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Question

Up to how long prior to harvest can organic crops be sprayed with Bt?

Up to how long prior to harvest can organic crops be sprayed with Bt ?

Submitted by: peter777


Answer

Expert response from Rob Wallbridge

Farmer

Thursday, 05/02/2015 16:21

Pesticides labels generally specify a "pre-harvest interval" (PHI) which indicates how much time must pass between pesticide application and crop harvest in order to ensure that the crop doesn't have pesticide residues that exceed the safe limits. In the case of Bt (Bacillus thuringiensis) sprays for organic crops like cabbage, broccoli, cauliflower, kale and many others, there is no pre-harvest interval (more on that later!). Theoretically, this means that crops could be sprayed with Bt up until harvest. Practically speaking, this is unlikely. Here's why:

 

When Bt bacteria is sprayed on a crop, the crystal protein it creates must be consumed by the target insect larvae. Then, the insect's alkaline digestive system "activates" the protein, creating a molecule that is toxic to that particular insect. The larvae (in most cases a little green caterpillar that looks more like a worm) then stops feeding and dies 2 to 5 days later. To keep crops growing and looking their best, organic farmers want to prevent as much insect feeding damage as possible (and they certainly don't want little green "worms" showing up in the broccoli or cabbage you've purchased and brought home to eat!). Farmers walk through their crops regularly, assessing insect populations, crop damage levels, crop maturity, the presence of natural predators, the weather, and other factors in order to determine when and if they need to spray. Because Bt takes time to work and because the damage would likely already have been done, spraying right before harvest would make little sense.

 

But why is there no pre-harvest interval on the label? It's due to the unique nature of Bt's crystal protein. Unlike insects' alkaline digestive systems, humans and other warm-blooded animals have an acidic digestive system. This means that the Bt protein is never "activated" into its toxic form. Even if it was, different strains of Bt are specific to different "receptor sites" in different types of insect, and humans don't have any of these receptors. Studies have shown that Bt ingested by humans and animals just passes through the digestive system, with no ill effect. Because of this, both the United States and Canada exempt Bt from residue limits on food. You can learn more about the form of Bt used in organic farming here.