To insist that insects—good, bad, or indifferent—should be banished from the garden is to begin a skirmish that leads to incessant warfare, a warfare that is unwise, unwinnable, and virtually unnecessary.
—Eric Grissell, from Insects and Gardens (Timber Press, 2001)
If only aphids, slugs, and cutworms were as dreadful as their names sound, and honeybees, butterflies, and earthworms were as beautiful, or dutiful, as their Saturday morning cartoon caricatures. Then it would be easy to distinguish between which creatures are beneficial for your garden and which are truly pests. In reality, however, bugs are much more complicated. Though this much we can be sure of: Insects and other small terrestrial and airborne creatures play a vital role in any garden’s overall composition.
The arrival of nonnative insect species makes it even harder to know which creatures are “good” or “bad” in a particular setting. And as with many other territorial battles, you can choose between two paths: Wage all-out war no matter what the collateral damage, or know the issues on the ground and engage in only the occasional, well-aimed assassination. To help you decide, here’s a quick primer on four common denizens of western Oregon’s gardens.
Slugs thrive in a moist environment, which our climate provides for nine months of the year and our watering habits typically maintain for the remaining three. Western Oregon has about 10 species of slugs that are considered horticultural pests, only one of which is native (the banana slug). The tiger slug is one of the most common species seen in our Portland gardens.
Pros and Cons: From the gardener’s perspective, there isn’t much good to say about these voracious terrestrial mollusks except that they eat and recycle plant matter, and serve as a food source for some birds and beetles. The trouble is that pest slugs will devour just about anything in your garden. So if you want your veggies to make it to your plate, be proactive. The major exception to this rule is our homeboy, the banana slug, which lives in wooded areas and rarely bothers garden plants. When taking up arms, go easy on this unfortunate soul.
Battle Plan: Deter slugs with paths and mulches composed of rough gravel or hazelnut shells—slugs don’t like to cross sharp or hot surfaces. Wide copper strips surrounding planters or raised beds also create a mild electric shock when slugs touch them, but they must be kept untarnished. If deterrents don’t work, try search-and-destroy tactics: At night, when slugs are feeding, hunt them down and snip them with scissors or drop them into a bucket of soapy water. Or try a commercial iron phosphate product such as Sluggo to bait the slugs and cause them to stop feeding. It’s safe to use around animals and pets, and the material breaks down into a soil amendment that plants can use.
The best time to catch or bait slugs is in September or October, before they lay eggs, followed by March to June, when they are most active and plants are most tender.
The honeybee, introduced to North America from Europe in the early 1600s, receives most of the credit for commercial crop pollination. But honeybee populations are declining sharply because of the diseases and parasites that spread when they’re transported around the world.
Thankfully, the orchard mason bee (Osmia lignaria) is also a great pollinator and a native of the Northwest. Insect conservation organizations like Portland’s own Xerces Society (4828 SE Hawthorne Blvd, xerces.org) recommend that we all pitch in to provide habitat for native bees to help these populations grow.
Pros: There are no real cons to having bees around. These pollinators often evolve in close relationship with the plants they pollinate, so they play an essential role in fostering native plant communities. Many Pacific Northwest native bees are solitary rather than hive-forming and are only active during the time their host species are in flower. In the case of the orchard mason bee, this period lasts just six to eight weeks, from early April to early June—the ideal time for pollinating many fruits and berries.
Helping Hands: Support pollinator biodiversity by avoiding the use of insecticides and creating habitats and refuge for both nesting and feeding bees. Good bee-nesting habitats include well-drained, undisturbed ground; tree snags; and hollow branches.
Encourage bees to visit your garden by growing nectar- and pollen-producing plants. Native plants are the best source of nutrients for native bees. These include Oregon grape (Mahonia), flowering currant (Ribes), wild lilac (Ceanothus), and wild buckwheat (Eriogonum). Some garden plants are also helpful to these insects, particularly old-fashioned flowers rich in nectar and herbs such as Agastache, hyssop, lavender, and oregano.
Universally loathed, particularly when they make their way into a salad bowl, aphids feed on a wide range of domestic and commercial crops. The green peach aphid (Myzus persicae) is a common pest in western Oregon, afflicting more than 100 of our plants, including staple garden vegetables like cabbage and kale.
Pros and Cons: Small insect-eating birds, like bushtits, feed on aphids, as do beneficial insects such as hoverflies, green lacewings, and certain parasitic wasps. So on the plus side, aphids’ presence encourages these helpful birds and insects in your garden.
But that’s about the only virtue these critters can claim. Aphids weaken plants by sucking juices from their tissues, sometimes spreading disease. They reproduce at an alarming rate in May and June—each female can produce three to six fully formed young per day for weeks on end. Many aphids also attract small ants that “farm” the aphids, protecting them from predators in exchange for the sweet honeydew the aphids excrete. This honeydew forms a sticky residue on plants’ leaves, which leads to unsightly black mold that can interfere with photosynthesis.
Battle Plan: Most disease and insect threats affect plants that are already weak or stressed. So if you have aphids, consider the plants’ overall health and address any problems with soil, light, or other conditions. To treat the problem, first dislodge the aphids with a strong stream of water—they’re not very savvy at finding their way back. Then, count on birds and predatory insects to do their job. If that doesn’t work, step things up and plant alyssum and coriander, whose flowers will attract and provide nectar for aphid-eating hoverflies.
CATERPILLARS AND BUTTERFLIES
The Willamette Valley grasslands and woodland margins host lovely native butterfly species, including many that are endangered. But these small and vulnerable creatures are threatened by urban development, mass agriculture and farming, herbicide use, and the spread of invasive species.
Pros and Cons: In addition to being beautiful creatures, butterflies are excellent pollinators, making them valuable players in the food web. And there are even a few native butterflies that will overwinter in urban garden settings. But city gardens also provide havens for nonnative fluttering insects such as the cabbage white butterfly (Pieris rapae), which, when still a caterpillar, is one of the most common garden pests. They adapt well to urban habitats and enjoy our penchant for growing their favorite larval foods: cruciferous vegetables such as cabbage, kale, and broccoli.
Battle Plan: Remove unwanted cabbage white caterpillars from your plants in the evening. It’s easiest to pick them off by hand. Remove the eggs too, which overwinter on cabbage, broccoli, brussels sprouts, and other crucifers, and are ivory and spindle shaped. Don’t apply Bacillus thuringiensis (Bt), though. This naturally occurring bacterium will kill desirable butterflies as readily as it will the cabbage white.
Helping Hands: The thrilling anise swallowtail (Papilio zelacaon) is the butterfly you want in a Pacific Northwest garden. Like most butterflies, it needs sunny areas for basking and a water feature or puddle in which it can drink. To attract it, plant good hosts for its larvae, like fennel, angelica, parsley, carrot, and dill. But remember that if you allow the butterfly’s eggs to hatch in your garden, you’ll have to sacrifice some plants for the larvae to feed on before they undergo metamorphosis. Adult anise swallowtails feed on the nectar of lilac, lupine, camas, columbine, fireweed, balsam root, manzanita, penstemon, and more.