersides of leaves, scrape them off. For larger areas, treat with the biological insecticide Bacillus thuringiensis (Bt). Clarke suggests applying Bt at the first sign of holes in the leaves.

Tomato hornworms

These large, green caterpillars with a horn-like appendage at their tail end (scary looking, but harmless) can consume a lot of tomato leaves.

Thankfully they've never been a major pest in my garden, and I just hand-pick the few that I do get. They also dine out on tomato relatives, such as eggplant and peppers. At Dundurn, both tobacco and tomato hornworms are problems. "[They] feast on our heirloom tomato plants, stripping them of leaves and eating immature fruit," Bick says.

But Bick has had an unexpected helping hand. "Hand-picking the hornworms is the historically accurate way of dealing with them, but we have found an ally in the garden: parasitic wasps. These wasps hunt the hornworms better than we can, lay eggs in them, and only become apparent to us when the wasp larvae hatch, emerge from the caterpillar and spin cocoons on the outside," she says.

For gardeners with a plague, a Bt treatment is an option.

Vine weevils

The black, long-snouted adults munch leaves on a wide variety of plants leaving irregular notches on the edges of foliage; the white, soil-dwelling larvae eat roots. They also eat the roots of plants in containers.

Zammit has noticed the adult weevils prefer leaves of heuchera, climbing hydrangea and bergenia, but he's not especially concerned with the nibbled leaf edges. "My main worry is the larvae feeding on the roots," he says.

While he hasn't treated for vine weevils to date, next year he intends to apply nematodes to container plants and to those in the garden. Commercially prepared nematodes, mixed with water, can be applied to the soil. Once the nematodes encounter the larva, they infest it, causing death.

Cutworms

These hairless caterpillars live underground and out of sight. While you're not likely to see this pest in action, you'll see their devastation in the morning: young plants on their sides, chopped off at ground level.

Transplanted tomatoes and peppers are usually the most affected in my garden. Luckily, preventing cutworm damage to transplants is easy. Wrap a newspaper collar — a physical barrier — around the stem of transplants, one to two inches (2.5 to 5 cm) high and extending slightly under the soil.

Bick had cutworm challenges at Dundurn this past season. "Gardeners would go down into the garden in the morning and find precious seedlings separated from their roots, tipped over and dying. Almost every species was affected, from the coleus to the gomphrena, radishes to peppers," she says. While replanting to replace sawn-off seedlings, they unearthed and disposed of many cutworms.

A nematode treatment is another option.

Colorado potato beetles

The adult is yellowish with black stripes, while the homely larvae are reddish with black dots, often trailed by their own droppings. Clusters of orange eggs on the undersides of leaves are easy to spot and rub off by hand.

Both the adult beetle and its larvae make themselves a nuisance in the vegetable garden, targeting mainly tomato and potato plants.

Bick says it's a recent arrival at the Dundurn garden: "[This] was the first year we encountered this potato-killing pest, which was well known to our 19th-century gardening counterparts. Small-scale gardeners hired young boys to hand-pick the bright orange-red larvae off the plants, so we did a similar thing. Children (and non-squeamish adults) visiting the garden were encouraged to look for the larvae and put them into a jar of soapy water, where they quickly died."

Lily leaf beetles

These attractive, shiny red beetles with black antennae are real eye-catchers. Their larvae, in a protective case of their own excrement, are the sort of thing only a parent could love. Both the adults and larvae, as the name indicates, eat members of the lily family. They also enjoy fritillaria.

I hand-pick the beetles and larvae. Tschanz says, "I do what everyone else does — either squish them or knock them off the plant into containers of soapy water."

When to let live

Only a small number of garden bugs cause heartache for gardeners — most do no harm at all, although a few may cause minor cosmetic damage.

Leafcutter bees, which cut little half-circles from the edges of leaves, are important pollinators. People pay money to buy them to pollinate vegetables. Swallowtail butterfly caterpillars arrive every year in my garden to dine on dill, as well as parsley. I consider it a fair trade. I have lots of dill, and I like the look of both the butterflies and caterpillars.

Other insects, while not particularly problematic, simply have a yuck factor. Think of pillbugs and sowbugs, those mini-armadillos that come out from under rocks and pots. They feed on decaying organic matter. Yucky, perhaps, but not really pests.

And finally: the earwig. Without a doubt, the earwig is a repulsive, revolting, repugnant bug in the garden that occasionally chews holes in dahlias and lettuce leaves. Yet it's also a pest-eating omnivore and helps control small, soft-bodied insect pests.

When I toss a head of lettuce into a sink full of water to clean it, earwigs do the doggie paddle, trying to escape from the sink. However, I don't engage in full-fledged earwig control in my garden, because for what I grow, their damage is below my threshold. I have a neighbour, though, who leaves out bundles of twigs in which the earwigs congregate overnight. After shaking out the bundles over his driveway the next morning, he does an earwig jig to squish them before they scurry away.

Part of successful pest control is choosing an approach that is practical for the gardener.