The Bagrada bug is back in low desert California and Arizona farm fields this spring, according to research entomologist John Palumbo of the University of Arizona (UA).

Palumbo discovered adult Bagrada bugs on cabbage in Yuma, Ariz., in mid-April. A pest control adviser found a large aggregation of adults feeding on a single London rocket weed in the middle of a watermelon field in California’s Coachella Valley.

“The Bagrada bug is back in 2010,” said Palumbo, based at the UA’s Yuma Agricultural Center in Yuma.

The Bagrada bug was first found in the low desert last August. Baffled growers, PCAs, and entomologists found chewed up plants in some cole crop fields in southern California and southwestern Arizona.

Adult and nymph Bagrada bugs suck sap from young leaves. Feeding causes small puncture marks visible as white patches on leaf edges. A heavily-attacked plant has a scorched appearance. The damage can result in plant death, plants without heads (“blind head”), or multiple heads too small for the commercial market.

The Bagrada bug is a stinkbug native to Africa, India and Pakistan.

Palumbo and PCAs quickly scouted cole crop fields seeking solutions.

“We kept seeing this bug, thinking at first it was a trash bug, and figured it had blown in from an adjacent cotton field,” Palumbo said.

Palumbo and Rick Klicka, PCA, Southwest Ag Service Inc., Brawley, Calif., carefully inspected organic cole crop transplants.

“The Bagrada bug was knocking the snot out of the plants,” Palumbo said.

Nine months have passed since the initial outbreak. Palumbo shared his knowledge of the bug during a standing-room only seminar at the 2010 Southwest Agricultural Summit in Yuma.

Palumbo’s knowledge is based on his experiences and YAC short-term greenhouse and field trials. In addition, Palumbo has read online accounts from scientists from other countries.

The Bagrada bug is a member of the Pentatomidae stinkbug family. The Bagrada bug is a major pest of Brassica crops, including seed crops and canola.

The Bagrada bug was first identified in the United States in California’s Los Angeles County in 2008. The pest quickly spread into neighboring Orange County. Last fall the Bagrada bug was found simultaneously in California’s Coachella and Imperial valleys, and Yuma County in Arizona.

“Bagrada bugs were everywhere in the low desert; we didn’t even have to look for them,” Palumbo said during last fall’s outbreak. He estimates crop losses from the Bagrada bug in some fields during the last crop season at about 45 percent to 50 percent in organically-grown cole crops and about 20 percent to 25 percent in conventionally-grown cole crops. Organic production represents about 5 percent of total cole crop production in the low desert.

Adult Bagrada bugs are 5-7 millimeters long with black, shield-shaped bodies with distinctive white and orange markings, according to the University of California’s Center for Invasive Species Research. The Bagrada bug has a strikingly similar appearance to the Harlequin bug.

Leaf damage from the Bagrada bug can be mistaken for herbicide damage.

Palumbo noted two distinct characteristics of the Bagrada bug. First, the insect is a true stinkbug, but smaller in size than the average stinkbug. The female is larger than the male. Second, Bagrada bugs are often found inside soil cracks. The bug tends to move in and out of the soil particularly in September and October seeking moisture and shade.

Palumbo conducted the YAC greenhouse trial by placing the Bagrada bug on potted plants. Eggs were found in the soil and on the plants. A single female can lay up to 100 eggs in two to three weeks. The incubation period is four to eight days.

The Bagrada bug goes through five nymphal instars. September and early October is the ideal period for development when temperatures average around 86 degrees.

The host range includes all cruciferous crops. Palumbo found the Bagrada bug last fall in canola, broccoli, broccoflower, cabbage, cauliflower, arugula, mustards, and rutabaga.

Online accounts list agronomic hosts including potato, maize, sorghum, cotton, wheat, pearl millet, capers, and possibly legumes. Weed hosts can include field bindweed, purple nutsedge, lambsquarter, black mustard, perennial sowthistle, and possibly sheperdspurse.

In Palumbo’s greenhouse tests, the worst damage from the Bagrada bug was to the terminal leaf, especially on transplants.

“The fast-growing terminal leaf was essentially desiccated by the Bagrada bug in as little as three days,” the entomologist said.

Palumbo’s trial included nine individually-potted broccoli transplants, each enclosed in a cage. A single Bagrada bug was placed on each plant.

“The Bagrada bug focused mostly on the young tender growth,” Palumbo said. “About 40 percent of the plants had wilted terminals after two days. About 30 percent of the plants had fully desiccated terminals after three days. Every terminal was desiccated after six days.”

What puzzled growers and PCAs last fall was the high number of damaged plants found in fields compared to the low bug numbers actually found. Palumbo has a theory.

“Many stinkbugs are more active at night than in the daytime,” Palumbo said. “I’m suggesting that this particular bug may be more active at night particularly liking the temperatures during September and October.

Palumbo compared the feeding time of day for the Consperse stinkbug which attacks cotton. Based on published research, the peak aggregation or nighttime scotophase for the Consperse bug is from 7:00 p.m. to 3:00 a.m. Findings are similar for the southern green stink bug in laboratory studies.

What is the recommended economic threshold for treatment for Bagrada bug?

“The economic threshold on crops grown in Africa is about one bug per-square-meter for seedling crops and three bugs per-square-meter for larger plants,” Palumbo said. “Until we know more about the bug’s behavior in the desert, we’re recommending a nominal threshold of 5 percent of infested plants.”

Available chemical control for Bagrada bug in the desert is a major issue. While pyrethroids are the predominant chemical option against the insect overseas, many of the pyrethroids are not registered in the U.S. Seed treatments, while not popular in conventional fields in those countries, are popular in organic cole crops.

Palumbo conducted half-season-long field efficacy trials with current and experimental insecticides. While the trials were initially designed to measure flea beetle control, Bagrada bugs were also present.

“Basically my first choice for Bagrada bug control would be carbamate and pyrethroid-based insecticides,” Palumbo said. “Neonicotinoid products also provide good control. Soil- and foliar-applied stink bug insecticides currently used by the cole crop industry can probably manage the Bagrada bug.”

Palumbo thinks the Bagrada bug has been around in the low desert for awhile.

“The question that baffles me is the Bagrada bug showed up (last fall) in Coachella, Imperial, Yuma, and Blythe at about the same time; it wasn’t an initial flight in August and they all blew in,” Palumbo said.

“I suspect they have been here for awhile in low numbers, perhaps coming into the low desert in 2008 in transplants,” Palumbo said. “We know cotton and other crops are hosts.”

“I encourage growers and PCAs to watch cotton and melons closely this summer for bug numbers and movement.”

email: cblake@farmpress.com