Cotton beneficials: Three approaches EDITOR'S NOTE - In this second of a two-part series on the increasing importance of beneficial insects in Southeastern cotton production, University of Georgia entomologist John Ruberson looks at the three general approaches to using natural enemies.
The three general approaches to using beneficial insects in cotton production include conservation, importation and augmentation, says John Ruberson, University of Georgia entomologist.
With the diverse complex of natural enemies present in cotton, there is great potential for getting help from conservation biological control, says Ruberson. The most readily available method for many growers is to use selective insecticides or selective rates, he says.
"Good materials are available for managing pests that also are less disruptive to natural enemies. Formerly, only Bt's and growth regulators were available to growers interested in conserving natural enemies. However, because these materials were not always effective, growers continued to use broad-spectrum insecticides," he says.
The growing threat of pyrethroid resistance in budworms and bollworms, and the availability of effective new materials, have enhanced grower interest in selective materials, adds Ruberson.
"The use of Bollgard varieties has contributed to the reduced use of pyrethroids, especially during the early and middle portions of the season. In addition to Bollgard, many insecticide manufacturers are actively pursuing insecticides with positive environmental profiles.
"Many of these compounds show substantially better selectivity than their predecessors. Currently, Tracer is available and has a very favorable profile. Other materials, like Steward and Assail, are in the pipeline and provide good levels of pest suppression without causing serious damage to the natural enemy complex," he says.
Unfortunately, notes Ruberson, the cost of these new materials is higher than the old standards. Nevertheless, conservation of natural enemies can reduce the number of insecticide applications needed and enhance profit margins.
Sometimes, beneficial species can be conserved to some extent by reducing the rate of insecticides, he says. Reducing the rate by 15 to 20 percent of a pyrethroid or organophosphate can make a big difference in the survival of some natural enemy species.
"This approach is a bit problematic, however, as its success depends on the presence of at least some effective natural enemies in the field. This may or may not be the case. If it is not, then reduced rates may provide less than the desired suppression of the target pest, risking additional loss and possibly requiring additional insecticide treatments close on the heels of the first treatments."
In addition to the selective use of insecticides, opportunities exist to create a cotton field that encourages natural enemies and their activity, notes Ruberson. "There's still a lot of work to be done in this area, but some technology now is being used which can help promote biological control. Cover crops and conservation tillage provide an excellent opportunity to encourage natural enemies and biological control.
"The presence of cover crops during the winter provides a habitat in which natural enemies can over-winter. Cover crops also can support alternate hosts or prey for natural enemies that are important in cotton."
Various cover crops and field plantings are being investigated for their potential to encourage natural enemies in row crops, he says. As more information is obtained, there may be new opportunities for using field and border plants to improve biological control in the field, he adds.
The importation of exotic natural enemies may provide some long-term biological control of certain pests, says Ruberson. In the West, for example, a parasitic wasp that attacks plant bugs has been introduced from Europe. This parasite has potential, he says, and is being considered for release in the Southeast.
"If this parasite successfully establishes, we shouldn't expect all of our plant bug problems to end. However, establishment of the parasite would contribute to widespread, permanent reductions of plant bugs, which would certainly help growers."
Another possibility for introduction, although further into the future, is a parasitic fly that attacks Southern green stink bugs. This fly comes from South America and has been introduced successfully into South Africa and Australia, according to Ruberson.
Augmentation of natural enemies in cotton has been studied for many years with very mixed results, he says. This method requires users to release natural enemies, typically purchased from commercial suppliers.
"Much of the work has concentrated on the parasitic wasp Trichogramma species - parasites of moth eggs - and green lacewings, Chrysoperla species - predators of aphids and other soft-bodied insects.
"Numerous studies with Trichogramma have failed to demonstrate consistent improvement in egg parasitism when these parasites are released. These releases simply have not provided consistently predictable results. Results with lacewings have been similar."
Augmentation currently cannot be recommended as a consistently reliable approach, says Ruberson. The approach has merit, but it requires timely releases of high-quality natural enemies to be successful, he adds.
The best current approach to using beneficials is through conservation, he says. "Our best option is to conserve what we have, particularly in the early to mid-season period. Allowing the natural enemies to build in this window can help reduce the need for frequent insecticide applications. And, if broad-spectrum insecticides are needed later in the season, the natural enemies will be sufficiently abundant around the fields to re-colonize quickly."
Several issues need to be addressed before growers can make biological control a consistent and predictable component of cotton pest management, says Ruberson.
"First, we must get a better handle on what the natural enemies are doing. If they have multiple prey, which ones will they prefer and how much? How much do natural enemies eat one another? And, how does natural enemies eating natural enemies indirectly affect pest populations?
"Secondly, we need to better understand which natural enemies occur when and where so we can better predict their occurrence. Third, more information is needed on the interactions of natural enemies with cropping practices, such as pesticides, varieties and tillage. Fourth, continued education is necessary to be aware of what is out there and what it is doing. As more information becomes available, we need to put it in the hands of growers and other users."