Ligustrum plants (a.k.a. 'privets') are exceptional pollinator plants. Treating these for "bugs" isn't advisable, unless a scale populations is excessive. I would need to see the specimen for that determination. Hopefully, you have not hired a company who, unnecessarily, applies chemicals to the environment.

First question is to find out what chemical was sprayed and at what concentration? ... Second question is "why" was the chemical used? Was there evidence of a disease or insect that warranted spraying? Diseases and insects are two different elements and require two different control methods. Also try and find-out who their next customer was and if they are also experiencing a decline in plant health. It is possible the spray tank was not cleaned-out prior to filling for your treatment. If there was a herbicide mixture in that tank ... your plants may have received a lethal dose. Once you find-out the chemical (s) used, contact your local Department of Agriculture or University Cooperative Extension office to discuss the application and further measures. Without leaves, your plants cannot photosynthesize sunlight into needed nutrients - the plant will decline. It may not be too late to provide some recovery tactics (i.e. flushing the root zone with water - depending on the chemical application). First, find-out the chemical that was used and go from there ... ~DOT