The Truth about Organic Pesticides

[Second of two Takoma Voice columns revealing what Jeff Gillman has to say in his The Truth about Organic Gardening, with some commentary.]

One look at the product shelves of local nurseries or even at Home Depot will tell you that green is in.  We’re in the midst of a media blitz in service of "green" garden chemicals – and that’s a good thing.  I think.  At least they’ve noticed that we care, but is "green" always good?  Caveat emptor, indeed, and the more we know about these products, the better.  So last month I covered the weed-and-feed advice in Jeff Gillman’s much-talked-about new book, The Truth about Organic Gardening, because it’s environmental information that’s science-based, not slave to any marketing labels.  This month let’s tackle the products with the potential to do the most harm – pesticides.

Preventing insect problems

First, I like Gillman’s philosophy and follow this myself: "One of my favorite ways to deal with pests is by ignoring them and concentrating on getting plants to grow as vigorously as possible."  And I’ll add: choosing the right ones.  Organics aren’t very effective at killing insects, anyway, so it’s best to focus on prevention.  He assures us that most plants can deal with the loss of about a third of their leaf area to insect damage, no problem.  And hey, don’t we want to provide for wildlife anyway?  Nature-loving gardeners these days understand how the plants they grow can contribute to wildlife, especially those songbirds we all love so much.  So more and more of us don’t mind feeding the insects that will feed the birds, as long as our plants can survive the meal.

Killing insects

There are some seriously destructive critters out there, like Japanese beetles, and if they’re in such numbers as to threaten the survival of a plant or make it look butt-ugly (to you), the first tactic is to pick them by hand and bag them or dunk them in soapy water.  Or try a good hard spray with the hose to wash them off.  Traps for Japanese beetles don’t stand up well in tests, though; they end up attracting even more insects, resulting in more damage than if they weren’t used at all. 

Beneficial insects, like ladybugs, eat the insects that damage our plants and you can buy 1,500 of them for about $20 online.  The bad news is that they usually disperse as soon as they’re released.  So the best way to have beneficial insects in your garden is to grow plants that attract them, and avoid the use of broad-spectrum insecticides, to which they’re very vulnerable.  And if you do want to try introducing some, Gillman calls the lacewing a "little-known and underutilized predator" that’s a good choice for controlling aphids, mites, and other small soft-bodied insects.  Unlike ladybugs, they’ll stay in your garden and eat the destructive insects there.  Nematodes,  little worms that eat insects from the inside out, are effective at killing Japanese beetles and can last in the soil for years, providing long-term control.  Sometimes people with Japanese-beetle-plagued plants try to kill them with a bacterial disease called milky spore, but Gillman says it fails more often than it works, with beetles developing resistance to them over time.  They’re most effective when applied to a larger area than just one yard. 

Insecticidal soaps like the brand Safer do kill small, soft-bodied insects, such as aphids, although results can vary.  They can also hurt beneficial insects and aquatic creatures and if the concentration is too great, they can damage the foliage of your plants.  Commercial mixes are safer than home concoctions, which should always be tested on leaves first.  At least soaps are safe for humans. Spinosad is a promising new fungal product that’s effective in killing flies, thrips and caterpillars.  It’s safe for humans and safer for beneficial insects than soap.  Bt (bacillus thuringiensis) is Gillman’s "favorite pesticide of all time."  It’s brewed from a soil-dwelling bacterium and contains a protein that acts as a stomach poison when eaten by beetles, caterpillars, mosquitoes or flies, depending on which type you choose.  Still, when it’s used repeatedly it can be toxic to some beneficial insects, probably due to the soap and oil it’s mixed with.  Neem, which comes from a tropical tree, kills aphids, many caterpillars and beetles by inhibiting their growth hormones so that they don’t develop normally, and it’s also an excellent fungicide against powdery mildew and leaf spot.  Unfortunately, there are human health concerns with its use, and it’s toxic to many aquatic animals.  Pyrethrum, which comes from mums,  is the most commonly used organic insecticide, especially in wasp sprays, and it’s so effective it knocks them right out of the sky.  It also works on aphids, whiteflies and scale, and it breaks down rapidly.  Unfortunately, it’s often sold in compounds that include synthetic, even carcinogenic products, so should be used with the same respect afforded to any potent poison. Rotenone may be organic but it’s Gilman’s least favorite pesticide.  It’s been found to cause Parkinson’s disease-like symptoms and also kills aquatic life.  "Green" or organic companies that rail against the herbicide Roundup because it kills frogs yet sell Rotenone are accused by Gillman of plain old hypocrisy.  His bottom line is: "Why would any sane person use this pesticide?"

