Flies on Astilbe

(Question)

I have a new pink Astilbe spirea this year and noticed small flies on the flower.  Is this normal or do I have to spray the bush?

I live in Ottawa, Ontario which is Zone 4 or 5.  My plant is in sun & shade.  I have added black earth with mushroom compost.

(Answer)

Astilbes, also called False goat’s beard or False spirea, are generally easy care plants in most gardens, but can be susceptible to several diseases and pests.  Diseases such as powdery mildew, bacterial leaf spot or Cercospora leaf spot are things to watch out for.  Pests such as tarnished plant bug, white fly, black vine weevil or Japanese beetle can also be a problem.

From what you describe, a white fly infestation is most likely what your new plant has contracted, either in your own garden or in the nursery greenhouse.  Generally, these insect pests are controlled in our gardens by natural enemies such as lacewings or lady beetles.  It may be that these natural biological control populations are diminished this year due to a severe winter, insecticide or pesticide use in your area, dusty environmental conditions or interference by ants.

There are a few things you can do to reduce and control the outbreak.  If you are a patient gardener, then hand removal of any larvae along with a good water spraying from a hose might do the trick.  You have to be sure to clean off all insects from the entire plant.

The alternative is use of an insecticidal soap.  Follow the directions on the package carefully when mixing up any solution.  In order for sprays to be effective, the solution must come in direct contact with insects, be used when plants are not drought stressed,  sprayed when temperatures are under 80 degrees F / 26.5 degrees C, and covers the whole plant thoroughly including flowers, upper & under sides of all leaves and stems.

Also, check any plants around your astilbe for signs of infestation spread and deal this these at the same time.