Trimax, Centric, and Intruder are classified in the neonicotinoid class of chemistry and are applied across the Cotton Belt to control tarnished plant bugs and cotton aphids.
Cotton aphids have a long history of rapidly developing resistance to insecticides throughout the world. Therefore, eventually these insecticides will also fail to provide satisfactory control as a result of resistance.
Due to the unique mode of action of this class of chemistry, it has been difficult to evaluate toxicity to cotton aphids in the laboratory. However, two years ago USDA-ARS scientists developed a method for rapidly testing cotton aphid susceptibility to the neonicotinoid class of insecticides.
Field reports from Angus Catchot (Mississippi Extension entomology specialist) in 2005 indicated less than adequate control of cotton aphids with the neonicotinoids.
This year, these reports are more widespread and range from poor control to the perception of no effect on cotton aphid populations in cotton fields. In each circumstance during 2006, instances of unsatisfactory control were reported after the second or third application of insecticides.
Apparently, the initial application provided acceptable control of susceptible individuals but the survivors were tolerant to the product being used. Subsequent applications of the same insecticide or product within the class were less effective.
In addition, multiple applications of the same or similar products induce additional selection pressure that allows only the most resistant individuals to survive.
The testing methods have been able to detect a change in cotton aphid susceptibility to a candidate neonicotinoid.
Colonies were collected from commercial cotton fields near Tchula, Miss., in north Alabama (Barry Freeman, Alabama Extension entomologist), and in south Arkansas (Gus Lorenz, Arkansas Extension entomologist).
All fields had received multiple applications of neonicotinoids. The samples were exposed to three rates (low — medium — high) of acetamiprid (Intruder). The mortality levels of those colonies were compared to a cotton aphid colony collected from a field near Stoneville, Miss., that had not been treated with a neonicotinoid.
Mortality of the Tchula and south Arkansas colonies ranged from 4 percent to 16 percent at the highest labeled rate of Intruder (1.1 ounces per acre, 0.05 pound active ingredient per acre) compared to 92 percent to 98 percent for the Stoneville collection.
No data were obtained for the colony from Alabama.
Additional collections will be tested, but these data illustrate the level of selection that occurs in individual fields following multiple applications of neonicotinoids.
Fortunately, producers will be receiving some relief from this pest as the cotton aphid fungus becomes established and reduces populations.
Currently, these and other data are being reviewed to determine if changes in recommendations for 2007 will be warranted.
Regardless of the products being used, the cotton aphid is being selected each time an insecticide is applied. In all likelihood, cotton aphid populations have been evolving and becoming resistant for several years.
In 2006, sprays were applied earlier than in previous years and more intense selection pressure has occurred. There is no way to gauge the severity of this problem in the future.
Cotton aphid susceptibility to the neonicotinoids may increase by next season, but it is very likely that some producers will continue to experience this problem, at least on a local level.
Many cotton producers in 2006 obtained excellent control with these products. Therefore, even though cotton aphid control problems with neonicotinoids have occurred in several states, these products will still be very important insecticides next year.
These data provide a justification for concern and diligence to proper insecticide use strategies. For example, it will be critically important to rotate chemistries (classes) while cotton aphids are present, not only when they are the primary target.
There are few options available for controlling aphids, so spraying only when absolutely necessary will be important to reduce the frequency of applications.
Currently, one of the only alternatives is flonicamid (Carbine), which has a slightly different mode of action from that of the neonicotinoids.
The OPs such as Bidrin, have been hit and miss on cotton aphids during the last few years and should not be relied upon as a single effective treatment.
Another factor that may compound the cotton aphid control issue is the use of the neonicotinoid seed treatments, Gaucho and Cruiser. The initial populations of cotton aphids that move into fields may occur as early as the first leaf stage and are exposed to levels of the neonicotinoids in plants. This is another level of selection that will promote resistance development.
Most of the labels for these products recommend a resistant management strategy that prohibits a foliar application of a neonicotinoid over a field of neonicotinoid-treated seed within 30 days of planting.
In the future, pest management strategies that can reduce applications of acephate or pyrethroids during the early season will be especially important to reduce the probability of flaring cotton aphids.