Mode of action or mode of activity refers to how a pesticide — in this case, an insecticide or miticide — affects the metabolic and physiological processes in an insect or mite. Greenhouse producers, in order to sustain successful integrated pest management programs and preserve the longevity of currently available insecticides/miticides, need to practice rotating insecticides/miticides to reduce the likelihood that plant-feeding insects and mites in greenhouses will develop resistance.
First of all, it is important to understand what resistance is. Resistance is the genetic ability of some individuals in a pest population to survive a pesticide application. In other words, the pesticide no longer effectively kills the target pest, which may be an insect, mite or fungal pathogen (if discussing fungicides). This is primarily due to the intensive mortality placed on insect/mite populations from frequent applications of insecticides/miticides, resulting in the amplification of already-existing genetic traits and/or potential genetic changes that lead to mutations in the remaining individuals. Due to the selection of individuals in insect and mite populations to overcome this burden, insect and mite populations are then able to tolerate applications of insecticides and/or miticides. The rate at which insects and mites may develop resistance to insecticides or miticides is influenced by a variety of factors:
- length of exposure to a single pesticide,
- level of mortality (high v. low),
- presence or absence of refuge sites or hiding places,
- relatedness of one pesticide to another,
- generation time (short v. long),
- number of young or offspring produced per generation, and
- mobility of individuals.
It is often argued that insect and mite populations will develop resistance faster to pesticides that are used in greenhouses than when these same materials are used outdoors. The primary reason for this mind-set is that the material will “hang around” or persist longer in greenhouses than outdoors, whereas pesticides used outdoors will generally break down from exposure to ultraviolet light or rainfall, which may reduce the amount of selection pressure placed on a pest population.
In order to reduce the possibility of insect and mite pests from developing resistance it is important to design a rotation program that involves insecticides and/or miticides with different modes of activity — not chemical classes. The reason for this is that some chemical classes have similar modes of activity.
For example, organophosphates and carbamates, despite being different chemical classes, both have identical modes of activity. Both chemical classes block the action of acetylcholinesterace (AChE), an enzyme that deactivates acetylcholine (ACh) thus allowing nerve signals to stop, which results in the total loss of nerve functions. So, using acephate (Orthene, Valent USA) for two spray applications during a generation and then switching to methiocarb (Mesurol, Gowan Company) does Á not constitute a proper rotation scheme. Similarly, although pyridaben (Sanmite, Scotts Company) and fenpyroximate (Akari, SePRO Corp.) are in different chemical classes — pyridazinone and phenoxypyrazole, respectively — they both work on the mitochondria electron transport system (responsible for energy production), so these materials should not be used in succession.
The chemical class, neonicotinoid (also referred to as chloronicotinyl) is relatively new and contains a number of systemic insecticides that are registered for use in commercial greenhouses including imidacloprid (Marathon, Olympic Horticultural Products), thiamethoxam (Flagship, Syngenta) and acetamiprid (Tristar, Cleary Chemical Corp.). Another neonicotinoid, dinotefuran (Safari, Valent USA) will eventually be registered, which means there will be four neonicotinoid insecticides available. Because all neonicotinoids have similar modes of activity it is important to not use them in succession, as this will increase the selection pressure on the target pest population and may potentially enhance the development of insecticide resistance. Use an insecticide with a different mode of activity either before or after using a neonicotinoid-based insecticide.
Another essential strategy is to rotate insecticides and miticides with non-specific or broad modes of activity — such as insect growth regulators, insecticidal soap, feeding inhibitors, horticultural oil and beneficial fungi and bacteria — with materials having specific modes of activity. This will alleviate the possibility of insects and mites developing resistance.
It is important to rotate common names (active ingredient) not trade names. For example, both Azatin (Olympic Horticultural Products) and Ornazin (SePRO Corp.), despite having different trade names, contain the same active ingredient — azadirachtin. In general, rotate different modes of activity every 2-3 weeks or every 2-3 insect/mite generations. The actual length of rotation will depend on the time of year, as temperature and season influence the duration of the life cycle. For example, high temperatures that typically occur in greenhouses during the summer months shorten the developmental time (egg to adult) of most of the major greenhouse insect and mite pests, including aphids, thrips, two-spotted spider mites and whiteflies. This often leads to overlapping generations with variable age structures (eggs, larvae, pupae and/or adults) present at the same time. As a result, more frequent applications of insecticides or miticides are needed, and they must be rotated more often. In contrast, during the winter months, the developmental time of most greenhouse insect and mite pests is extended (due to the cooler temperatures and shorter daylengths), which means that insecticides and miticides may not need to be rotated as frequently.
Below are examples of insecticides or miticides that have dis-similar modes of activity and may be used in rotation schemes for aphids, thrips, two-spotted spider mites and whiteflies:
- Aphids: Endeavor (Syngenta), Marathon (spray application), Ultrafine Oil (Whitmire Micro-Gen), Orthene and Insecticidal Soap
- Thrips: Conserve (Dow AgroSciences), Avid (Syngenta), Mesurol (Gowan), Orthene and Pedestal (Crompton/Uniroyal)
- Two-spotted Spider Mite: Floramite (Crompton/Uniroyal), Pylon (Olympic), Avid, Akari or Sanmite (Scotts) and TetraSan (Valent)
- Whiteflies: Marathon (spray application), Endeavor, Distance (Valent), Talstar (FMC) and Orthene
One of the continual problems that greenhouse producers contend with is that the insecticide or miticide label does not state the mode of activity, so it may be difficult to access this information unless you directly contact the manufacturer. Olympic Horticultural Products has produced a useful brochure (Chemical Class Chart) that briefly describes the mode of action of insecticides, miticides, fungicides, herbicides and plant growth regulators used in greenhouses. This information has also recently been posted to the USDA Web site at www.usda.gov . In Figure 1, page 68, I present a listing of the major modes of activity, concentrating on insecticides and miticides, with detailed descriptions for each along with the insecticides and miticides (common name and trade name) that may be categorized under each specific mode of activity. This information can be used to develop sound rotation programs that will help deal with insect and mite pests more effectively.