edited from Applied Bio-nomics
Pictured top: Whitefly present on the underside of a leaf
Pictured right: Encarsia pupae packaged in cards.
Few pests are as subtle and insidious as whitefly. While the adults are easily seen and, for some species, easily trapped on yellow sticky cards, the early larval stages are mostly clear and too small to be seen by the naked eye.
When you see one whitefly, you have thousands. In the case of the Greenhouse whitefly, each female can lay about 300 eggs, mostly in the first few weeks of becoming an adult. It doesn’t take much imagination to see why whitefly can quickly get out of control.
The chemical insecticides available to combat whitefly are showing alarming rates of resistance and combined with the whitefly’s
rapid reproduction rate, this pest can be one of the most serious problems that we face for some crops. There are many species of whitefly, but for most crops the Greenhouse Whitefly is the most common.
The key thing to keep in mind is that one whitefly is one too many. While chemical strategies involve waiting for economic thresholds, biological strategies require prevention. Encarsia must be in place before the first whitefly is seen.
Beware using the trapped adults on the cards as relative indicators of changing whitefly numbers. In the cooler periods, Greenhouse whitefly adults can live for about 3 months. So, even if you have completely eliminated the eggs and larvae of the coming generations, the pest will still be evident.
Eggs of whitefly are surprisingly easy to see and scout. Depending on density and species, many whitefly lay their eggs in tight circles of about 10 eggs. Sometimes, the eggs are laid in a spiral. The eggs tend to be grey and mounted on stalks. Once the eggs hatch, they become transparent crawlers and are nearly invisible under high intensity growing lights.
Biological control of any whitefly species relies on prevention. As soon as your plants are placed in your facility, low levels of “fresh” (not refrigerated) beneficial parasitoids should be released. Any exposure to cold temperatures below 8 C (46 F) for even just a few hours completely and irreversibly eliminates their searching ability. Fresh Encarsia (Encarsia Max) can be released at rates as low as 0.25 wasps per square meter weekly to prevent a whitefly build-up. Start with more (up to 2-5 wasps per meter) if whitefly are already present, or if you are in a high pressure area for whitefly, or if you are producing plants that are especially susceptible (attractive) to whitefly.
When a whitefly is observed either in the crop or on a trap, the rate is doubled until the whitefly is no longer detectable. The doubling should only continue until you reach the maximum rate of 8 wasps per square meter.
The use of whitefly-trapping plants can be very effective. Eggplant is an extremely popular plant for whitefly. In many crops, just one or two eggplant per acre can prevent a buildup of whitefly in the crop. For crops such as pepper, eggplant can completely “pull” all of the whitefly off the crop and onto the eggplant. Trapping plants should become “banking” plants by focusing the biological controls onto the eggplant.
The tendency of whitefly to gather on the growing tips, especially in the winter, also makes it possible for relatively easy mechanical removal. If you have spray booms, try attaching tickling ribbons or string that will gently disturb the plants as the boom passes overhead. This will force the whitefly to disperse and fly about for a bit. If you can add a vacuum to the trailing edge of the boom, you can remove a significant level of whitefly adults. When this technique is repeated, you will remove the adults that continually move up on to the softest plant material.
Click here to go to Applied Bio-nomics handbook.
Whitefly info on page 22.
Encarsia (Encarsia formosa):
Encarsia are parasitic wasps that are sold in cards hung directly on your plants. They are excellent searchers, and will be able to sense where other Encarsia are already present and search for better places to parasitize whitefly. We recommend that you re-apply the strips every two weeks, 2-3 times.
Strips of 1,000 (100/card)(22725): $22.50 for areas up to 1,000-10,000 sq feet
Buy 3 or more: $15.00 / 1000