Battle of the whiteflies

Whiteflies were in the headlines recently. One doesn’t often (and prefers not to) hear about insects making big news, but apparently one epic infestation was severe enough to cause a major grower in the community to shut down the range, clean out and start again, and bring in a reporter. Fortunately employees were retained, but the action suggests a high level of frustration and desperation, to say nothing of economic loss. Having helped other growers though similar situations, the feelings are all too familiar.

Whiteflies have unfortunately been bothering greenhouse growers for generations. Literature from more than 100 years ago reports greenhouse whitefly, Trialeurodes vaporariorum, an important pest, the same species I encountered my first day on the job with Cornell. Soon after, in the mid-1980’s a new species appeared, the so-called B-biotype sweetpotato whitefly (SPWF, Bemisia tabaci). Its broader host range, difficulty to control, potential for virus transmission and association with poinsettia distinguished this new biotype from the otherwise identical sweetpotato whitefly that had never really been a problem for us. It also was associated with a distinctive silverleaf symptom on summer squash. By the early 1990’s B-biotype sweetpotato whitefly had become a destructive pest outdoors in the southwest U.S. on lettuce, cotton and melons resulting in produce shortages and newspaper headlines.
 

This “new” whitefly impressed upon us, again, the value of monitoring to detect infestations early, as well as the need to identify the “problem” since the usual insecticide controls were not working well. Some growers using biological control for greenhouse whitefly had the same experience, as the parasitoid wasp Encarsia formosa is not nearly as effective against B-SPWF. We were then reduced to timing sprays for peaks in the youngest nymph stages, using tank mixes including, in some cases, products with potential for plant injury, and removing older leaves that harbor populations. With the arrival of several new insecticides the situation turned around quickly, until a second new strain, the Q-biotype SPWF, appeared in late 2005 on poinsettia in Arizona. This one unfortunately came equipped with resistance to some of these insecticides, but thanks to new chemistry and biological controls we’ve again prevailed – for now. The take-away lesson is that victory can be short-lived.

The battle plan
Check new plants on arrival and regularly thereafter. Scan older leaves for whiteflies of any stage – learn to recognize them and how to tell the two kinds apart. The University of Florida’s Lance Osborne operates a website with helpful photos (http://mrec.ifas.ufl.edu/lso/Whiteflies.htm). Note the several new species now in Florida that we should watch for.

Put out a couple of yellow sticky cards. While not completely reliable, they work long hours without pay and are like another set of eyes to help in detection.

Release biocontrols early. Growers have had good results using biocontrols for whiteflies. Releasing them regularly, it’s best to start even before whiteflies are seen. Be sure to check a portion of every shipment to see insects arrive alive and in the stated quantity. Place a sample card with the biocontrol tags in a sealed jar with a small piece of sticky card suspended above to trap wasps as they emerge.

Match the biocontrol(s) to the whitefly. Use Encarsia formosa for greenhouse whitefly and Eretmocerus for SPWF; there are other kinds available, too. Parasitized pupae (last immature stage before adult whiteflies emerge) will be black (Encarsia) or tan (Eretmocerus). However, many whiteflies are killed as these biocontrols feed upon (without parasitizing) the immature stages – often just a few parasitized pupae are seen with Eretmocerus.
 

Treat early if needed. Insecticides can be quite effective depending on what the whitefly population is resistant to, which is something difficult or impossible to assess ahead of time. But even effective products perform poorly when applied late. M-Pede or a horticultural oil like Suffoil-X work well on contact with no resistance issues. Good coverage is absolutely essential and expect no residual activity, but some care is needed on sensitive plants and they should not be applied too frequently even on more tolerant varieties. These products can be very helpful for reducing populations if whiteflies are already at high levels. There is a long list of labeled products for whitefly control, including insect growth regulators (Enstar AQ, Talus, Adept, Pedestal, Distance, and azadirachtin products like Azatin, Aza-Direct, Molt-X, Ornazin, AzaGuard, etc.), insect-killing fungi (Preferal, BotaniGard, NoFly), neonicotinoids (Safari, Marathon/Discus and generics, Flagship, TriStar), and others (Kontos, Judo, Aria, Sanmite, Magus, Avid, Endeavor, Orthene, pyrethroids like Tame, Triact, Decathlon and Talstar). Discuss best options for your particular situation with a consultant or extension specialist, considering objectives, plant material and sensitivity, uses for other pests, compatibility with biocontrol, cost, and personal preference. With all the options and expertise available we should be able to keep whiteflies from being headline material again.

Daniel Gilrein is a frequent contributor to Greenhouse Management and an extension entomologist at Cornell University.