The more things change

“Where the Sam Hill,’ he blazed, ‘do all these footy little devils come from, anyhow? Where am I to put a beast of a bug when the next one that’s exactly like it is entirely different the next time you look at it? There’s too much beginning and no end at all to this game!’” (M. Oemler, 1920. Slippy Magee, Sometimes Known As The Butterfly Man)

One indelible image I retain is the 1992 photo of USDA Plant Pathologist James Duffus on the cover of USDA ARS’s Agricultural Research in a snowstorm — in a southern California cotton field. The “snow” is actually whiteflies, then decimating cotton and other crops around the southwestern U.S. A few years before tha,t John Mejia in The Grower, a trade magazine, noted lettuce prices reaching record levels at $42 per carton due to losses from lettuce infectious yellow virus, transmitted by this new whitefly. Though not the first time whitefly outbreaks had been seen, the scale of the problem around that time was much larger and included more crops, including greenhouse plants and poinsettia in particular.

All but the whippersnappers remember this as the sweetpotato whitefly, now more specifically termed ‘B-biotype sweetpotato whitefly’ (SPWF). It appears to have originated outside the U.S. and acquired notoriety on arrival in Florida in the mid-1980s, where it had a broader host range compared with an older SPWF, was difficult to control, became associated with large infestations in poinsettia crops, and found guilty of virus transmission and other problems in outdoor agriculture. For a short time it was declared a new species, the ‘silverleaf whitefly,’ for its effect on infested squash leaves. Whether it is simply a genetic variant of the SPWF or should be considered a new species is under discussion.

Before this new pestilence descended, greenhouse whitefly had been our most common whitefly problem. Despite causing occasional trouble it was very susceptible to biocontrol with Encarsia formosa, then receded in importance as SPWF became more widespread. Unfortunately, Encarsia formosa proved not highly effective against SPWF and some insecticides were not working well either. Tank mixes of acephate (Orthene TTO) with pyrethroids (such as Tame and Talstar) were found synergistic and fairly effective, so widely used on poinsettias until imidacloprid insecticides (Marathon) became available in 1995 and then pyriproxyfen (Distance) in 1999.

Then, in early 2005, a new strain of this sweetpotato whitefly was reported in Arizona. Now widespread, this “Q-biotype SPWF” came enhanced with reduced susceptibility to insecticides like imidacloprid and pyriproxyfen, causing widespread frustration among greenhouse growers having trouble managing an apparently familiar pest. Fortunately there has been some good news: this new strain isn’t (so far) a problem outdoors, some insecticides (Safari, Judo, Kontos, M-Pede, horticultural oil, and some others) can be effective to highly effective, there have been good results with biocontrol (Eretmocerus in particular), and according to a recent survey (McKenzie, et al. 2012), Q hasn’t taken over — the B-biotype is still common.

Thanks to Lance Osborne at the University of Florida and collaborators on the Ad Hoc Whitefly Task Force, management guidelines are available online at bit.ly/Nymu2u.

There are some very important lessons from this experience: be alert for new pests or control problems, pests may appear outwardly the same but be fundamentally different with serious economic impacts, and depending exclusively upon chemical control comes with risks — management should incorporate biological and non-chemical methods. So remember greenhouse whitefly? It hasn’t gone away either, but now we’re anticipating its next move.

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