You’ve researched your seeds, tested your soil, weeded and planted watered and fussed. Your tomatoes are top-notch, and your peppers are popping — but what is this? A hornworm? An infestation of aphids? You’re not going to just hand over your bounty to a bunch of ravenous bugs. You put too much into your garden. Do you know what to reach for? Do you know the best insecticide for your vegetable garden? It might be the most valuable tool in your shed.
What is an Insecticide?
An insecticide is a chemical mix used to kill or repel certain insects. Insecticides can be formulated in a lab (synthetic), found naturally occurring in nature (organic), or combine natural and lab-made ingredients. Some insecticides kill a wide range of insects, while others target specific pests. Some are ‘contact’ toxins that can be sprayed directly on the pests and their homes. Others are ‘systemic’, which means that they are absorbed by the plant and then transferred to the insect when the insect feeds on the plant.
Most work by acting on the insect’s nervous system or by weakening the chitin that makes up their exoskeleton. Their use in gardens, however, is a double-edged hoe: what is toxic for insects is often toxic to other living things as well — including us. When your garden is geared towards growing fruits or vegetables, knowing what you are putting on your crops becomes essential.
A Brief History of Insecticides
The history of humans’ efforts to exterminate bothersome bugs goes back millenia. Earliest efforts stemmed from the use of common plants and minerals. The use of sulphur as an insecticide was common in ancient Greece. Chrysanthemums (pyrethrum) was being used 2000 years ago. Neem oil, which began use as an ingredient in traditional Indian medicine, was also adopted by farmers to fight insects in Asia over the last millenia. And American farmers have used tobacco (nicotine) to eliminate pests since at least the 17th Century.
Modern farmers’ interest in chemical insecticides was born with the adoption of DDT in the 1930’s. At the onset of WWII, the U.S. military used the insecticide to protect troops from malaria and other insect-borne tropical diseases. Farmers adopted it as an easy and effective way to eliminate insect populations.
Originally thought mostly harmless to mammals, DDT became widely available in post-war America. But as the years passed, the dangers of the chemical became clear. Beginning with insects sprayed with DDT, the presence of the chemical became magnified as it moved up the food chain. Apex predators began accumulating toxic levels. Birds of prey were hit especially hard as their diet consisted mainly of fish that ate DDT-sprayed insects. Fortunately, a ban on its use in 1972 has allowed species to begin recovering.
Another big discovery in insecticides happened in the same era as DDT. In the 1920’s a bacterium called Bacillus thuringiensis (Bt)was found to kill insects when sprayed on crops. In 1976, the mechanism by which Bt kills insects was finally understood, and its use (in the form of a protein extract) became widespread. Stung by the DDT debacle, various agencies scrutinized Bt. By 1991 scientific consensus was that Bt is harmless to mammals.
Today, Bt is widely used in vegetable gardens and commercial farms alike.
The 1960’s and 70’s saw the rise of ecology and environmental awareness. Rachel Carlson’s book, Silent Spring, brought the dangers of toxic pesticides into public awareness. The organic farming movement bloomed, and growers searched for natural insecticides. Age-old solutions, such as neem oil and pyrethrum returned to prominence as effective pest killers.
As recently as the 1990’s, another insecticide came to market with great promise only to be found unacceptably toxic. Agriculture’s new darlings, neonicotinoids, have been applied to over 140 crops for three decades. Yet it is now recognized to be lethal to honey bees and ladybugs. Furthermore, it remains in the soil for a long time. Neonicotinoids remain a common ingredient in insecticides, but their use is currently contested in court.
Today, the U.S. uses about 100,000 tons of insecticides a year. And — perhaps surprisingly, more is used by home and garden enthusiasts than by commercial agriculture.(EPA) What you choose to spray in your vegetable garden will affect not only your crop, but you and your environment as well.
The Collateral Toxicity of Insecticides
The history of farming is partly the history of learning how to poison pests without poisoning the crops and environment. Yet time and again, nature has shown that there is an unfortunate inverse relationship between an insecticide’s effectiveness and its toxicity to the environment. Agriculture has seen successes and failures. Insecticides have made it possible to feed millions, yet they have also damaged swaths of our ecosystem.
Doubtless our scientists are doing their best to make insecticides safer. Indeed, modern insecticides are safer and less toxic than ever. But when products are categorized by levels of toxicity (tables 7, 8), it makes sense to remain wary.
And when it takes decades to realize the consequences of using chemicals like DDT and neonicotinoids, gardeners can be forgiven for being wary of insecticides made in a lab. So when we garden at home, how do we protect our vegetables without unwittingly damaging our environment?
Environmentally Friendly Bug Spray
Plants have been evolving to protect themselves from insects for millions of years. It should be no surprise, then, that a number of plants have created their own chemical recipes to strike back. “Humans have probably known for a very long time that natural products such as nicotine from tobacco, turpentine from pines, pyrethrum from chrysanthemum species, and quinine from cinchona bark can provide protection from pests and parasites.” And farmers have been aware of the naturally occurring Bt strains in their soil for a century.
It is by turning to these natural extracts that a home gardener can protect their vegetables without creating collateral toxicity. Using chemicals that nature has already introduced into the ecosystem means no unexpected drawbacks or disruptions. This means that you have less-toxic options when choosing how to protect your vegetables.
