I’m a very reluctant user of pesticide sprays and powders. I try to avoid using them at all and even when I feel I have no other choice, I always feel a bit guilty. And that’s true even when I apply an organic one. Organic or not, all pesticides do some environmental damage, disrupting the natural course of things.
That’s why I’ve never jumped on the wagon of using pesticides to prevent diseases or pests. The “apply weekly to prevent problem X” recommendation I hear so often just doesn’t appeal to me, even when the product is a fairly harmless homemade treatment. I have no qualms with exclusion techniques, like covering a plant with floating row cover to keep its enemies at bay, but if you have to spray a pesticide again and again, I say: think twice! There is always collateral damage!
Look Before You Spray
Rather than applying a pesticide preventively, I suggest waiting to be certain you really do have a problem (just because you had an infestation last year doesn’t mean you will this year) and then dealing with it by using the least environmentally damaging product possible, like insecticidal soap (an insecticide that also has fungicidal effects, yet does not persist in the environment, so has few negative side effects).
Especially try to apply any pesticides you feel you do need when beneficial insects are absent. You can help protect pollinating insects, for example, by spraying when the plant is not in bloom or, if it is flowering, early in the morning before pollinating insects are present.
That way, the impact on the environment will likely be minimal and above all will be very localized.
If You’re Going to Spray …
If you’re going to be spraying a plant with a pesticide, organic or not, at least do it right. A diluted pesticide or spotty applications will only result in the pest coming to adapt to the treatment, eventually creating pesticide-resistant pests. Always read the label of the product and apply it at the full recommended dose and under the recommended conditions.
And if the problem comes back every year, learn to be a true laidback gardener and stop growing the plant that’s causing you to feel the need you have to treat it. Why grow problem plants when there are hundreds of others that will do pretty well with no treatments under your conditions?
Disease- and Pest-Resistant Plants
Note that there are now disease-resistant varieties (and to a certain but lesser extent, predator-resistant varieties) of almost all popularly grown plants: scab-resistant apple trees, white-resistant phlox, slug-resistant hostas, roses resistant to black spot, tomatoes resistant to just about everything, etc. Learn to use them instead of disrupting the environment with endless applications of pesticide!