I would like to do the routine pest management on my 2 mature flowering crabapple trees (previously paid a service to do this). When in the Spring do I start? What is the best chemical to use? How frequent are applications done? I live in the Chicagoland area (northern Illinois).
I am new to this site and in browsing questions, noticed that yours had no reply. Hopefully, you have found the needed information elsewhere by now, but in case not: I have always done my own maintenance on my Crab Apple, although it is a small weeping ornamental Crab so a two gallon 'home sprayer' is adequate. Here, where I live (Kamloops.B.C. Canada) the first spray ( horticultural oil and sulphur )was done several days ago (must be done before the buds open and on a windless, dry day when no danger of frost is on the horizon for two or more days). All parts of the tree, including the trunk should receive thorough coverage. This oil and sulphur comes packaged together and can be purchased about anywhere. I use this mixture on my rose bushes as well (before leaf out). In the coming months keep your eye open for any sign of catepillars (leaf rollers) - it's important to detect this problem when minimal and the insects are young. I try to minimize the environmental impact of pest control, so I use a product called BTK (kills only catepillars, but won't harm other bugs or animals) - Spray as directed on the bottle and spray again in 10 days to two weeks. This may need to be done more than twice.
Try Illinois Cooperative Extension for guidance. Specific pest and disease problems need to be identified and then given well targeted treatments, same as with medical problems.
