Poor fruit retention on the bottom five fruiting branches, the critical sources of about half the crop, has been a concern for cotton growers in California's Sacramento Valley.
Growers typically found high retention through early flowering, but by the end of the season, bolls — and yield — were reduced on the lower branches.
“We were puzzled as to what was causing the fruit loss,” said Doug Munier, farm advisor for Butte, Glenn, and Tehama counties, who began trials in 1997 to determine the cause.
Munier did testing on the Finch Ranch. Judy Brown, part owner of the ranch near Orland, at first saw good retention on the top branches, but as the crop matured and those same branches reached the lower half of the plant, the fruit disappeared.
“It seemed like insects were the most likely suspects,” she said.
Lygus was ruled out since it only damages the fruit on the top five fruiting branches, but Munier, after considering several possibilities, initially suspected the cucumber beetle.
“There were a lot of cucumber beetles out there in the field, so we tried Adios with Sevin. We controlled them, but it made absolutely no difference in the final yield,” Munier says. He concluded the cucumber beetle wasn't responsible for the yield loss and isn't a threat to cotton.
Meanwhile, Brown closely monitored the lower branches during bloom. In a two-day period, she discovered that fruit retention on the bottom five branches decreased from 90 to 70 percent.
The fallen fruit was collected and inspected. Small round holes were found in the flowers. “We identified it as the beet armyworm coming in mid- to late-flowering, and it was taking off a lot of the early fruit,” Munier says.
In California, beet armyworm moths may be found year-round. Female moths can deposit as many as 600 eggs over a three- to seven-day period, then die four to 10 days after emerging from pupae. The eggs hatch within two to three days. The entire life cycle is four to five weeks, and several generations occur each year.
The beet armyworm tends to be more of a foliage feeder in the San Joaquin Valley, although it can damage fruit there too, but in the Sacramento Valley, this pest doesn't appear to do much foliage feeding.
“They zero in right on the fruit,” Munier said. Older larvae will leave irregular holes in leaves and on squares, flowers, and bolls.
The adult beet armyworm moth has a wingspan of 25 to 32 millimeters. Its forewings are gray or brown with a pale spot near the center of each wing. Eggs are laid in masses and are white to pink in color, slightly peaked on top, and covered with grayish white, hairlike scales. Moths are found on the upper part of the plant and the top half of the leaf.
Clusters of small green or black caterpillars, with dark heads and three lightly colored stripes running the length of the body, emerge from the eggs.
A small black spot is on either side of the body, and the spot usually becomes visible when the caterpillar reaches seven to eight millimeters (the spot may be difficult to see on darker caterpillars). Larvae can reach lengths of 25 to 30 millimeters.
Brown tracked the armyworm moths with pheromone traps and used the information to determine her spraying schedule for the pest.
Timing was the key, Brown said. “If you have a sharp peak of moth flights, it means most of the eggs are deposited within a day or two of each other. Then you spray approximately three weeks later, and you get most of the newly hatched worms.”
If the flights aren't sharply peaked, it's more difficult to know exactly when to spray. “In that situation, a material that kills the eggs or has a longer residual will be a better choice,” Brown said.
“We've tested a number of different materials. A lot of it comes down to economics,” Munier said. “If you have another pest at the same time, one material might be better over another. If it's strictly armyworm, you can choose one of the least expensive.”
Munier used Larvin and Success during his tests on the beet armyworm. Two other insecticides, Confirm and Steward, not registered at the time but since labeled, were also used. Brown said she was prepared for the armyworm for the first time in the 2000 season. “We had a much better bottom crop. At least a quarter-of-a-bale increase. The best we've ever had.”