Late-Season Whitefly Control
Whiteflies can be difficult pests to manage, especially in greenhouses where they enjoy a high-quality food supply, favorable environmental conditions and minimal protection from natural enemies. Whiteflies damage a number of crops: verbena, fuchsia, regal geranium, lantana, nicotiana, ageratum and of course poinsettia. Often, growers can successfully control whiteflies all season with effective foliar or systemic treatments applied early in production; however, sometimes these fail, or uncontrolled populations re-infest the crop from other sources. With the loss of Dithio and Plantfume smokes, growers have been wondering about whitefly control options for late-stage poinsettia production. I am also seeing annual problems with Lewis mite, a relatively “new” pest of poinsettia, that often becomes apparent in fall well after sprays would normally be applied. Early symptoms of damage are very subtle and easily missed. From a distance it may be easily confused with a lack of nitrogen. If treatments are needed to control either pest during the sensitive bract stage, which ones are likely to be safe for plants?
It might be helpful to review some of the basic elements of whitefly management, since it is usually the primary pest in poinsettia production for most growers. Good management early on can carry the crop into color with minimal pest infestation and often eliminates the need to apply any insecticides after bracts form. Consider the following points when you evaluate your operation:
- source(s) of infestation: where the whiteflies are coming from;
- correct identification: which two common greenhouse species is/are present;
- a monitoring plan: detect pests early before infestations are obvious;
- prevention: use cultural and non-chemical controls to avoid infestation; and
- effective treatments: when needed, select and time appropriate insecticides or biological controls.
Source(s) of infestation
It was clear from past work in our IPM program that growers were often unaware of the sources of whitefly problems. These included older stock or “pet” plants kept around the greenhouse and weeds inside and outside the range. Cutting-grown starter plants (such as poinsettia) were sometimes infested on arrival, but often they were very clean and contributed minimally or not at all to the problem. We found whiteflies by checking under leaves of weeds inside and outside the range, examining lower leaves on stock plants and other older plants around the facility and inspecting new plant material on arrival.
While there are more than 1,200 species of whiteflies identified worldwide, silverleaf whitefly (Bemisia argentifolii) and greenhouse whitefly (Trialeurodes vaporariorum) are the two most common species in greenhouses. Distinguishing them is not too difficult and is important for growers interested in using biological control. It can also help in tracing the source of infestation. The easiest way to tell the difference is to examine the pupal stage, just before the whiteflies emerge. Greenhouse whitefly pupae are whitish, shaped similar to a cake with vertical sides and have a fringe of fine hairs around the edge. Silverleaf whitefly pupae tend to be yellowish, have a mound shape and lack the fringe of hairs. These features are visible under a hand lens, but if unsure send a sample to a diagnostic lab or entomologist for confirmation.
A Monitoring Plan
Sometimes businesses detect infestations when already severe, often just as plants are ready for sale. Early detection can make all the difference, enabling timely treatment before populations are high. A combination of yellow sticky cards, placed vertically just above the crop canopy, with regular plant inspection, is a simple way to detect whiteflies early. Place cards in areas of air movement and among older plants where the insects are likely to be found. Usually 1-3 cards is suggested per 1,000 sq.ft., but if this seems like too many or impractical even a few cards, thoughtfully placed, will be useful. Check the cards weekly if possible, and keep a record of whiteflies trapped since the last check. The cards also help to indicate activity of a few other pests, such as thrips and fungus gnats.
Sticky cards may not tell the full story, since we have missed some infestations by relying on them too heavily; therefore, be sure to examine the undersides of older leaves for whiteflies and their immature (nymph) stages. If infested plants are found, flag a few with tape or stakes to re-check after controls are applied.
Take care to avoid introducing clean poinsettias or other plants into an infested environment. Isolate and treat infested material, and eliminate all other sources of infestation, including the weeds and “pet” plants identified when monitoring. In instances where older and younger plants need to be put together in the same range or in houses with fans or other active airflow, locate clean plants upwind of older plants. Some greenhouses have eliminated weedy borders around the perimeter, replacing with turf or mulch underlain by landscape fabric or other weed barrier.
