Question
I have a newly planted (18 months) redbud in Zone 5B (Chicago, IL) that seems to have a heavy whitefly infestation. The leaves on fully half the shrub are yellowed and adult whiteflies are visible on the trunk.I have read that insecticides are not useful and that they are useful. What do you think? Aside from being unattractive, is the health of the shrub in danger?

Answer
Whiteflies damage plants by sucking out plant juices. Because large amounts of sap can be removed, primarily by the developing nymphs, heavily infested plants can be seriously weakened and grow poorly. Leaves often turn yellow, appear dry and drop prematurely.

Also, because whiteflies suck out more plant juice than they can digest, the excess is excreted as a sweet, sticky substance called honeydew. The honeydew covers leaf surfaces and acts as a growth medium for a black, sooty mold. Both the removal of plant juices and the presence of the black, sooty mold growing on the honeydew can interfere with photosynthesis.


Insecticides will work but they will need to be sprayed at least three times about a week apart. Make sure both sides of the leaves are covered with the spray. Recommended insecticides are:
acephate--Ortho Isotox Insect Killer, or Ortho Orthene Systemic Insect Control

malathion--Ortho Malathion 50 Plus Insect Spray, Dragon 50 percent Malathion Spray, K-Gro Malathion 50 Insect Spray.

permethrin--Spectracide Lawn and Garden Insect Control.

bifenthrin--Talstar 10 WP

imidacloprid--Merit 75 WSP.

These are contact insecticides and will kill the adults present.

Since these insects are sucking insects the systemic insecticides will get into the plant juices and as the whiteflies suck the juices they will get the insecticide.  


Some insecticides can be applied to the soil and taken up by the roots of the plants. These are called systemic insecticides. The most recent, Imidacloprid, is sold under the trade name Bayer Advanced Garden Tree & Shrub Killer Concentrate. (Merit is the trade name of imidacloprid used by professional tree care companies.) It is applied as a drench over the root zone. An older梐nd much more toxic梥oil systemic insecticide that is still available for some ornamental plant uses is DiSyston (disulfoton). DiSyston is sold as granules or in plant food mixtures for soil application.