The best way to ward off insect and disease problems is to grow a healthy plant. Just as a wolf pack will target the weakest member of a herd, insects seem to zero in on a plant that is under stress. Good gardening practices—choosing the right plant for the spot, soil preparation, proper planting, feeding, watering, mulching, and the like, all go a long way to keep our gardens free of damaging pests.
But cultural control goes further than just having a green thumb. Sometimes our yards are invaded by insects no matter how good a gardener we are. In that case, it pays to know the enemy.
What’s eating my garden?
The first step in dealing with damaged plants is to identify the culprit. When I worked at our master gardener help desk, we’d frequently get clients who wanted to know what was eating their plants—when the problem turned out to be something else entirely (hail, herbicide overspray, root damage, etc.). Insect damage tends to be spotty, with chewed leaves, honeydew, and frass (bug poop, top). Usually the insect itself is caught in the act.
Many insects are host-specific, so identifying the plant can help us to identify the bug. If you don’t recognize it, your local extension office can help.
Then you have to decide if the damage warrants action. Defoliation of a deciduous tree in early September isn’t serious, since the leaves would drop anyway. Are there dozens of caterpillars eating your broccoli, or just one or two?
Next, consider the insect’s life cycle. Some bugs emerge hungry early in the spring, others don’t do damage until August. Some have one life-cycle per year, others have three, four, or more. Knowing when the bugs are around helps us create our strategy.
Sometimes, we can change our timing, growing a crop when the insect isn’t present. Or, we can hang pheromone traps, monitoring insect populations so that we spray only when the pest is present, instead of indiscriminately applying chemicals on an arbitrary schedule. Colorado State University issues bulletins throughout the growing season that alert gardeners to current pests and appropriate control methods.
Marigolds deter nematodes. We’ve heard it so often, we take it as truth. But in fact, only one type of marigold (the stinky “African” kinds) deters nematodes, and only in its immediate root zone. Planting a ring of marigolds around a garden only succeeds in making your plot look pretty (which is great). And besides, Colorado doesn’t have harmful nematodes in the first place.
Nearby plants can act as “trap crops,” however. This is when you plant something a pest really likes so that it leaves alone the plants it only sort of likes. For example, Swallowtail Butterfly larvae (aka “parsley worms”) like parsley, but so do we. Happily, the caterpillars like lovage even more than they like parsley—so plant some lovage, too. Most gardeners will gladly give the hungry caterpillars their lovage in exchange for an undamaged parsley crop.
Trap crops work well for other pests, too, especially flea beetles, which are hard to control. I sometimes sacrifice an early planting of bok choy in order to get intact cabbage. (Later plantings aren’t as susceptible to damage, as there are fewer beetles around at the end of the summer.)
Near-by plantings can also cause pest problems. For example, Cabbage Aphids overwinter as eggs on any plant in the cabbage family, including weeds such as wild mustard.
Using cultural controls to control pest populations take a bit more knowledge than just reaching for a spray bottle. You might have to contact your local master gardener help desk, or spend some time at your computer to discover just which technique will solve your specific problem. The result, however, is a pesticide-free yard and a sense of accomplishment that comes from working with Mother Nature, rather than against her.