There are many products labeled for control of whiteflies, including aerosols, sprays and systemics. Many people use Marathon (Olympic Horticultural Products) media treatments, but there are also reports of good results with foliar sprays such as TriStar (Cleary Chemical Corp.) and Flagship (Syngenta Professional Products), two newer neonicotinoids. Distance (Valent USA Corp.), an insect growth regulator (IGR), has shown good results in trials as well as in actual production settings, spraying once or perhaps twice relatively early in production before color shows. It has translaminar activity, as do some of the neonicotinoid insecticides, so coverage may not be quite as critical. Note it should not be applied after bract formation. Enstar II (Wellmark International) and Azatin (Olympic Horticultural Products) (Ornazin, SePRO Corp., and Aza-Direct, Gowan Á Company, have the same active ingredient as Azatin) are other IGRs that have worked well for us too. Apply weekly with good foliar coverage. Endeavor (Syngenta Professional Products), a relatively new product, has shown good knockdown of adults in our tests. Orthene/pyrethroid, which includes Tame (Valent U.S.A. Corporation), Talstar (FMC Corp.), Decathlon (Olympic Horticultural Products), Scimitar (Syngenta Professional Products), Mavrik (Wellmark International) and Astro (FMC Corp.) combinations are synergistic, working better than either alone, and became the standard in the industry shortly before Marathon was marketed. Several new products are in development, so check occasionally with your regional extension specialist on the current status of products for whitefly control. Attending regional or national grower meetings and subscribing to extension or trade publications is helpful for keeping up-to-date.
After Bract Formation
Sometimes growers still find late-season surprises, so we have looked at a number of possible alternatives to Plantfume and Dithio for whitefly control during and after bract formation. We have also treated plants with miticides that have been effective in our trials against Lewis mite. Although these were fairly unscientific, we looked at a large number of cultivars and made repeat applications in “worst case” scenarios. Treatments were usually applied late in the day and at maximum (or above) label rates. I do not advise growers to try this.
In 1998, 41 poinsettia cultivars in full color were treated twice three days apart with TriStar at 0.09 oz. per 100 gal. (this is much lower than the current high label rate of four packets or 2.3 oz.), Talstar (40 oz.), Sanmite (Scotts Company) (4 oz.) or Marathon 60W (the liquid formulation was not yet available). We also tried treating these same plants with a space mix of the aerosols Attain TR (Whitmire Micro-Gen) + PT 1300 Orthene TR (Whitmire Micro-Gen) three times at 2- to 3-day intervals at rates exceeding normal label use. Duraplex TR (Whitmire Micro-Gen) was also released three times two days apart. For each spray, attention was given to full coverage and heavy deposition on bracts in particular. We saw no injury in any case, although residue was very noticeable following use of Talstar or Sanmite. We will be looking at more realistic (higher) rates of TriStar this year.
In 1999, we tried applying Flagship 25WG (8 oz. per 100 gal.) or Endeavor (10 oz.) sprays twice five days apart to 42 cultivars in full bract. Endeavor left noticeable residue; note the label specifically prohibits application to poinsettia bracts. There was no injury in either case. Two years ago we looked at Endeavor again (5 oz.), and also at Avid (Syngenta Professional Products) (8 oz., twice the label rate for mites), Azatin XL (16 oz.) or Orthene TTO 97 (Valent USA Corp.) (0.25 lb.) + Decathlon (1.9 oz.) sprays applied four times at weekly intervals during bract formation on ‘Freedom Red’ poinsettias. There was again no injury with any of these, although Endeavor showed a fair bit of residue especially on darker colors.
Finally, we tried the same Avid and Orthene + Decathlon sprays, applied once to several varieties in full color, but this time we combined them with either Strike (Olympic Horticultural Products) (2 oz.) for mildew or Decree (SePRO Corp.) (24 oz.) for Botrytis as tank mixes (i.e., Avid + Strike, Avid + Decree, Orthene + Decathlon + Strike or Orthene + Decathlon + Decree). In no case did we see symptoms of injury to flowers or bracts, although there was a noticeable brown residue on light-colored bracts where Decree was used. The residue would be much less apparent if normal rates were applied as recommended on the label. As a side note, we have also found that repeat applications of Floramite 50W (Crompton/Uniroyal) at the highest label rate (4 oz.) were not injurious to a variety of cultivars.
Keep in mind these trials are done for experimental purposes; growers should always follow label directions and restrictions. I still advise conducting small-scale tests in your own greenhouse under local conditions since there are so many variables from one operation to another. However, our experience suggests several possible rescue treatments to consider should a whitefly or mite problem arise at the worst possible time. We plan to continue additional tests this winter with some of the newer products, so stay tuned for future updates.