It doesn't sound like there would be much chance of the chemical to get into the shrubs if the imidacloprid is applied close to the base of the tree. Also, another thing to consider is unless you're eating solely certified organic food, there's a really good chance you've consumed very small amounts of imidacloprid at some time.
The United States Department of Agriculture (USDA) Pesticide Data Program monitored imidacloprid residues in food and published their findings in 2006. Imidacloprid was detected in a range of fresh and processed fruits and vegetables. It was detected in over 80% of all bananas tested, 76% of cauliflower, and 72% of spinach samples. In all cases, however, the levels detected were below the U.S. EPA’s tolerance levels. Imidacloprid was also found in 17.5% of applesauce and 0.9% raisin samples, although percentage of detections were greater in the fresh unprocessed fruit (26.6% of apples sampled, and 18.1% of grapes sampled).
These findings were on direct application to the plant, which would indicate higher concentrations than that which you would find on plants nearby those which were treated. Additionally, it is advised that if homeowners want to use a product with imidacloprid as the active ingredient that they use one specifically labeled for edible food crops. The product label will contain specific instructions on application, pre-harvest intervals, and special instructions on limiting harm to pollinators.
Purdue Pesticide Programs
Office of Indiana State Chemist
Lindsey Purcell, Urban Forestry Specialist
Department of Forestry and Natural Resources, Purdue University