My good friend Dave Robson who is a horticulturist in Illinois reminds me that while poinsettias are great for holiday gift giving (for your friends/family/neighbors and yourself!), sometimes pests tag along on the plants.
One of the most common tag-alongs are whiteflies. These insects commonly go unnoticed, usually because they are small and feed on the undersides of the leaves. Then, after a week or 10 days of feeding, their host plant starts to appear lackluster and weak. These symptoms develop after a week or two of the immature nymphal stage of the insect sucking plant juices from the leaves. As feeding continues, many of the green and red/pink/white colored leaves turn yellow and fall off. It’s common for a sticky substance called honeydew to develop on the leaves that remain, which is common with pests such as aphids. Actually, whiteflies aren’t flies at all and are more closely related to aphids, which explains this behavior.
The first step in controlling them is to see if you’ve got them, aka, a thorough inspection. Once you turn over a few leaves, and focus your attention a bit, they’re relatively easy to spot, even though they’re only about a tenth of an inch in size. If you rummage around in your desk drawer and find a magnifying glass, you can use it to observe the white color, (they’re not called whiteflies for nothing!) and 2 flat wings in a triangular shape. You’re likely to see a few fly away as well. Keep looking and you’ll probably see small roundish to oval beige eggs, which will turn into adults in due time.
If the undersides of the leaves are covered with whiteflies, it may be wise to simply throw the the plant away rather than trying to control them. Trying to control heavy infestations is akin to pushing an uneven rock up a steep hill. After all, once you know what they look like, you can do the inspection at the garden center and choose healthy, non-infested plants.
If, on the other hand, there are only a few, control can be achieved by donning nitrile gloves, taking the plant outside or into an attached garage, turning it upside down and spraying an insecticidal soap product on the undersides of the leaves. Garden centers and hardware stores alike sell ready-to-use trigger sprayer formulations that require no mixing and can be stored easily. Be sure to read all product instructions and keep them out of the reach of children. Products such as permethrin and deltamethrin can also be used, but may result in a bit of damage to the leaves.
After the spray application, set the plant down on the garage floor or lawn for a few minutes to allow the excess product to drip off and then bring the plant back inside to enjoy. Whiteflies are tenacious insects and thus it may be necessary to make several applications at weekly intervals for complete control.
[Thanks to Dr. Jody Green for her review of this post.}