Avoiding Pests and Disease in the Garden
2008, Dorling Kindersley Limited
Slug damage is most common on vulnerable plants like hostas. There are many slug remedies on the market but there are plenty of home rememdies as well.
"Whenever I take people on a tour of my landscape," says master gardener Paul James, "they're invariably amazed at how few pest and disease problems I have despite the fact that I rarely use any type of organic controls and, for nearly 30 years, I've never used any synthetic products."
To maintain his relatively-pest-free landscape, Paul never uses broad-spectrum insecticides. "They're formulated to destroy virtually every bug they come in contact with. Such products, however popular, will invariably backfire on you. And in many cases, they're also toxic to humans as well as other critters like birds and bees." And, he says, even nontoxic pest controls like garlic and pepper sprays can backfire on you as well.
Broad-spectrum insecticides backfire because they destroy beneficial insects as well as problem pests, and as a result, they upset the natural balance of bugs in the landscape. It's the balance between good and bad bugs that's critical to achieving the biodiversity that allows both kinds of insects to coexist.
"In other words, it's okay to have bugs that prey on your plants as long as you also have bugs that prey on them," he says. "When — and only when — the population of problem pests gets out of control should you even consider control measures, and even then, you might want to think twice before spraying or dusting."
The population of beneficial insects can't rebound at the same rate of problem pests. Nature designed the system that way to make sure there would always be an adequate food supply — or plant-eating bugs for the beneficial insects to consume. So if, for example, you spray a broad-spectrum pesticide on a plant plagued by aphids, you may actually do more harm than good. You might destroy 80 percent of the aphids, but you would almost certainly destroy 100 percent of the lady beetles that were busily eating all those aphids. And what's more interesting is that aphids don't have to mate during the growing season. In fact, female aphids are likely carrying unborn females that are already pregnant. The lady beetles, on the other hand, are at a distinct disadvantage. They must find a mate and reproduce in the more familiar sense, which takes time. And in the meantime, the aphids have a huge jump-start on whatever plant you've just sprayed.
Insects can, and very often do, develop a resistance to certain garden chemicals, and the results of such genetic resistance can be even more devastating. Each time you apply a pesticide, a few pests will survive. In the next generation that they produce, the offspring may develop a genetic resistance to that pesticide. And genetic resistance can occur very quickly in the insect population because insects are capable of producing many generations over a short period of time. In mammals, genetic changes may take thousands, perhaps even millions, of years.
"This is all reason enough to avoid the use of garden chemicals," Paul says, "but I am willing to admit that there are times when you need to control pests." He offers some tips:
Don't panic at the first signs of attack. Realize that plants can sustain a fair amount of pest damage — up to 50 percent of their leaf surface — before the pest needs to be controlled. Instead, monitor the damage for a day or two, and then develop your own plan of attack. For aphids, first try a strong blast of water from the hose. While the water blast works amazingly well, it's something you'll have to repeat every other day or so until the aphids are gone. If that doesn't solve the aphid problem, then consider using insecticidal soap. This concoction is highly effective against all soft-bodied insects — including aphids — as well as mites and whiteflies, but it won't harm mature, beneficial insects. And finally, consider releasing beneficial insects — in this case, lady beetles to control the aphids.
Rather than reach for a product that's likely to destroy every bug in sight, try to target the specific pest with a specific product. Paul recommends BT for caterpillars; Neem for cucumber and Colorado potato beetles; horticultural oil or insecticidal soap for scale, mites and aphids. In fact, with BT, Neem, horticultural oil and insecticidal soap, you can control virtually any and every pest problem, he says.
Disease control is a slightly different situation than pest control, but aggressive intervention isn't always the answer. Plant diseases, especially fungal diseases, are very often the result of factors you can actually control:
Proper placement. Site plants so they get plenty of air circulation around them to prevent fungal spores from attaching to leaf surfaces. Planting too close to the house or crowding plants will set the stage for fungal invasion.
Good landscape sanitation. If you have a plant that's prone to fungal disease, be sure to remove and discard fallen leaves from the base of the plant on a regular basis to prevent the spread of the disease. More importantly, take steps to prevent the onset of fungus. Once a fungal disease arrives, it's likely to stick around for a while.
Practice prevention. To prevent fungus, water the bases of plants rather than the leaves, unless of course you're trying to control aphids. Apply fresh mulch now and then throughout the growing season to prevent spores from bouncing back onto leaf surfaces. And use horticultural oils, baking soda solutions — perhaps even milk — to prevent fungal diseases from attacking plants in the first place.
Finally, the most important thing you can do to prevent pests and disease problems is to give your plants optimum growing conditions — the particular type of light, water and soil that each plant needs. And maintaining healthy soil is important too.
"Make no mistake, healthy plants are far more able to withstand attack by pests and diseases than unhealthy ones are," Paul says. "So don't skimp on the compost; it can do more to maintain healthy soil and produce healthy plants than most people realize."