Managing Pesticide Resistance in Tobacco

Resistance is the ability of a pest to survive exposure to a pesticide at a rate or concentration that previously controlled it. Resistance to a pesticide develops after repeated exposure to a specific chemical or chemical family. A few naturally occurring resistant individuals survive after each application, while the susceptible portion of the population is killed. These resistant survivors multiply and gradually replace the susceptible ones. Eventually, the resistant population dominates, and the pesticide loses efficacy. Resistance to pesticides can develop very quickly. Do not use the same chemical repeatedly unless it is used in rotation with a different chemical or used in combination with other chemicals having a different mode of action.

Many chemicals with the same active ingredients are marketed under different brand names. For example, the fungicide azoxystrobin is marketed under more than one brand name, including Quadris and Azoshy.

Different chemicals may also have the same mode of action, or way of controlling the pest. Pesticides are grouped into chemical families or groups, based on their modes of action. For example, the insecticides Up-Cyde, Decis and Pounce have different active ingredients, but the same mode of action, and are both in the same insecticide family, Group 3. Alternating between different active ingredients within one chemical family is essentially the same as using the same active ingredient repeatedly. This is because resistance to all chemicals within that family develops at the same time, even though only one active ingredient may have been used repeatedly.

Certain pests are more prone to develop resistance to pesticides than others. Pests with a short life cycle and many generations per growing season, such as aphids, are more likely to develop resistance. Pests are more likely to develop resistance to pesticides that have a single mode or site of action than to those with multiple modes of action.

To reduce the risk of pests developing resistance, use the following strategies:

• Know the active ingredient of a pesticide. Many chemicals with the same active ingredient are marketed under different brand names. Do not alternate between chemicals with the same active ingredient.
• Know the pesticide group or family. Avoid repeated use of pesticides from the same chemical group or subgroup by rotating between products from different chemical groups. Pesticide labels and the product cards in the Ontario Crop Protection Hub indicate the chemical group or family for each product. See the table below for a list of insecticide and fungicide groups and their modes of action.
• Only use chemical controls when necessary and consider implementing other forms of control (e.g., crop rotation, use of resistant varieties, etc.) or biological control to help reduce the risk of a pest or disease developing resistance to a pesticide. Major strategies for managing diseases should minimize the conditions that favour development of disease. These include sanitation, maintaining optimum levels of moisture and nutrients, and manipulation of various parameters (e.g., appropriately timed sucker control). For further details on insect pests and diseases, as well as integrated pest management (IPM) strategies for managing them, see OMAFRA Publication 842, Guide to Flue-Cured Tobacco Production.
• Do not exceed the total number of applications or total amount of product allowed per year for each product. Do not apply the product at rates lower than the rate on the label.
• Monitor recently treated pest populations for signs of resistance.
• If you experience a control failure after using a registered product, do not reapply the same pesticide.

For general information on resistance management in all crops, see Managing Pesticide Resistance.  See the pesticide label for product-specific information on resistance management.

The tables below summarize the mode of action and group number for insect and disease control products registered on flue-cured tobacco in Ontario.

Table 1 - Insecticide Groups Based on Mode of Action.

The classification scheme listed below is adapted from information developed by the Insecticide Resistance Action Committee Mode of Action Working Group. Products with the same group number have a similar mode of action. For details on this classification system, see www.irac-online.org/.

Table 2 - Fungicide Groups Based on Mode of Action.

This classification scheme is adapted from information developed by the Fungicide Resistance Action Committee to distinguish fungicide groups according to their cross-resistance behaviour. M = multi-site inhibitor, U = unknown mode of action and unknown resistance risk, NC = not classified. For further details on this classification system, see www.frac.info and search for “frac code list.”