Treatment Thresholds

Karen Delahaut, Outreach Specialist, UW-Madison

If you detect pest problems in the greenhouse while scouting, you must decide whether or not action should be taken. To do so, you will need to determine your action threshold. In production agriculture, this is often referred to at the “economic injury level” – the level at which the amount of money lost because of a reduction of yield if nothing is done to manage the pest is greater than the cost of the treatment. There is a form of economic threshold that can be used in the greenhouse and it uses the increased revenue from improved crop quality and yield verses the adverse impacts of control such as phytotoxicity from too many pesticide applications. The economic threshold method isn’t always practical when assessing pest problems on plants that are being grown for their aesthetic appeal. In this case an “aesthetic injury level” is often used to determine when treatment is necessary. The difficulty with aesthetic injury levels is that they are subjective and don’t have a numerical value associated with them so some people tolerate more damage and higher pest levels than others.

Treatment thresholds are fundamental to an IPM program. However, few thresholds are available for greenhouse crops because of the limited amount of research being done on them by university specialists and because of the aesthetic nature of the commodity. Thresholds are necessary to reduce the development of pesticide resistance, limit the amount of disruption to cultural practices, minimize phytotoxicity from pesticide applications, and increase profit in plants grow for sale by reducing the expenditures related to pesticide applications.

Thresholds are based on the type of pest and the damage caused, crop species and cultivar and how susceptible it is to damage, the stage of plant growth, the time until sale, and what controls are available for the pest – fast acting synthetic pyrethroids vs. slow acting insect growth regulators.

To establish thresholds you should regularly monitor the plants and keep detailed records. Evaluate the acceptability of the finished crop and if it’s being sold, whether it’s acceptable to your customer. You can use quantitative or numerical thresholds such as the number of insects caught in a sticky trap of the number of pests dislodged when you tap foliage over a white card or clipboard. You can also get a numeric threshold by looking at the percentage of plants or leaves infested or damage by the pest.

If you are growing stock plants from which cuttings are made, be sure to establish a zero tolerance on these plants. They should be kept virtually pest free all of the time. Young seedling plants and cuttings will also have a much lower threshold level than more mature plants.

Thresholds are an important part of a monitoring and pest management program despite the fact that they are difficult to establish. With time and experience you will be able to establish your own thresholds for the crops you regularly grow.