I have been monitoring tarnished plant bug resistance to acephate since the 1998 growing season.
Last season, some of the highest levels of resistance to acephate were recorded in several populations throughout the Mid-South during August and September. Those high levels were detected when the majority of our cotton crop was beyond the stage where we would spray for tarnished plant bugs.
Generally, insecticide resistance declines during the winter months. This occurs due to fitness costs associated with the genes that confer resistance, so we essentially start each season with what would appear to be a clean slate.
The genes for resistance are still in the population, but at relatively low levels in the spring.
Once applications of the insecticide are made during the season, resistance levels can increase rapidly because all of the susceptible individuals are killed and those that survive the application are likely to have the genes for resistance.
Those resistant tarnished plant bugs mate with each other and pass the resistance genes on to their offspring, thus increasing the overall resistance level in the population.
Subsequent sprays with the same chemical or class of chemical will be less effective and further increase the level of resistance.
We have experienced this in the past with tobacco budworms, and more recently tarnished plant bugs, with resistance to the pyrethroids.
Growers and consultants in the Delta generally accept the fact that no more than one application of a pyrethroid will provide any level of control of tarnished plant bugs. After that one application, the level of control declines rapidly.
It appears that we are now headed in a similar direction with the organophosphates.
Not too long ago, growers and consultants were pleased with the results they were getting with 0.5 pound of acephate. During the past couple of years, we may have made our first application with 0.5 pound, but with subsequent applications that rate steadily increased to 0.75 pound to 1 pound of active ingredient per acre.
Monitoring during May of this year suggests resistant populations survived the winter and resistance is still at alarming levels, particularly for this time of year. Most of the tarnished plant bug populations tested last fall had similar or higher levels of resistance this spring.
Those populations may not be effectively controlled with acephate, even during the pre-flowering and early flowering stages of cotton plant development.
Resistance ratios are measures of how resistant a population is compared to a susceptible population. Simply stated, the higher the resistance ratio, the greater the level of resistance.
In the hill region of Mississippi, resistance ratios of tarnished plant bugs to acephate averaged 2.73-fold; those from the Delta regions of Mississippi, Arkansas, and Louisiana averaged 2.72-fold during May.
In the hill region, five out of 10 locations had resistance ratios of 3.0-fold or greater. In the Delta region, six out of 20 populations had resistance ratios of 3.0-fold or greater.
We generally believe that a 3.0-fold level of resistance to acephate is the breaking point at which control failures can be expected. Therefore, based on results of tests in May, acceptable control may not be achieved at some locations, even if acephate had not been previously applied in the area.
Generally, acephate provides effective control during the early flowering period, but efficacy declines later in the season. What we may see at some locations is that even the first application of acephate will not effectively control tarnished plant bugs.
During the later part of the 2005 growing season, field tests were conducted on a population with a 3.6-fold level of resistance to acephate.
Orthene (0.5 and 1.0 pound active ingredient per acre), Bidrin (0.5 pound active ingredient per acre), Baythroid (0.033 pound active ingredient per acre), and Vydate (0.33 pound active ingredient per acre) were applied to cotton in the field.
Tarnished plant bugs from the resistant population were caged on the terminals of those plants.
None of those insecticides were effective at controlling the bugs. The highest levels of mortality were observed with 1 pound of Orthene and 0.5 pound of Bidrin. Mortality never exceeded 50 percent by 48 hours after treatment with any treatment.
The results are alarming from the standpoint that acephate and Bidrin are the standards for tarnished plant bug control. Even more alarming is the fact that few new chemistries have been labeled in recent years for tarnished plant bug. The few that have been labeled are more expensive and do not provide the same level of control that was expected with acephate and Bidrin.
The increase in resistance to acephate may be the result of multiple applications throughout the season.
Also, once unacceptable control begins to be experienced with recommended rates, the general reaction from growers and consultants is to follow up that application with a second application and generally at a higher rate. This will compound the problem by increasing the selection pressure for resistance.
A general recommendation would be to delay applications of the organophosphates until they are absolutely necessary for tarnished plant bug control.
Rotations with other classes of insecticides will be a key component to managing resistance in tarnished plant bugs.
Generally, insecticides from the neonicotenoid (Trimax, Centric, and Intruder) class are applied during the pre-flowering stages of cotton plant development. They generally provide good control of tarnished plant bugs; however, we must avoid making repeated applications of these insecticides as well or we are likely to end up in the same boat with this class in a few years.
Diamond and Carbine are two newer insecticides with different modes of action that have tarnished plant bug activity. These insecticides will likely prove to be valuable for managing resistant populations.
Gordon Snodgrass is a research entomologist with the USDA Agricultural Research Service, Southern Field Crops Insect Management Laboratory.