Adventures in Gardening: Biological Pesticides

by David Soper

When faced with the challenge of insect infestation in your garden, think first about the biological pesticides. As a general rule, they will NOT kill the good guys working in your garden.

The first is Bacillus thuringiensis, commonly called Bt. It is a bacteria specific to leaf- and flower-feeding caterpillars. To be effective, Bt must be eaten by the targeted pest. Once it is consumed, it works by abrading the lining of the stomach so the caterpillar starves to death and usually within three days. As soon as the eat the Bt, they stop eating the plant. Two problems with Bt. . .it only works if eaten and only works on caterpillars (including butterflies).

Another, Sabadilla comes from the seeds of Schoenocaulon officinale, the Sabilla lily. It is sold under the tradenames Red Devil(tm) and Natural Guard(tm). This is a contact and stomach poison that is effective against caterpillars, thrips, leaf hoppers, stink bugs and squash bugs. It is approved for use by organic vegetable growers. It leaves no residue. It does break down quickly in the light and it is highly irritating to the eyes so exercise caution with this one.

By far the most popular botanical insecticides are those that are pyrethrum-based. Pyrethrum's source is the chrysanthemum, Dendrathemum cinerariaefolium. This is what is know as a "fast knockdown" product. Most insects are highly susceptible to it in low concentrations. Flying insects drop immediately on exposure. Pyrethrums can be used as flushing agents because of the very high irritability to insects. They flee as soon as they detect its presence. You will often find it combined with other natural synergists to increase its killing power.

This product is a contact poison affecting the central nervous system. When using this product, you should protect your skin since it can be quite irritating. Pyrethrum is pretty expensive so labs have developed a synthetic and cheaper version called Pyrethroids. This man-made version is more persistent and will affect a broader range of insects. If you are growing organic vegetables for sale, you can't use this, but agriculture is a big user.

My personal favorite is Neem. Neem is manufactured or, more accurately, derived from India's Neem tree seeds. It has been used effectively for more than 4,000 years. It not only works against insects like gypsy moths, leafminers, thrips, mites, loopers, caterpillars and mealy bugs but is also effective against certain fungi. Neem works on insects by interfering with the bug's ability to grow and molt which, in turn, will interfere with egg laying. I've used in with roses and on my buddlea with good effect.

You'll see lots of insecticidal soaps for sale and they have been around for more than two centuries. Unfortunately, they only work on soft-bodied insects and must be directly applied. You need to completely cover the insect, too. Oddly, no one seems to know exactly why they work. The best theory is they remove protective waxes on the insects thus causing death thru loss of water.

Lastly, some gardeners have had good luck with traps. These usually have bait attractive to a particular type of insect like yellow jackets. Most of the baits are based on chemicals used by females to attract males. When the guys come sniffing around, they get stuck. Life is tough in the insect world.

There are lots of ways to control insects in your garden and many don't involve heavy duty chemicals. Good luck and great gardening.

David Soper, The Garden Guy, writes and lectures on gardening topics. Check out his books at

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