Resistance to pesticides or antibiotics is inevitable if and only if the substance is used incorrectly. By incorrectly I mean either not long enough (sporadically) or too long (continuously). To avoid resistance it is paramount to use the product according to the directions which are based on the best research we have. Let me explain how this works. If you use a pesticide (or antibiotic, which is really a type of pesticide) for a short period of time, you kill only the most susceptible pests. Lets say that by using terramycin for only for days you kill the most susceptible 25% of the foulbrood bacteria. That leaves you with the most resistant 75% which will reproduce happily until your next treatment. The next time, four days of treatment will not kill as many bacteria because all the bacteria come from those more resistant 75%, so you may only kill 10% of the bacteria in the hive. Once again you've killed the most susceptible and left the most resistant to continue to grow. In a very short time you'll have a totally resistant strain of foulbrood. This is why it's so important to complete the full course of antibiotics when your doctor prescribes some for you. Now, if you use the pesticide continuously you'll also develop resistant pests. This is because no treatment is ever 100% effective. There will always be a few bacteria which are totally resistant to the treatment. If you leave the terramycin in the hive all the time you will kill off all the susceptible bacteria, and the totally resistant ones will reproduce freely. Now you've again got a resistant strain of foulbrood. It may at this point seem that resistance is inevitable if you've only got one method of treatment. This may be the case, but if you treat properly you can make it take a very long time to develop if it ever does. The key is to kill off enough of the disease that it doesn't easily bounce back, but not so much that all you have left are resistant organisms. You also rely on the host (in this case the bees) to be able to fight off the remaining disease on its own. In addition, you hope that there remains in the wild a reasonable quantity of susceptible disease, and that what remains in your treated host may mutate and become less resistant during the long periods without treatment. It's a tricky balancing act. So, the long and short of it is, if you use extender patties don't leave them in there any longer than you'd treat with the dust method. This info applies to other beekeeping treatments too. Don't leave apistan or menthol in the hive longer than it says on their packages. In fact, for diseases like varroa which you can actually see, it's a good idea not to treat at all until you actually see signs of a reasonable infestation. I find that I usually have no varroa in the spring, so I don't treat then (unless I find some). By the fall, I usually have a good crop of varroa in the hive, so I treat for the recommended time period at the recommended dosage and then remove the strips. The bees generally do fine until the following fall. With foulbrood you can't treat that way because once you have a real infestation the terramycin won't work. So you need to treat preventively, ideally once in the spring and once in the fall. Please don't treat more than this! I'll stop ranting now. -Alan Goldblatt