Home asparagus plants do not need to be treated with insecticides as a precaution. If you discover an insect infestation on your asparagus plants, you can try hand picking the pests if your patch is small. If more than 10% of your plants are infested with insects you could try an insecticide with one of these active ingredients: Pyrethrins, Neem Oil, Spinosad, Malathion, Carbaryl, or Pyrethroids. The following article from the Michigan State Extension Service can help to identify asparagus diseases and insect pests and provide management information. The article can be found here: Disease and Insect Pests of Asparagus - MSU Extension

A pre-emergent product like Preen can be used on asparagus before the spears emerge from the ground. Once the weeds appear the product will not work. A better alternative would be to apply mulch of straw or chopped up leaves. Grass clippings can also be used unless you have applied a weed killer to your grass.

Thank you for your question and have a great gardening summer.