Some species of ornamental plant pests, including the green peach aphid, the silverleaf whitefly and the serpentine leafminer, have developed resistance/tolerance to pesticides. When a pesticide loses effectiveness, some managers try using higher concentrations or more frequent applications to obtain acceptable control. These measures encourage resistance, however. Resistance is most likely to develop during long-term exposure to a particular compound, particularly in organisms with short life cycles and high reproductive rates.
To prevent resistance, managers should use pesticides (particularly nerve toxins) only when needed to prevent economic losses or when pest populations become intolerable or exceed action thresholds, and should use the lowest acceptable doses of pesticides. Once resistance has become evident, commercial ornamental plant and turfgrass growers or landscape managers should rotate pesticide classes (pesticides with different modes of action) between pest generations. For instance, if a pyrethroid insecticide treatment fails to provide control, avoid using another pyrethroid insecticide (e.g., permethrin, cypermethrin, cyfluthrin, bifenthrin, lambda-cyhalothrin, deltamethrin, etc.) to try gaining control because the mode of action is the same or very similar. For more information, see the Insect Resistance Action Committee (IRAC) website.
Tank-mixing of insecticides is generally discouraged because it may cause pests to become resistant to all products in the mixture. However, there are times when tank mixtures can increase pesticide activity and lower the dose required. When a tank mix provides better control than the additive effect of each product applied separately, the mixture is said to be synergistic.
In most cases, tolerant or resistant arthropod strains are less fit than are susceptible strains. In the absence of the pesticide selection pressure, susceptibility will return to the population.
Get more information on insecticide resistance management at the Web site of the Insecticide Resistance Action Committee (IRAC).