What about aphids? It's a question that was on the lips of many cotton producers this past year as the tiny insects settled in for a lengthy stay in Southeastern cotton fields.
“In the past, growers have ignored aphids, and the insects tended to go away,” says Mark Abney, a University of Georgia entomology graduate student. “Other areas of the Cotton Belt haven't been as fortunate. In other regions, cotton aphids have become a major concern. And, we've had some prolonged, severe infestations in Georgia.”
The infestations seen in Georgia this past year raised questions from growers about the impact of aphids on the cotton crop, says Abney.
“Early season aphid infestations have the potential to stunt seedling cotton. Even larger, older cotton can be stunted by aphids, especially if other stresses such as drought conditions are involved. Reduced lint quality is a problem typically encountered in more arid regions,” he says.
The biggest concern for Georgia growers from aphid infestations would be reduced yields, notes Abney. Unfortunately, it's difficult to correlate aphid numbers and aphid feeding with cotton yields, he adds.
“This makes it very difficult for us to know when or if to treat aphid infestations. We really don't know how they're hurting us,” he says.
Georgia researchers, says Abney, have evaluated the natural enemies of aphids and looked at how these might be incorporated into management decisions. Part of this research included tracking aphid and natural enemy populations in a Georgia cotton field. This tracking included insect natural enemies and the entomopathogenic fungus Neozygites fresenii.
“The fungal pathogen is our Number One natural enemy of aphids both in the Southeast and the Mid-South, and it has been since the mid-1980s. It's been extremely effective, and that's why we've been able to ignore aphid populations over the course of the past couple of years,” says Abney.
Once cotton aphid infestations are established in a field, there are options for suppressing them, he says. “One option is using the insecticide imidacloprid or Provado. We have seen very good suppression with this material. Other chemistries are available, but imidacloprid is soft on beneficial insects. Some of our other chemical options can be devastating on natural enemies.
“In terms of biological control — from a suppression standpoint — insect beneficials aren't going to work very well. But the cotton aphid fungus actually works as well as our insecticide applications in suppressing aphid populations.”
Turning to cultural control methods, Abney says the severity of an aphid infestation in cotton can be made worse by drought stress. “If you can irrigate cotton and avoid stress, you'll help yourself in terms of decreasing the amount of damage caused by aphids.”
The most effective biological control of aphids is the cotton aphid fungus, he says, but many growers are asking if they can afford to wait for this fungus to take effect.
“In past years, this hasn't been a problem. But this past year, before we saw the fungus, we had prolonged aphid infestations that caused real damage to cotton. In the future, can we continue to ride out the infestations and wait for the fungus, or should we use an insecticide?”
Research in the past two years has attempted to answer these questions, says Abney. In both years, the imidacloprid treatments were effective at suppressing aphid populations.
“Our populations increased in late June and early July. We made Provado applications on July 5 in 1999 and on July 6 in 2000, and aphid numbers were suppressed quite well. But about a week after we applied the insecticide, aphid populations in our untreated plots also declined.
“This is the effect of the aphid fungus or parasitic wasps. This leads to the question of whether the insecticide treatments can be justified economically.”
Tests conducted in 1999 and 2000 concluded that when the aphid infection rate reached 12 to 15 percent in the field, the aphid populations declined rapidly.
“This is not to say that we shouldn't be concerned with conserving natural enemies — we should.”
“That would indicate that if we knew we had 12 to 15 percent infected aphids in the field, then we wouldn't want to apply an insecticide. The problem is knowing when aphids reach this point of infection without having to send off a sample to be tested.
Can't see it
“At this point of infection, you can't see the fungus in the field. If you get an identification kit, you may be able to save yourself an application. If not, it'll be another week or so before you actually can see signs of the fungus in the field.”
Research also looked at how aphid infestations impacted the yield and quality of Georgia cotton, says Abney. “We didn't see any differences in yield in 1999 or 2000, regardless of the treatment. It didn't matter that we sprayed an insecticide and killed the aphids. “There also were no differences in lint strength or micronaire. It didn't matter if we sprayed for aphids, or if we did nothing.”
A conclusion of the research, he says, is that the cotton aphid fungus is an effective way to control aphids. “We already knew that. The big question is if we can rely on the fungus in the future if we continue to have these prolonged aphid infestations early in the growing season.”
A second conclusion, he adds, is that insect natural enemies were not reliable in terms of suppressing aphid populations. “This is not to say that we shouldn't be concerned with conserving natural enemies — we should.
“There is potential for beneficial insects to prevent aphid infestations later in the season. They also may prevent flare-ups early in the season.”
Good suppression of aphid populations was achieved through the use of imidacloprid. But the insecticide treatments were not economically justified in this particular study, says Abney, because the treatments were made early in the season when the fungus was effective.
On the question of whether aphid infestations cause cotton yield loss, Abney says there still are a lot of gaps in the knowledge, and researchers will continue to explore the issue.