…with some words about systemics
The number of different pestiicides on a garden center’s shelves can be overwhelming so here’s a very basic guide to making a choice.
First, of course, is to answer the question: “Is the damage severe enough that you really need to use a pesticide.” Perfect really isn’t natural, and it comes at a cost. If “yes”, be very sure that you’ve identified the culprit, the type of insect or disease that is damaging your plants, so you know which shelves you should search. Insecticides don’t do much to cure a fungal or nutritional problem. And an herbicide would eliminate the problem by killing the plant. The large print on the label gives you this information.
Now, put your glasses on. Just like a contract, you’ll need to read the fine print. Find a product that lists your problem on the label. Then try to determine how the material will be applied. Do you need a sprayer? This information may be on a part of the instructions that is folded or wrapped so that you can’t get to it without breaking some type of seal. In that case, ask! You want to be able to apply it when you get home.
Unless you have used the product before buy the smallest amount possible. See if it works. You don’t want too much as it’s best not to store a pesticide for any length of time.
Ready to use (RTU) versus the concentrated mix-it-yourself: With RTU you’re paying for convenience in not having to do the measuring and mixing, and in many cases the sprayer is provided.
Pesticides are also classified by mode of action. Some must contact the bug or fungus directly. Others need to be ingested by the insect. Or be repulsive. Systemics though are absorbed by the plant and actually get into the circulatory system of the plant. Glyphosate (as in Roundup) is a systemic weed killer. You need to use special care with such formulations.
Another very important example: Imidacloprid is a systemic insecticide that has been quite successful in controlling the Hemlock Wooly Adelgid as well as many other insects that chew on leaves and stems ornamental plants. Unfortunately that includes those that use 
pollen from buds and flowers including, most importantly our friends, the bees and other insects who carry that pollen home and feed it in some form to their young ones. Our farms and gardens will be in sorry shape without the pollinators. In fact we’re already approaching the critical point with those populations.
The North Carolina Agricultural Chemical Manual in a table labeled Relative Toxicity of Pesticides to Honey Bees lists imidacloprid along with other systemics under the heading of Group 1 Highly Toxic, warning that “Severe bee losses may be expected if these pesticides are used when bees are present and foraging in the flowers, or the product is applied near beehives.” The same warning is in the label.
Bottom Line: Read the label before you buy and again before you apply. And follow it when using any pesticide!
Article written by Glenn Palmer, Extension Master Gardener Volunteer.
For more information:
Pesticide Environmental Stewardship Website
http://pesticidestewardship.org/Pages/default.aspx
Pesticide Use and Safety
http://content.ces.ncsu.edu/pesticide-use-and-safety-information.pdf
Disease & Insect Management in the Home Orchard
http://www.cals.ncsu.edu/plantpath/extension/clinic/fact_sheets/index.php?do=disease&id=7