Bottom line

Pesticides are never completely safe, whether they’re organic or not.  Consider the case of nicotine, a popular folk remedy that indeed kills insects – the good and the bad, as well as beneficial microorganisms, and has been banned for all uses by the EPA.  Sure, organics as a group are less toxic than synthetics as a group, but the best choice is to not use insecticides at all.  Your defense should start with prevention and continue, if necessary, with the more benign controls, like mechanical ones and beneficial insects.  Then if you do use a product, remember that the word "organic" affords you and the environment no protection at all.

Disease control

When it comes to leaf diseases, most organic controls are ineffective.  Copper compounds are one exception, but they can be quite dangerous to humans and the environment. The one product that Gillman does recommend is sulfur, the oldest pesticide still in common use in Western culture.  It’s relatively safe and just as effective as most synthetic fungicides.  However, it needs to be reapplied frequently and under just the right conditions or it’ll damage plants.  A synthetic fungicide that’s also "relatively safe" and very effective against black spot and powdery mildew is acibenzolar-S-methyl.  It’s a plant activator and chemically very close to aspirin.  That said, Gilman still recommends against using any fungicide because plants can develop resistance to them, so that you could control the disease you’re going after but end up getting another one.

Bottom line

Prevention is even more essential in combating disease than in combating insects, and fortunately even more effective.  It starts with giving any disease-susceptible plant excellent air circulation.  But here I’ll add this snarky question:  Do your garden really need garden phlox, monarda or hybrid tea roses, all plants that suffer from fungal diseases?  There are thousands of plants to choose from, so why not select plants that are trouble-free?  And if you already have a plant that’s routinely afflicted, I usually suggest just getting rid of it.  But if and when disease does appear, it’s important to get rid of the infested plant parts and keep your pruning tools sterile.  If the problem persists and you want to buy a product, take a sample to a good independent nursery and have it diagnosed first so you know you’re at least buying the right one. 

Beyond organic – toward sustainable

While principles of organic gardening dictate the avoidance of synthetic pesticides, sustainable gardening is all about creating a self-sustaining garden where no or very few inputs are required.  Now that’s easier said than done – certainly the labor of the gardener is required just to keep our yards from reverting to forest.  But when it comes to pesticides, the sustainable gardener aims to avoid them all.  Recently Gillman’s book was discussed on the blog GardenRant and comments like these were common: "I refuse to use any chemical garbage on my garden, green or not."  And I agree completely with this one, from the Horticulture Department at Cornell: "I’ve never been a huge fan of the ‘input substitution’ approach to organics — trading one bag or bottle for a greener product while not changing the practices that led to the problems the bags or bottles are meant to solve.  Grow a diversity of plants.  Keep them (and the soil supporting them) healthy."

I couldn’t have said it better myself.  I’ve only resorted to products a couple of times over my decades of gardening and only to save a plant that’s large or essential – like an entire full-grown hedge.  Even to save the hedge I found spraying so unpleasant (even with insecticidal soap) that I tried pruning them for better air circulation and voila – the problem went away.  Which goes to prove the wisdom that healthy gardening practices are the best way to avoid toxins, organic or otherwise.

Photo courtesy of United States Postal Service.