No one knows how many whitefly species there are, but most of the ones we are concerned about seem to arrive to North America via Florida, and the bad ones stay.
If you’re not a total bug-geek like me and you have chosen to read this article it is likely because you are familiar with whitefly being a pest in your crop or garden and want to know how to get rid of them. However, if you are a bug-geek looking for lots of information on different and interesting whiteflies I can only direct your attention to the University of Florida pest publications at mrec.ifas.ufl.edu/lso There you can contact some of the greatest whitefly researchers in the world.
This article will speak specifically about the most common whiteflies and how to control them without chemicals. (The link above leads to chemical treatments for whitefly). So, get ready to learn about Greenhouse Whitefly and Bemisia (silver leaf or sweet potato whitefly).
Whiteflies are “phloem-feeders,” meaning they tap into the sap of the plant with piercing mouth pieces. While being “plugged-into” the plant they excrete honeydew – much the same as aphids. They honeydew droplets are tiny and spray from the whiteflies allowing the honeydew to spread easily. Most of the honeydew (with low air velocity) will eventually settle onto upward-facing surfaces. It is the honeydew that will often damage or kill your plants the quickest. They honeydew is a perfect habitat for molds and bacteria that can infect the plant or simply exclude the light.
The actual sucking of the whiteflies rarely kills a plant and seldom shows visible damage. Only under extreme conditions can the whitefly population truly kill a plant.
Other than the honeydew, which can easily be rinsed-off with water, the greatest potential economic damage of whitefly is their visibility; most growers cannot sell a plant with whitefly scale on their leaves – nor should they.
How to Scout:
This is most important. I get regular calls about whitefly that turn out to be leaf-hoppers, or even the shed exoskeletons of aphids. Grab a lens and take a look.
(Above: Leaf hopper (left) vs Whitefly (right))
Check under leaves. These whitefly will rarely be seen without disrupting the plant or flipping leaves over. Seeing them on tops of leaves may indicate a sever problem or a different type of whitefly. What you’re looking for is, well, white flies. Some have yellow body parts and the shapes of the wings may differ, thus an indication of particular species.
When disturbing a plant you can often distinguish between greenhouse whitefly (GHW) and Bemisia by their flight. GHW will fly up and spread-out with the lightest touch of the plant whereas Bemisia flutters low and quickly before re-establishing.
Under the leaves will also be the eggs or scale. Look for tiny (sometimes microscopic) white to translucent dots – usually accompanied by adults.
Often the first sign is the unsightly, and sticky honeydew residue on the tops of leaves. This will indicated whitefly are on the undersides of leaves above.
Typically, the time from egg to adult is approximately 3 weeks. Adults can live up to one month, laying hundreds of eggs in that time. Whitefly are scale insects so the various life stages before becoming an adult are often miss-identified as “eggs.” The actual eggs are near-microscopic and often just look like a light dusting on the leaf. The first instar is equally difficult to see. It is technically a “crawler stage”, but its movements are limited, and it’s more likely just to shift around or create a little distance from one another. The second and third instars are visible, but white-translucent and can sometimes be missed when scouting. Fortunately, for scouting, often all life-cycles are often seen together. This is especially true of Bemisia, which seems to stay more compact, using the protection of honeydew.
Controlling Whitefly with Bio-Control Agents.
Prevention is Key! Prevention of both GHW and Bemisia is easily accomplished with the parasitoid wasp Encarsia Formosa at rates as low as 0.25 wasps per square foot (2.5 per m2) with a single release point per acre for Fresh Encarsia, or 5 wasps per square foot (50 per m2) with release points every 10 square feet for refrigerated or long-stored Encarsia. These releases need to take place every three weeks, or weekly for whitefly-prone plants. Encarsia parasitizes 3rd instar whitefly turning the scale black before emerging as a new wasp. Male Encarsia are rare so almost all Encarsia are female, capable of parasitism immediately. They search via smell, so they need reduced wind-velocity to achieve their full capability. Each wasp will host-feed 3 and parasitize 5 whitefly daily over their (approximately) two-week life cycle.
Prevention can also be achieved with small releases of the whitefly destroying beetle Delphastus catalinae. Because they also search via smell and fly, a reduction of wind-speed is often necessary, but this also allows for a single release point per hectare. Release rates vary, but often 0.25 per m2 is sufficient.
Once you see whitefly, it is a different story. Encarsia can be applied weekly at a rate as high as 4 wasps per square foot, and Delphastus can be released at a similar rate. Delphastus, while more expensive than encarsia, is a ferocious feeder of whitefly. It easts eggs first, then works its’-way up towards eventually eating the adults. It can reverse a growing population of whitefly efficiently.
There are more options, however:
Aphidoletes aphidimyza – the aphid predator – finds food via the smell of honeydew. When there are little to no aphids, Aphidoletes will lay eggs in honeydew near whitefly populations. They larva, finding no aphids, will eat whitefly scale. This is easy to see, as Aphidoletes larva feeding on aphids are dark orange, but when feeding on whitefly they turn yellow. A huge amount of Aphidoletes are required for this type of knockdown – often 5,000 per hectare with one release point, with windspeed dropped significantly.
Neoseiulus cucumeris is an incredible in-expensive generalist predatory mite. It is typically sold as a thrips predator, but it is a generalist, surviving on all sorts of microscopic mites, pollen, and other pests. Because it is cheap, a large amount can be applied to your crop or garden to knock-down whitefly as well. (I used cucumeris to control broad mite on plants that were going to be used to host whitefly – the cucumeris nearly wiped-out the whitefly culture!)
Swirskii is an excellent predator mite and will also eat whitefly. However, it will eat Aphidoletes eggs first, which may disrupt your aphid control. Do not use Swirskii with Aphidoletes.
Eretmocerus is another whitefly parasitoid. Do not apply both Eretmocerus and Encarsia together except on extremely fast growing plants (like tomatoes or cucumbers). They parasitize different stages of the whitefly, but if they are on the same leaf they will fight and cancel each other out.
Home gardeners will often not see whitefly until late in the summer, and typically only one nightshade-family plants and a few others.
It is rarely worth applying beneficials unless you have particular plants you want to protect. Whitefly is mostly a greenhouse pest, and with the absence of natural predators, and their resistance to chemical applications, using beneficial insects and mites is a proven, and inexpensive strategy to controlling whitefly.
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