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Desert cotton – insect sweeping, identification

Entomologist Peter Ellsworth excitedly peered into his sweep net to check for good and bad critters from the southern Arizona cotton field he had just swept.

“This is a Drapetis fly which is an excellent whitefly predator in cotton,” said Ellsworth as he magnified the insect with his 8X lens. “There is a false chinch bug which can become a real pest as a nymph early in the season, but is of no concern as an adult in cotton later.”

Ellsworth, University of Arizona (UA) integrated pest management specialist based at the Maricopa Agricultural Center in Maricopa, Ariz., sweeps crop fields regularly looking for beneficial and bad insects.

He shared his insect sweeping and identification wisdom on the road this summer during hands-on field days for cotton growers, pest control advisers (PCAs), and others in cotton fields in Buckeye, Marana and Safford, Ariz.

The UA-sponsored cotton field day in Marana, located just north of Tucson, was held in mid-July at Clark Farms, owned and operated by Tom and Miley Clark. The 40-acre field, planted in the Stoneville 5458 upland variety, was about three weeks behind schedule in development due to the cool, wet spring.

“The human eye, hand lens, and correct sweep techniques are all crucial to becoming a good bug man,” Ellsworth told the Marana crowd. “The sweep net is invaluable.”

Visually inspecting cotton plants for whiteflies is critical. First, find the top unfurled leaf on the plant at least a quarter in size. Then move down five leaves to the fruiting branch.

The branch underneath is the subtending leaf; it is attached directly to the main stem of the cotton plant. Ellsworth says this leaf provides for the best estimate of whitefly population levels.

“The fifth leaf is the one you want to turn over and inspect for adult whiteflies,” Ellsworth explained. “This is the first physiologically mature leaf. If there are three or more adults on it, count the leaf as infested.”

It is important to know which plants to inspect. Check plants that have not been brushed up against and are moving. Do not cast a shadow over the plant during inspection as insects may fly off. Do not check plants where the cultivator has recently passed. Check plants not recently swept with a net.

Inspect the leaf with a “special bug man magnifier lens,” Ellsworth says. The lens focuses on a quarter-sized disk area. Wedge the lens between the main vein and the left lateral vein so that the circle touches the sides of those two veins and check for nymphs. Inspect for any live third and fourth instars; only one counts the leaf disk as “infested.” Identification is possible with the naked eye, but a lens provides more accurate counts.

“If you are a bug scout you should have a lens with you,” Ellsworth said. “I have been walking cotton fields for 20 years. I cannot walk through, kick the bushes, look in the net for adult whiteflies, and determine if a field needs an insecticide application. The lens allows for accurate whitefly nymph counts; information critical for timely spray decisions.”

Ellsworth passed around several leaves taken from different parts of the plant. With magnifiers in hand, the participants closely identified insects. The most common insect found was the banded-winged whitefly, Trialeurodes abutilonea Haldeman, native to the Marana area and the first whitefly to colonize in these local fields.

The banded-winged whitefly is a typical disrupted pest like aphids, mites and thrips. At about 105 degrees most banded-winged whiteflies cycle out completely. Normally this whitefly serves as good food for other insects and is beneficial to help maintain pest control in fields.

“The banded-winged whitefly generally stays below any economic density until you spray inappropriately, especially early in the season, and kill the parasitoids and the predators which normally keep the pest in check.”

Ellsworth then found the more common sweetpotato whitefly, Bemisia tabaci. Whiteflies are more commonly found in stressed areas of fields, for example plants yellowed by hypoxia from water-logging. Whiteflies are attracted to the color yellow. Another major factor that makes for attractive plants is water stress. Consistently late irrigations can do two things: drive good insects out of the field while bringing more whiteflies in. Ellsworth says scout the plants leaf-by-leaf.

Field scouting and the up-close lens examination delivers key details on good and bad bug thresholds.

“A person can become proficient in one hour of training and it’s easy. The hardest part is determining the fifth leaf. It takes more familiarity with cotton and more practice before you can grab and turn the right leaf over as you rapidly walk the field,” Ellsworth said.

It takes about seven minutes on average to scout a 40-60 acre cotton field for nymph and adult whiteflies. Better decisions are made when the actual insect types and numbers are known.

“You can’t accurately scout cotton without using a sweep net in Arizona,” Ellsworth said.

Ellsworth suggests these sweeping points. Allow a fist-sized air pocket distance between the top of the net and the top of the plant during the sweeping motion. Move the net vertically across the plant. A quality sweep requires physical strength.

“Sweeping is a good way to cut down on gym memberships,” Ellsworth noted.

A 15-inch diameter net is standard. A wooden handle, versus a metal or plastic handle, is best in excessive summer heat.

Ellsworth recommends 100 standard sweeps per field on which all Arizona guidelines are developed and reported. Fifty sweep increments are more common for control guidelines in California cotton fields. When a tough decision must be made on insecticide control, Ellsworth says 100 sweeps is necessary to avoid making the wrong choice.

Be careful on how much plant debris is collected by a sweep. If little debris is collected in the net, the sweep motion is not hard enough. A too strong “Paul Bunyan” sweep can break plant terminals. Ellsworth suggests sweeping cotton plants 18-inches and taller.

Do not sweep the edge rows. Sweep at least 50 feet into the field. A sweep across the row counts as one sweep and the next sweep returns backwards on new undisturbed foliage with the backhand across the same row as a consultant briskly walks a field.

“I normally do 25 sweeps, and then walk a zigzag pattern further into the field,” Ellsworth said. “I take another 25 sweeps in a new area and then move across the field to an entirely different area for two additional 25 sweep sets; 100 total sweeps per field.”

It is best to limit sweeps to 25 at a time. Too much plant material in the sweep net makes sorting more difficult and may crush some smaller insects making identification very difficult.

The physical force of the sweep can vary depending on the plant’s growth stage, seed variety, branching status, and row filling.

Ellsworth prefers a two-handed sweep approach. One-handed sweeps are tough on the body.

Several members of the crowd armed with sweep nets took off into the field to practice the sweep techniques. An inspection of one grower’s net yielded three species of lady beetles, a minute pirate bug, and cotton’s nemesis insect — the lygus bug.

Adult lygus bug adults are about .25 inch (6 mm) long and .1 inch (2.5 mm) wide, and flattened on the back, according to the University of California IPM Web site. The color varies from pale green to yellowish brown with reddish brown to black markings.

Adult lygus have a triangle in the center of the back. Nymphs are uniformly pale green with red-tipped antennae early on as a first instar. Five black spots are on the upper body of larger nymphs.

Lygus bugs threaten cotton from early squaring through cutout and final boll set. The insect pierces cotton squares and damages anthers and other tissues, causing square shed and reduced yields.

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TAGS: Cotton
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