Whitefly are an infamous greenhouse pest. One of the best controls for whitefly is Dicyphus hesperus (referred to as dicyphus from here on), a true bug that will feed on a wide variety of greenhouse pests.
Dicyphus works best as a preventative from whitefly (and other insects) establishing, so hasn’t been used traditionally (as it is expensive to keep re-introducing preventively). However, if you establish it on a mullen banker plant (Verbascum thapsus) it becomes an inexpensive, effective preventative system.
If Dicyphus eat insects, why do you need a mullen plant?
Dicyphus eat a wide variety of invertebrates including spider mites, russet mites, predatory mites, thrips, aphids, and moth eggs in addition to greenhouse whitefly (Trialeurodes vaporariorum), amd silverleaf whitefly (Bemisia tabaci).
They drink, however, from plants (with a preference for mullen and eggplant). They can drink from other plants too, but, in my experience, strongly prefer mullen/eggplant. Note that they don’t damage the plants that they drink from unless there are hundreds of them per plant.
For a healthy, breeding population of dicyphus, you will need to supply both food and drinking sources which usually means supplying mullen plants.
For a long time I had a very healthy population of dicyphus in the greenhouse. It would be difficult to find an eggplant leaf without one. I’ve seen a real decline in the last few weeks, so I am not sure if they have found new homes, or if they just had too little to eat in the eggplant zone. I have been surprised by the scouting in the zone where I have the best dicyphus population. All of the insect counts there are very tiny (one card has not had an insect on it in 3 months which is astonishing).
This may also be related to no spider mite populations. A few months ago, we had spots of spider mites and we could find them fairly easily because every spider mite colony would have a dicyphus or two at it. They apparently will not give full control of a mite situation, but I have witnessed them actively hunting spider mites many times, so I assume that they provide at least some control of that pest as well.
We originally tried to build up the dicyphus population to keep whitefly from establishing in our greenhouse. We ended up testing this when we brought in a small amount of plant material that turned out to have whitefly on it. We were worried about the introduction of this pest (which we had never had up to that point) into the greenhouse. Luckily the whitefly did not establish, and we have never seen one since that week over a year ago. The dicyphus population, however, went through tremendous growth and they appear to be good at eating a wide range of pests.
If you grow in a greenhouse (or are not squeamish about insects in your house) I strongly recommend trying to grow a few of these bugs to provide a healthy preventative program in your indoor garden.