Beneficials help manage pests EDITOR'S NOTE - In this first of two parts, University of Georgia Entomologist John Ruberson takes a look at the increasing importance of beneficial insects in Southeastern cotton production.
Cotton producers come to realize the true value of natural enemies whenever they are disrupted, causing an insect pest explosion, says John Ruberson, University of Georgia entomologist.
"A classic example has been the beet armyworm during the Boll Weevil Eradication Program in some regions and years," says Ruberson. "Elimination of the armyworm's natural enemies through widespread organophosphate applications opened the door for this pest."
Natural enemies and biological controls, he adds, comprise a "silent service" in cotton. "They're out there quietly doing their job, even though we may rarely acknowledge them. But they become apparent by their absence and failure. Therefore, we must find ways to improve the impact of what we already have," he says.
By virtue of its complex structure and available nectar, cotton is an outstanding crop for supporting natural enemies and encouraging biological control, notes Ruberson. "There are numerous places to hide in a cotton plant, with the diversity of structure in the terminal, the fruiting structures and the large and intertwining leaves. The available nectar also is valuable for many natural enemies, as they readily can find the carbohydrates so often needed for egg production.
"From the plant's perspective, there's considerable potential for biological control to succeed in cotton. But how do natural enemies respond to this potential?"
In a survey of untreated cotton conducted in the early 1960s in Arkansas, more than 600 species of predators were identified in cotton, says Ruberson, and almost one-third of these species were spiders.
"Predators kill their prey outright, immediately removing them from the environment. In this respect, predators are a grower's best friend in the field. Many different predators can be found in cotton, but they vary considerably in their overall contribution to cotton pest management.
"Some species are generalized, attacking a variety of pests. For example, fire ants and big-eyed bugs attack a wide range of prey, including eggs, caterpillars and other natural enemies. On the other hand, some predators have a more restricted range of prey that they'll attack."
In addition to the predators, there also is a substantial number of parasites, he says. Parasites, he explains, contribute to overall pest mortality by attacking their hosts and killing them sometime afterwards.
Thus, unlike predators, parasites typically have a delayed effect on the target pest. But even though many parasites fail to assist growers immediately during pest outbreaks, it's still very worthwhile to conserve them, says Ruberson.
"Some of the parasites that attack caterpillars slow down the development and feeding of the host. For example, the wasp Cotesia marginiventris attacks young caterpillars - mostly armyworms - and greatly reduces their feeding. It also kills the armyworm after a short time, so that plant damage by the parasitized armyworms is greatly reduced."
Such a diversity of natural enemies has great benefits, notes Ruberson, but it also can be very confusing. One of the most common questions asked by growers is, "Which beneficials are most important?"
"Unfortunately, we don't know. The diversity of natural enemies makes it difficult to assess what each one can and does contribute, and then to fit all of the pieces together. A further difficulty is that many of the predators are quite as content eating other natural enemies as they are eating pests.
"So, when a predator is faced with a choice between a bollworm egg and a juicy young spider, it may opt to eat the spider. Added to this confusion is the variability in the numbers of species of natural enemies from field to field. The range of responses makes it very difficult to predict how natural enemies will respond to a given pest density."
Despite the confusion, there are some natural enemies that clearly can be counted on for support, says Ruberson. Perhaps the most prominent of these is the big-eyed bug.
"Two species dominate in cotton - one is silver and the other is black. These bugs can be found in cotton from the seedling stage until harvest and are active predators of a variety of pests, including eggs and larvae of bollworms, budworms, loopers, armyworms and cutworms. They also attack plant bugs, fleahoppers, stink bug eggs and young nymphs, aphids and whiteflies."
Another important contributor, he continues, is the red imported fire ant. In the southern reaches of the Mid-South, this ant can make a substantial contribution to pest management.
Lady beetles can be very effective against cotton aphids, says Ruberson. In combination with parasitic wasps, lady beetles can dramatically suppress cotton aphids. "On a side note, lady beetles are poor indicators of the effects of pesticides on beneficials. There are numerous reports of lady beetle populations increasing after a pyrethroid or other harsh insecticide application."
One of the most important parasitic wasps is the small braconid Cotesia marginiventris, he says. This wasp is highly effective against beet armyworms and Southern armyworms. It also can parasitize bollworms, budworms, fall armyworms, cabbage loopers and soybean loopers.
Another valuable parasite is the aphid parasite, Lysiphlebus testaceipes. "When this parasite is abundant, it can cause rapid and dramatic reductions in aphid populations, and their activity is easily detected as healthy aphids are replaced by swollen and hardened aphid `mummies.'"
There are three criteria of importance when selecting a species to scout for and incorporate into decision making, says Ruberson. These include, 1) visibility - scouts must be able to detect it; 2) consistency - the species must be a rather consistent component of the landscape; and 3) relevance - the species must play a relevant role in the system.
For example, he explains, lady beetles satisfy two of these criteria, but their relevance is limited mostly to cotton aphids. The big-eyed bug satisfies all three but will require additional training of scouts to know what to look for and when and where to look, he says.
In the next issue of Southeast Farm Press, Ruberson looks at how producers use natural enemies.