Natural and Organic Insecticides
Rosemary, clove, garlic, vinegar, eucalyptus, and other common plants have been touted for their ability to repel pests naturally. But these at-home solutions have limited results, even under the best conditions — repellants only keep insects at bay for a time, not kill them. If you are serious about your vegetables, you need something more effective.
When researching the right insecticide for your vegetable garden, please remember that natural and organic does not necessarily mean safe. Even approved organic insecticides can later be found harmful. Rotenone, for instance, was reclassified after the EPA determined that it was reaching levels in water that were toxic to humans.
Thankfully there are some natural and organic sources for insecticides that aren’t hard on the environment. A handful of candidates stand out. Here is the short list of best insecticides for vegetable gardens.
Bacillus thuringiensis (Bt)
Over 80 strains of this bacterium supply the essential ingredient in this class of insecticide. The insects ingest the bacteria, where it prevents digestion and starves the pests. The right strain will specifically target the specific pest you’ve identified. However, locating and identifying harmful insects takes some experience. If you are uncertain which kind of insect is munching on your precious cukes, you may be better off with a wide-spectrum insecticide.
“Insecticidal soap is made from the salts of fatty acids.” It attacks the cuticle (outer layer) of insects. Bt is proven to be highly effective against a large number of soft-bodied pests, but keep in mind that those with exoskeletons can be resistant.
Neem oil comes from the seeds of the neem tree. It has half the toxicity of the next safest natural insecticide, and is 10 times safer than pyrethrum. Neem oil works by coating the insect, which disrupts its ability to exchange gasses (breathe). It also interferes with the maturation of juveniles. Some neem oil products contain an additional active ingredient, azadirachtin, which is considered an insect anti-feeding agent.
Horticultural oil has many different names, but they are all made of petroleum (paraffin or mineral oil.) It works in a similar manner to Neem oil, but may be more viscous and impurities in petroleum cause damage to tender or drought-stressed plants. You will also get less consistent results than you would with a single-source oil like neem.
A mixture of silica and the shells of microscopic creatures called diatoms, this insecticide tries to abrade its targets into non-existence. The sharp crystalline structures in diatomaceous earth scratches up insects’ outer layer, causing them to dehydrate. This type of insecticide is especially effective against slugs. Unfortunately, this insecticide does not discriminate, and will kill beneficial insects along with the pests. It is also a lung irritant.
Derived from chrysanthemums, pyrethrum-based insecticides are a quick and efficient killer. Unfortunately, it affects a wide spectrum of arthropods, including honeybees and others arthropods that are beneficial to vegetable plants.
There is no perfect insecticide. Different pests require different solutions. Consider your choices, weigh their pros and cons. Bt-based insecticides are your best option if you are a skilled entomologist, or are working on a large scale with a single crop.
For me, the choice is neem oil. This year I am sowing bush beans, peas, winter squash, and (for the first time) brussels sprouts. Watermelon and carrots will be growing nearby. And I expect a variety of insects attempting to taste my wares. I want a wide-spectrum insecticide, so I can focus on the parts of gardening that I find more rewarding.
Bonus: Neem oil has also shown to be an antifungal, especially against those that attack roses, fruit trees, and vegetables.
This is not to say that I am forswearing Bt insecticides, forever! If a specific pest invades in large numbers, it may be worth my time to target it. But until that swarm arrives, I’ll be spraying with neem oil.
Neem Oil Spray Recipe
If you want to study the effects of neem oil in your vegetable garden before buying a commercial mix, you can make a small batch at home. Here’s how to do it:
- Gather water, cold-pressed neem oil, and dish soap. You will also need a spray bottle.
- Add one or two drops of dish soap to a manageable amount of water (around 1 liter or 1 quart). As we all know, oil and water don’t mix. So we use a little soap to help the neem oil and the water base cling together.
- Add neem oil. Begin with a small amount — 0.5 to 2% of the water volume.
How To Apply Neem Oil For Insecticide
Now that you have your sprayer full, it’s time to apply it to your precious vegetables. It is a good idea to choose a ‘test’ plant and wait a day before spraying more broadly.
Although neem is non-toxic, you may experience skin irritation, so don your gloves, mask (not difficult to find these days!), and eye protectant. Give the leaves a thorough spray. Remember, insects like to hide on the undersides of foliage, so be sure to spray the bottoms of the leaves as well as the tops! If you have blossoms or vegetables, do not spray them unless you are certain they are the target of your pest.
Don’t be stingy when you spray! You want an even coating of oil everywhere an insect wants to chew. Simply spattering the leaves like a passing rain does little to inhibit pests.
Repeat the process every 2-3 weeks. Another advantage of neem oil is that it disrupts the growing cycle of newly hatched pests, so fewer will reach egg-laying maturity. With consistency, you should see smaller numbers in successive generations — and with a bit of luck you may not have to spray at all by the time your harvest comes.
We’ve needed insecticides since we began tending crops. There have been a few hard lessons along the way, and our environment has paid the price. Now we know that the best insecticides are not those invented in a lab. They come from a little bacterium we like to call Bt, and from the seed of a neem tree. This season, when it comes time to protect your vegetables from insects, remember how effective and non-toxic these natural ingredients are.