For Convenience’s Sake Don’t Sacrifice Efficacy
February 26, 2009 For convenience’s sake, growers are adding insecticides for thrips, cotton aphids and tarnished plant bugs to over-the-top applications of herbicides such as glyphosate. For efficacy’s sake, this method of co-application is not recommended unless action thresholds to trigger insecticide applications for target pests have been reached.
“We need to underline those recommendations in red,” says Dr. Roger Leonard, research entomologist, LSU AgCenter, Winnsboro, LA. “The use of a convenience application, as an attempt to manage weeds and insects at the same time, will frequently fail. The treatments usually do not provide as effective control, compared to the strategy of controlling either the weed or insect pest problem independently. The farmer tries to tie the timing to both, and weeds and insects are not always in sequence.” One or the other may end up as too early, or more importantly too late.
Even worse, an unnecessary application that increases production costs and may flare populations of a more expensive-to-control pest, such as cotton aphids or spider mites.
Seed Treatment Acceptance
The acceptance of seed-applied insecticides is undeniable, and Leonard estimates that at least 80% of Louisiana’s cotton acreage in 2007 will be planted with insecticide-treated seed. And that could lead to problems associated with using subsequent applications of the same class of chemistry, or even identical active ingredients on cotton seedlings that emerged from insecticide-treated seed. As an example, the active ingredient thiomethoxam is the insecticide component in Cruiser and Avicta seed treatments, and in Centric, a product used as a foliar spray; the active ingredient imidacloprid is found in Gaucho Grande and AERIS seed treatments and in the foliar-applied Trimax-Pro. Both thiomethoxam and imidacloprid are classified in the neonicotinoid class of chemistry.
A potential problem could develop when, for instance, Gaucho Grande is used as a seed treatment, Trimax-Pro is used for cotton aphids in June, and then Trimax-Pro is sprayed for tarnished plant bugs after that. What you have done is use three applications of the same active ingredient, imidacloprid. A similar situation would be using Cruiser- or Avicta-treated cotton seed and subsequent early season applications of Centric.
“Even if one pest at a low level is not the primary target,” Leonard continues, “that population is being inadvertently selected or exposed to the insecticide. So if you use the same product two or three times for one pest, and then come back and use it again for another pest populations that has been previously exposed to that chemistry, the probability of consistent and successful control is decreased.”
Generally Speaking
Generally, the neonicotinoid class of chemistry is still maintaining effective levels of control. But a number of field reports in recent years suggest that effective control against some target pest populations has been inconsistent. Says Leonard, “In 2006, there was an indication in Louisiana, Mississippi and south Arkansas that some populations of cotton aphids expressed decreased sensitivity to some of the neonicotinoid insecticides.”
Dr. Jeff Gore, a research entomologist with USDA/ARS in Stoneville, MS, agrees, saying, “We gathered some aphids from fields near Tchula, MS, and brought them back to run some tests in the laboratory. We compared them to aphids we had collected here at Stoneville. What we did was start with the highest rate of Intruder” – Intruder is a neonicotinoid with the active ingredient acetamiprid – “and we found at Stoneville, the mortality rate was 95%. The two field populations had mortality rates of 4% and 16%.”
As for plant bugs, Dr. Gordon Snodgrass, also a USDA/ARS research entomologist at Stoneville, looked at 15 locations in 2005 to determine the LC50 (a measure of the lethal concentration it takes to kill 50% of the target pest) for that year. In 2006, he returned to those areas and found that nine of the 15 locations showed an increase in tolerance to imidacloprid. Tolerance in 2006 was 2.5 times greater than in 2006.
“That may not mean anything yet,” says Snodgrass. “But it shows a general direction and the LC50 is not going down or staying the same; it is going up.”
So what is the answer if you used an imidacloprid or thiomethoxam seed treatment? A good option for the first insecticide application, says Gore, would be Bidrin, an organophosphate. “Bidrin will show some suppression of cotton aphids, but more importantly, it will not flare cotton aphids and is very good on thrips, too. It is more expensive than pyrethroids and Orthene, but it will save us money in the long run.”
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