When applied correctly, most synthetic pesticides have come a long way, in terms of toxicity to humans and environmental damage, compared to their predecessors. However, many gardeners would prefer not to use synthetic pesticides in their garden at all. In addition to the possible health risks to humans, pesticides can also kill insect predators and insect pollinators (including bees) — insects that help a healthy garden. There are a number of alternate means to fight insect garden pests. So, doing away with pesticides doesn’t mean surrendering the garden to the munching insect hordes.
As a college student, I majored in chemistry and biology. As a gardener, I rarely use any pesticide — organic or synthetic — in my garden. This is because I like to use biology, not chemistry, to solve my garden pest problems. And, there are a variety of ways you can keep your garden completely pesticide free.
Before I get to biological controls, there are numerous methods to keep insects at bay.
Agricultural netting places a physical barrier between your plants and insects. Once the plants have sprouted, the gardener installs a series of hoops — they look like large croquet hoops or wickets — straddling the row. Then, ag netting is draped over the hoops and affixed to them. Done properly, it is 100 percent effective in keeping outside insects away. It cannot, however, guard against pests that live in the soil. (In other words, this won’t work against squash vine borers if you plant squash in the same patch as last year.) Ag netting allows almost all of the sunlight through, but is woven tightly enough that even the smallest insects cannot get through. It works wonderfully, but can get damaged in high winds or strong downpours.
Geology can also help you defeat insect pests. Diatomaceous earth, sometimes called D.E., is sedimentary rock formed by the compression of fossilized diatoms. Diatoms are a type of algae with a hard shell. D.E. has a wide variety of uses, including as a liquid absorber, a filtration agent for beer, an ingredient in cat litter, and a stabilizing component of dynamite. It also kills insects. When insects crawl across D.E., it absorbs lipids from their exoskeletons and the insects dehydrate and die. Diatomaceous earth is not toxic, but it is abrasive, and contact with the dust will irritate your eyes and lungs. As such, wear a mask and goggles when applying D.E.
Insect repelling plants
There are a variety of plants that repel certain insects. Encircling your vegetable garden with these plants will help deal with pests. The list of plants suspected of repelling certain insects is long, but here are some of the better-known examples.
Many herbs will repel specific insects. These include basil (asparagus beetles and tomato hornworms), borage (tomato hornworms and cabbage worms), chamomile (flying insects), coriander (Colorado potato beetles), dill (squash bugs and cabbage loopers), fennel (slugs and snails), oregano (broad spectrum), parsley (asparagus beetles), peppermint (cabbage loopers and squash bugs), rosemary (cabbage loopers), spearmint (beetles, ants, and rodents), and summer savory (bean weevils). Catnip will repel ants, aphids, and the Japanese beetle with the tradeoff of attracting cats.
Almost any strongly scented herb will have some ability to repel certain insects. In fact, if a plant smells strongly, it is likely a form of chemical defense against pests (although it could be a chemical attractant to pollinators). Herbs make great companion plants in a vegetable garden and can be used in the kitchen.
Many ornamental flowers likewise repel certain insects. Plants in the genus Artemisia repel a variety of common garden pests including cabbage looper, cabbage maggot, and the cabbage butterfly. Other useful plants include chrysanthemums (Japanese beetles), cosmos (corn earworms), dahlias (nematodes), geraniums (leafhoppers), larkspurs (aphids), lavender (moths), marigolds (nematodes), nasturtiums (squash bugs), petunia (tomato hornworms,) and tobacco (flea beetles, with the tradeoff of attracting hornworms). Planting flowers around your garden will not only add a dash of color, but will attract pollinators as well as keep some insect pests at bay.
Plants related to onions — onions, scallions, leeks, garlic, shallots, and chives — are aromatic and also ward off a wide variety of pests.
