I walk through my gardens daily, to enjoy them, see what’s in bloom, pluck a few weeds, trim a few deadheads, and scout for signs of trouble.
Leaf damage such as holes, distortion or discoloration may be signs of insect problems. But before reaching for pesticides, it’s important to determine exactly what you’re dealing with. Spraying an all-purpose bug killer on everything that crawls or flies is expensive, ineffective and can be harmful to natural insect predators and the environment.
If you see bugs on the plant, observe them to see what they are doing. If they’re chewing on the leaves or massed on damaged areas, you have enough evidence to act. Consider that they may be just passing through, or be beneficial insects looking to eat the pests causing the damage.
Some insects are active primarily at night. The most common of these are slugs, shell-less snails that hide in the ground during the day and feed under cover of darkness. Low-growing, shade-loving plants are most vulnerable, but in wet seasons any plants can be attacked. To see if slugs are responsible, set a saucer of beer at the base of affected plants in the evening. Slugs are attracted to the yeasty smell, crawl in and drown. If your trap has slugs in the morning, you’ve found your problem. You may use the beer traps to reduce the population, or opt for a chemical control.
Common daytime pests include aphids, small, pear-shaped insects, about one-eighth of an inch long. They may be green, black, white or red, and attack soft new plant growth. Usually discovered when they multiply to the point of covering flower buds or emerging leaves, they suck out plant juices, and in sufficient numbers can dry up those buds or new leaves, or cause distortion of new growth. They can also transmit diseases such as tobacco mosaic virus. These soft-bodied insects are easily eliminated with insecticidal soaps, which break down the insect’s protective shell, dry them out and smother them.
This is also the time of year when plant bugs take their toll. Seemingly overnight, robust, healthy growth turns brown at the tops of the plants. On closer inspection, you might see small beetles darting out of sight, and notice that the newest, most tender growth is speckled with brown dots. Plant bugs pierce the leaf surface and suck out juices, leaving behind a tiny, round, dried out spot. Many insects repeating the process cause browned and burned foliage. Usually the plants will grow faster than the insects can work, and look fine in a week or so. But if the damage seems to be increasing, use a dose of insecticidal soap here as well.
Spider mites are almost impossible to spot with the naked eye. Leaves look bleached or faded, sometimes yellowed. Armed with a magnifying glass and a 3-by-5-inch index card, pluck off one of the damaged leaves, and rap it against the card. Using the magnifying glass, examine the tiny specks, which will look like spiders. Fortunately, these small creatures are not terribly mobile. Often a well-targeted spray from the garden hose to the underside of the leaves will wash them off of your plants, and they will be unable to get back to re-infest. Severe infestations may require insecticide to control.
If you don’t recognize the pests, ask another gardener, who may know it well. Take a photo or sample of both the damage and the bug, and take them to a greenhouse or nursery for help. And the MSU Cooperative Extension Service is an excellent resource (call (888) 678-3464 or ask online at ask.extension.org/ask).
Make sure that any chemical controls list both your pest and your plant on the label. Always follow package instructions to the letter.