Nursery, Greenhouse, and Landscape News

Costs of Chemical vs. Biological Control for Silverleaf Whitefly

The failure of many growers to adopt biological control practices against silverleaf whitefly is due more to the economics of implementation than an inability of natural enemies to control this pest. Economic advantages to chemical control are likely to be temporary because they are conferred by the low cost of only one material, Marathon 1G. Furthermore, silverleaf whitefly has a history of rapidly developing resistance to pesticides. The long term activity of Marathon will speed the development of resistant strains. In the absence of Marathon, biological control can be economically competitive with other tactics.

The challenge to extension educators is to understand why growers are using Marathon and to keep growers aware of biological control alternatives that can be implemented when this product fails. The economics of different whitefly management strategies over a 16 week growth cycle were reviewed in 1993 for a 20,000 square foot greenhouse. Since 1993, costs for some pesticides have increased, but the cost for Encarsia formosa has dropped. Updating costs for 1996, weekly applications of high volume sprays cost $0.122 per plant; weekly aerosol applications or aerosols cost $0.083 per plant. In comparison, an IPM program with weekly scouting and applications cost $0.139 per plant. In contrast, weekly scouting and releasing one Encarsia formosa wasp per plant for 8 weeks costs the same as the aerosol program ($0.083 per plant).

When one or two applications of insect growth regulators are subsequently applied to the crop, the respective costs per plant ($0.096 and $0.109) are still lower than the cost of a weekly high volume spray. Assuming growers could purchase effective strains of Encarsia formosa, this analysis suggests that biological control is very competitive economically. When other, less quantifiable advantages of reducing pesticide use are considered, biological control becomes even more attractive. Costs of each of the above control strategies however, are well above that for Marathon. Assuming it takes 8 hours to apply Marathon, the cost is only $0.064 per plant.

This rate is $233 to $555 less expensive than for managing the entire crop with Encarsia formosa and one or two sprays of an insect growth regulator. Similarly, it is $233 less expensive than strict aerosol sprays and $713 less expensive than weekly high volume sprays. It is not surprising then, that growers are not very motivated to explore biological control for whiteflies at the present time. On the other hand, once resistant strains develop, biological control will be able to compete effectively against available pesticide options. The challenge for the biological control industry is to find a way to maintain effective strains of natural enemies so that they can be produced at a low enough cost to maintain their competitive edge against more conventional methods.

- Cliff Sadof, Purdue University. From a presentation at a Conference on Biological Control in the Midwest at Iowa State University, Ames IA, October 1996.

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