2010 was a year that many winter cole crop vegetable growers in the Desert Southwest would rather forget, thanks to the bagrada bug which attacked plant seedlings en masse.
Since then, research conducted at the University of Arizona and the University of California has led to a better understanding of the pest, its biology, and has helped reduce yield and income losses for growers.
When the bagrada bug made its 2010 grand entrance, winter vegetable growers, pest control advisers, and entomologists were stunned.
“The pest caught us blind. Suddenly the bagrada bug was everywhere in the desert,” says John Palumbo, University of Arizona (UA) Extension specialist and entomologist based at the Yuma Agricultural Center.
Palumbo’s phone lit up like a Christmas tree. He jumped in the truck and headed to the fields. The first time Palumbo saw an adult bagrada bug he thought it was a trash bug moving out of recently defoliated cotton.
That fall and winter, cole crop growers suffered an estimated 14-percent yield loss on average in direct-seeded broccoli, according to follow-up surveys Palumbo conducted with pest control advisers. The average maximum loss was 36 percent.
“Some growers disked their fields under,” Palumbo said.
Yield losses since then have averaged about 6 percent; still a significant financial loss for cole crop growers.
Fast forward to the present. Palumbo and the UA’s Ta-I Huang, along with researchers at the University of California, Riverside – including Darcy Reed, Tom Perring, Nilima Prabhaker-Castle, and Jocelyn Miller - have learned a great deal about the destructive pest, Bagrada hilaris.
The scientists have a better understanding of the insect’s biology; vital information to help develop effective management solutions.
Palumbo discussed the latest bagrada bug research findings during the 39th annual California Association of Pest Control Advisers Annual Conference and Agri-Expo held in Reno, Nev. in October.
Cole crops are members of the Brassica family which include cabbage, broccoli, cauliflower, and other vegetables. While many plants can serve as hosts for bagrada - including alfalfa, Sudangrass, and cotton - the pest prefers to chow down on cole crop seedlings – vulnerable transplants and young plants after seed emergence.
Bagrada a devastating pest
The pest enters the field when the seed is pegging before stand establishment.
Palumbo said, “I’ve seen them knock out plants and suck them dry resulting in desiccated cotyledons.”
He believes a volatile chemical in the plant attracts the bagrada insect.
On the biology side, the bagrada bug is a very small stink bug from the Pentatomidae family; also known as the painted bug and harlequin bug. Yet the bagrada is about half the size of the harlequin.
Bagrada is about the same size as the sevenspotted lady beetle and one fourth the size of the Say’s stink bug. The female bagrada is slightly larger than the male.
The bagrada has a typical five instar stink bug life cycle. The small egg is about the size of a cabbage looper or corn earworm egg. Through the instar progression, the insect’s body gets darker in color with more orange, white, and black.
The bagrada bug is a warm season pest which thrives in the heat. Optimum average temperatures for bagrada growth range from 86-95 degrees Fahrenheit in the Desert Southwest in July, August, and September. As the temperatures cool off, the insect’s biological development slows.
“In the desert, this is a warm-season pest that infests a cool-season crop,” Palumbo said.
The pest attacks the underside of leaves during the day, and hides at night in the soil and under dirt clods.
The bagrada bug can quickly destroy a seedling. In Palumbo’s trials, a single insect placed on a cotyledon killed the plant in about 60 hours under laboratory conditions.
In another lab test, small pots were lined up in a row, each containing one of 12 different vegetable seedlings. The bagrada passed right by the head lettuce to feast on cole crops. Its feeding favorites include green cabbage, red cabbage, and radish.
“We like to say the bagrada bug has never met a brassica plant species it won’t attack or feed on. There is a lot of truth to that,” Palumbo told the PCA crowd.
If the plant lives, the damaged plant develops multiple unmarketable small heads instead of a single large marketable head or floret.
Palumbo added, “One grower suggested developing a market for baby cabbage. This is a good idea, but unfortunately sometimes the plant won’t even grow a head the size of your fist.”
Pest marches into Central California
Since its 2010 debut, the bagrada bug has marched in heavy numbers into some Ventura County cole crop fields. Bagrada is found in isolated areas in the Salinas Valley and on the Central Coast. This year, Palumbo received numerous queries from the Fresno County area.
First found in South Africa, the insect arrived in the western hemisphere in the U.S. in 2008 in California; possibly as a stow-a-way on a cargo ship arriving at the Port of Long Beach. The insect then scurried into neighboring Orange County and kept moving.
Some desert PCAs call the starburst-shaped feeding marks left on plants “tattoos.”
The bagrada bug does not always feed on the cotyledon. It will attack the plant’s first true leaves. If the plant survives, then split, forked, or multiple terminals lead to the unmarketable produce.
Pesticide use during the first 20 days of stand establishment from August through November averaged 4.3 sprays in 2010; 3.4 sprays in 2011, and 4.1 sprays in 2012, according to the survey results from PCAs.
A common question growers pose to Palumbo is when does the bagrada bug stop damaging the plant so growers can discontinue sprays. Trials conducted in Yuma County and the Coachella Valley in 2011-2012 young broccoli plantings revealed a possible answer.
“The bottom line is once the plants get to about the 6th leaf stage then you are probably out of danger,” Palumbo said. “Most of the PCAs I’ve talked with in the desert agree with this.”
The late morning and afternoon are the best times for PCAs to scout for bagrada. Yet since PCAs must cover a lot of ground daily, early morning scouting should look for pale-colored feeding marks on plants. If found, look for damage on the surrounding plants.
Palumbo has conducted several trials with synthetic insecticides and natural predators. While he said bio-control is a ways off, pyrethroid insecticides currently provide the most effective control.
“Newer pyrethroids on the market appear to be more consistent with good knockdown and residual control.”
Residual activity usually lasts about five days. He estimates that the cost of each spray in the desert totals about $20-$25 per acre.
Looking to the future, Palumbo says the best insecticidal control of bagrada may lie in neonicotinoid seed treatments, based on trial findings.
“This might help minimize yield losses and the number of needed sprays,” Palumbo concluded.
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