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Florida Researchers Look at Whitefly Control

The Tomato Magazine
October 2004

The silverleaf whitefly (SLWF), Bemisia argentifolii, also known as the B strain of the sweetpotato whitefly, B. tabaci, remains the key pest of tomatoes in Southern Florida. Whiteflies cause losses by inducing irregular fruit ripening and by transmitting Gemini viruses. The most damaging is tomato yellow leaf curl virus (TYLCV).

So report David J. Schuster, Sandra Thompson and Phyllis R. Gilreath in a research study reported during the Florida Tomato Institute, held last fall in Naples, Fla. Schuster and Thompson are with the University of Florida/IFAS Gulf Coast Research and Education Center in Bradenton. Gilreath is with the UF/IFAS Cooperative Extension Service, Manatee County, Palmetto.

"Despite applications of the nicotinoid Admire 2F® to seedlings in plant production houses and additional soil applications of either Admire or Platinum®, another nicotinoid, at transplanting or up to three weeks after transplanting, tomato growers in every production area in southern Florida experienced larger-than-normal numbers of whitefly adults during the spring season of 2003," they reported. "This was particularly true late in the season. Because of the threat of transmission of TYLCV by these whitefly adults, numerous applications of additional insecticides were applied, sometimes weekly or more frequently, even though the soil applications of Admire or Patinum were still providing control of whitefly nymphs."

Among the insecticides applied were Fulfill®, Monitor®, Lorsban® 50-W, several different pyrethroids, Phaser® Thiodan®, soap and oil. Results were mixed, and residual control was short. Some growers made foliar applications of nicotinoids including Provado®, Actara®, and Assail®, again, with mixed results.

The research team feels that the latter practice could encourage the development of resistance to nicotinoid insecticides. Partly in response to these concerns, Syngenta Crop Protection has since removed tomatoes from the Actara label.

"Naturally, growers are asking why so many whitefly adults are present and persist despite these many insecticide applications. Are the whiteflies becoming resistant to the insecticides?" the researchers questioned. "Are there other cultural activities that are contributing? What can we do now, and what are the control prospects for the future?"

The pyrethroids, edosulfan (Thiodan, Phaser) and organophosphates (Monitor, Lorsban) are not as effective against the SLWF as they once were, they acknowledged, pointing out, however, that no systematic or long-term monitoring of efficacy or resistance has been conducted in Florida. As early as 1991, researchers observed up to an 80 percent reduction in endosulfan efficacy and up to a 60 percent reduction in Lorsban efficacy.

"There is no reason to expect a reversal of these trends, especially in light of the current heavy use of these insecticides," the team warned.

New Products for the Future
"In a spring 2002 experiment at GCREC, a single application of MT-02-03, now known as Oberon (spiromesifen; Bayer CropScience, Kansas City, Mo.), resulted in numbers of SLWF nymphs below the threshold of 5/10 leaflets for up to four weeks," the team noted. "Four applications of Diamond (novaluron; Crompton Uniroyal, Raleigh, N.C.) also reduced nymph numbers below the threshold."

Additional experiments were conducted during the fall of 2002 and spring of 2003. In the fall 2002 experiment, Oberon 240SC (8.5 ozs/acre) was applied, following an at-transplant application of Admire at 16 ozs/acre, when the threshold of 5 nymphs/10 leaflets was reached, the team reported. The treatment was compared to Admire alone, to Admire followed by a weekly alternatation of Baythroid 2 (2.8 ozs/acre) and Thiodan 50W (2 lbs/acre), or to a check. Whitefly nymph control with either Oberon or the weekly alternation of Baythroid and Thiodan were equivalent and significantly lower than the check; however, Oberon was applied twice while Baythroid and Thiodan were each applied five times. Both treatments resulted in SLWF nymph densities below the threshold for five weeks, while densities in the check plots were above the threshold.

In the 2003 experiment, Diamond 0.83E (14.5 ozs/acre), Oberon 240SC (8.5 ozs/acre) or Courier 70W (0.5 lb/acre) were applied following an at-transplant application of Admire (16 ozs/acre), when the threshold of 5 nymphs/10 leaflets was reached, and were compared to Admire alone or to a check. A single application of Oberon reduced nymphet numbers to the threshold or below for at least two weeks. A single application of either Diamond or Courier resulted in significant reductions in the numbers of nymphs compared to the check within one week of the application. Further evaluations were precluded by the effects of heavy rains on the tomato foliage.

"The high level of some RS50 values, especially in 2003, suggests a decline in the susceptibility of the SLWF to Admire; however, the apparent shift in susceptibility may be as much a reflection of changes in cropping practices and nicotinoid use patterns as a true trend in declining susceptibility," the researchers said. "Furthermore, the reduced susceptibility appears to be unstable when whiteflies are no longer exposed to Admire. Therefore, lengthening a tomato-free period in the summer to at least two months and making only a single application of a nicotinoid insecticide could help in managing the reduced susceptibility.

"The high numbers of whitefly adults observed in the spring of 2003 could be related to the reduced susceptibility of the SLWF to Admire, but this is probably not the whole answer. A shortened tomato-free period in the summer caused by economic concerns and increased grape tomato acreage and changes in suitability of pepper as a whitefly reservoir, undoubtedly, also have contributed. Although there are new products on the horizon for managing SLWF, growers are encouraged to redouble their efforts in implementing a nicotinoid resistance management program that was first outlined by Schuster and Thompson in 2001," the scientists noted.

Nicotinoid Resistance Management Recommendations
So what can tomato growers do to reduce their whitefly populations and the risk of disease?

Growers were advised to: (1) Plant whitefly-free transplants; (2) Delay planting new crops as long as possible and destroy old crops immediately after harvest to create or lengthen a tomato-free period; (3) Avoid planting new crops near or adjacent to infested weeds or crops, abandoned fields awaiting destruction or areas with volunteer plants; (4) Use UV-reflective (aluminum) plastic soil mulch; (5) Control weeds on field edges if scouting indicates whiteflies are present and natural enemies are absent; (6) Manage weeds within crops to minimize interference with spraying; and (7) Avoid u-pick or post-harvest pin-hooking operations unless effective control measures are continued.

Other cautions given were:

· Do not use a nicotinoid like Admire on transplants, or apply only once 7-10 days before transplanting; use products in other chemical classes, including Fulfill, before this time.

· Apply a nicotinoid like Admire (16 ozs/acre) or Platinum (8 ozs/acre) at transplanting, and use products of other chemical classes (such as the insect growth regulators Knack® (pyriproxyfen; Valent U.S.A. Corporation, Walnut Creek, Calif.) or Courier® (buprofezin; Nichino America, Inc., Wilmington, Del.) as the control with the nicotinoid diminishes.

· Never follow an application (soil or foliar) of a nicotinoid with another application (soil or foliar) of the same or different nicotinoid on the same crop or in the same field within the same season (i.e. do not treat a double crop with a nicotinoid if the main crop had been treated previously).

· Save applications of nicotinoids for crops threatened by whitefly-transmitted plant viruses or whitefly-inflicted disorders (i.e. tomato, beans or squash) and consider the use of chemicals of other classes for whitefly control on other crops.

© 2004 Columbia Publishing

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