You can make a homemade pesticide solution from almost any insect repellant plant. Popular home concoctions include extracts of garlic, onion, chili peppers, tomato leaves, tobacco leaves, chrysanthemum leaves, citrus oil, and others (including almost any aromatic herb). These can be made by chopping the appropriate part of the plant into small pieces and boiling them for 20 to 30 minutes. The mixture should then be strained through cheesecloth to remove the solids. Often, combinations of these will be used, including onion and garlic, garlic and pepper, or citrus and pepper. Likewise, dish soap may be added to the mix — at a rate of around 3 tablespoons per gallon — to increase its efficacy. Or it may be mixed in with diatomaceous earth.
If concentrated enough, these homemade pesticides can be moderately effective at repelling many insects. Your neighbors may stop visiting your garden, though.
The flip side of plantings aimed at driving away deleterious insects is a planting aimed at attracting beneficial insects. If you give your local insect predators a habitat near your garden, they will help knock down the levels of garden pests. Around my house, I have a variety of spider webs, spider egg sacs, and wasp nests that I do not disturb. Likewise, my garden adjoins an empty lot where native plants grow wild. As such, I see robber flies, praying mantises, ladybugs, and other predators throughout the gardening season.
If you do not have a lot of native plants nearby, planting a bed of native perennial plants near, or in, your garden is good idea. Plant perennials that grow fairly tall. Tall plants give flying predators a perch to look out from. They also give spiders a place to weave a web. If you live in the Northern Hemisphere, plant them on the north side of your garden so they don’t shade your crops. You can pick native perennials that additionally attract pollinators or butterflies or hummingbirds as an added bonus.
Perennials won’t need to be reseeded each year, and the insect predators that live on them won’t have to be “reseeded” either.
You would be surprised at how effective simply picking insect pests off your plants can be. If you suddenly have a large infestation of any medium sized or larger insect, directly removing them may solve the problem. To do this, get in your garden right before sunrise, during the coolest part of the day. Insects are more sluggish in cool weather. Bring a large cup filled with water and a few drops of dishwashing liquid. Pick the insects off the plants and drop them in the water. With the dishwashing liquid, they won’t be able to escape. For larger plants that are heavily infested, spread newspaper under the plant and shake it. Then, roll the paper and funnel the insects that fell off into the soapy water. Most insects hide on the underside of leaves overnight. To find them, look for leaf damage or little spots of frass (insect droppings). In some cases, throwing a bag over a severely infested plant and pulling it may be the best way to save the plants around it.
If you repeat your insect picking for three or four mornings, and are diligent, you can knock an infestation down to a point where your insect predators will clean up the rest. As garden tasks go, picking insects is probably the least glamorous, but I’ve cleared entire pumpkin patches of caterpillars in a few days by simply spending 20 to 30 minutes in the garden each morning.
Crop rotation has a variety of benefits, including a reduction in insect pests. (Search “crop rotation” on Grit.com for more on this topic.) If you vary what you plant each gardening season, avoiding planting crops in the same biological family, fewer pests will be waiting for them.
Of the things I’ve mentioned in the “Alternate approaches” section, one alone won’t eliminate your insect pest problem. However, using them all in concert can yield a garden where pesticide use is rarely needed. Crop rotation and a stand of perennial native plants near your garden alone set up a resident predator versus intermittently arriving prey balance that will keep pest numbers down. Companion planting some herbs or flowers to ward off specific insects may further help. When the inevitable infestation comes, a little physical picking combined with an application of “stink” (homemade garlic and pepper extract, or whatever your favorite brew is) can go a long way.
Then, if the insects still have the upper hand, you can choose a pesticide that will do the job with the least damage to you and your insect predator and pollinator friends. Even so, it pays to consider that pesticide use is a quick solution that can have long-term effects on your garden predators. With this in mind, I accept a small amount of insect damage each year in my garden as the price I pay for maintaining my garden spiders, wasps, robber flies, mantids, and other insect predators, as well as keeping my pollinators happy. I’m not going to let the whole garden get gobbled up, but I’m not going to freak out if a few leaves get chewed, either.
Chris Colby is an avid gardener who lives in Bastrop, Texas. His academic background is in biology — a Ph.D. from Boston University — but his main interest is in brewing beer.