Don’t bring back the hornworms war. Pesticide that’s safe for home use.

Only those old enough to remember growing tomatoes before Bt (Bacillus thuringiensis) know what a nightmare we faced every year. The huge tomato hornworms inevitably show up in our summer crop to defoliate whole plants in one or two day’s time. They are so voracious that virtually nothing but chemical pesticides could slow them down, but they never stopped the invasions. In those days, getting perfect tomatoes was not easy as hornworms waged a perpetual war of attrition all summer long.

Then Bt changed all that. It proved the first organic solution to the age-old problem of caterpillar larvae on tomatoes and all sorts of other plant species. It’s a naturally occurring bacteria that is toxic to the stomach of any caterpillar to eat. The idea is we spray the living bacteria on the tomato plant leaves so it is present in high concentrations. Worms feed on foliage, ingest the bacteria and they expire in 24 hours. All of them expire because larvae live to eat before they pupate into moths.

Recently, there have been concerns about dangers of Bt. But these are not related to this use of the bacteria in gardens to control caterpillars. They are about Bt in GMO seed varieties that are a bioconglomeration integrating Bt into the plant itself for caterpillar resistance. Using the impact of Bt on caterpillar stomachs as a given, a great deal of propaganda aimed at anti-GMO issues is overflowing into our gardening world. Ignore it. Bt applied for caterpillar does not enter our plants, it sits on the leaves for just a day or two before it dies or is eaten. If you ate your unwashed tomato after spraying, it would be perfectly safe to eat, though washing is recommended of course.

This is an example of why it’s important to vet new information on such dangers adequately. Nobody wants to go back to the pre-Bt hornworm wars, yet the irresponsible conflation of GMO Bt and home-use Bt for caterpillars is often presented as equal threats to people and the environment. This inaccuracy puts one of the greatest discoveries of organic pest control in the cross hairs, driven by irrational fears of ill health, toxics and contamination of our gardens. The truth is, with Bt organically grown tomatoes are an easy crop, but without Bt, most of us would have no tomatoes at all.

As the summer heats up, large moths look for nightshades, their favorite larval food plant. They lay eggs on the undersides of the leaves where difficult to spot. They later hatch out into tiny green caterpillars tough to spot when they are small. The first sign is the sudden disappearance of flowers as they consume them first, cutting off any new fruit in a subtle way never noticed by novices. Then a week or two down the road, the real damage is readily apparently as caterpillars the size of your little finger gobble up leaves, fruit and buds overnight.

Spray Bt on your tomatoes in the early morning so the elusive green worms can feed all day and into the night. Spray the whole plant, top and bottom and undersides of leaves to reach the entire population. By the next morning they are suddenly visible hanging dead from their feeding sites, black as coal. Better yet, all the new hatchlings that become the second wave of damage are also feeding and die. This is why Bt was the first near 100 percent effective product that eliminated the use of highly toxic chemicals, and toxic botanical pesticides on tomatoes altogether.

Today, there is so much agricultural propaganda on the internet driven by fear that the poor organic home gardener must be overwhelmed. When global issues flow into our local world, the results may obscure truths about growing plants and cause confusion. What matters most is people are successful at growing food plants, so they remain connected to the natural world amid too much technology.


Maureen Gilmer is an author, horticulturist and landscape designer. Learn more at

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