Chemical Resistance in Insects and Diseases
Resistance to pesticides can develop in insects and fungi. Resistant populations occur when the same chemical, same family of pesticides or pesticides with a similar mechanism of activity are used repeatedly at the same location. When a few naturally resistant organisms remain after a treatment, these contribute to development of a larger population of resistant organisms. Eventually the population that develops may contain mainly resistant organisms and will not be controlled with the recommended rates of the pesticide.
Some pesticide labels give warnings about possible development of resistant populations. Take such warnings seriously and follow label directions carefully. The label summaries in this guide include such information when it is available.
Fruit insects which have demonstrated resistance to some pesticides in various fruit growing areas include mites, aphids and oblique-banded leaf rollers. In Manitoba, there has been one suspected case of mite resistance to dicofol (Kelthane) in strawberries.
Strains of fungus resistant to fungicides have been known to develop especially where fungicides with systemic activity are used repeatedly (ie. Benlate). There are documented cases of fruit rots, especially gray mold (Botrytis), demonstrating resistance to some fungicides. To date there have been no known cases of fungicide resistance occurring in Manitoba fruit crops.
To prevent or delay development of resistance in insects and fungi try to design a spray program which includes pesticides from different chemical families especially if repeated sprays for the same problem are required. There are some insects and diseases for which choice of pesticides is limited and alternating pesticides will not be possible. If a fungicide or insecticide fails to control a pest when applied according to label instructions, especially if your previous experience has demonstrated good control, consider the possibility of pesticide resistance. Keep in mind that weather factors or application errors can also result in failure of a pesticide to control the target pest.
