Since the Bagrada bug’s first appearance in low desert cole crop fields three years ago, University of Arizona research on the quarter-inch-long stink bug is now yielding answers to help farmers and pest control advisers (PCAs) better understand and control the pest.
The Bagrada bug, Bagrada hilaris, stunned farmers, PCAs, and chemical company representatives when it first appeared in the fall of 2009 in huge populations in California’s Imperial County and neighboring Yuma County in Arizona.
Bagrada bug adults and nymphs feed on young cole crop plants including broccoli, cauliflower, cabbage, brussel sprouts, kale, turnip, mustard, and radish. The insect’s needle-like stylet mouthpart sucks the sap from leaves; essentially killing or maiming cotyledons and growing points.
Feeding at the growth terminal can cause branching which produces small, unmarketable multiple heads of broccoli and cauliflower. Heavily attacked plants have a scorched appearance. The bug feeding causes large stippled or wilted areas on leaves. Newly formed central shoots or heads often become stunted.
The Bagrada bug, also known as the painted bug or harlequin bug, is a serious pest of vegetable crops in East and Southern Africa, Southern Asia, and Southern Europe.
The insect was first found in the U.S. in 2008 in California in Pasadena (Los Angeles County). The Bagrada bug then crawled into Orange and Ventura counties before pole-vaulting into low desert cole crop fields.
So far, Bagrada bug numbers have been low this fall. It was the September-November period over the last two years when numbers quickly increased.
“PCAs and farmers should not become complacent just because they are not finding any Bagrada bugs yet,” said John Palumbo, University of Arizona research entomologist based at the Yuma Agricultural Center, Yuma, Ariz.
“It would be wise to assume the bug will eventually show up in some intensity and to prepare for the insect accordingly.”
Such pest challenges breathe fire into the bellies of entomologists including Palumbo. Since the first infestation, Palumbo has worked many long days and nights seeking answers to perplexing questions about the pest.
Palumbo shared the results of his most recent YAC Bagrada bug trials with farmers, PCAs, and industry members during the 2011 Preseason Winter Vegetable Workshop in Yuma.
Palumbo’s preliminary research conducted over the past year suggests that direct-seeded and transplanted crops are most susceptible to Bagrada bug infestations during stand establishment.
“Research to date also suggests that heavy Bagrada bug infestation control with insecticides is the most economically viable option to protecting stands and preventing significant yield losses,” Palumbo said.
This includes chemigation with pyrethroids, and using contact insecticides (pyrethroids, Lannate, Lorsban) once plants emerge and the irrigation pipe is pulled.
After stands are established and plant size increases up to the 3-4 leaf stage, or on growing transplants, PCAs may consider alternating to dinotefuron (Venom/Scorpion) to protect plants from Bagrada feeding. This neonicotinoid also provides knockdown of whitefly adults and nymphs.
Part of Palumbo’s research strategy is to better understand the Bagrada bug’s feeding behavior.
“The bottom line is the Bagrada bug in the feeding mode tears up the leaf tissue,” Palumbo said.
Many stink bugs, including the Bagrada, use a ‘lacerate and flush’ feeding method. The stylet, about half the length of the insect’s body, pierces the leaves and sucks out the juice.
In a laboratory tests, Palumbo placed a four-to-five-day-old broccoli cotyledon in a plastic tub and placed a single Bagrada bug on the leaf. Another plastic tub was placed on top to keep the insect from escaping. It was the perfect feeding opportunity for the hungry insect.
UA research specialist Marco Pena harnessed a microscope, video camera, and time-lapse photography to record the feeding activity. A UA-produced video features the process. Palumbo showed and narrated the video during his workshop presentation.
“The Bagrada bug finds a point in the leaf and then the stylet comes in and destroys the leaf cells on the top of the leaf surface,” Palumbo said. “Look here,” Palumbo shouted. “We suspect the insect is depositing salivary enzymes in the leaf - not a virus or a feeding toxin - to help break down the cellular tissue.”
Initial feeding creates starburst-shaped lesions on the leaves and circular scorched areas as the plant cells die.
“This rapid feeding means the grower cannot afford to have the insect feed very long or else the plant will be stunted or killed,” Palumbo said.
Pena’s video was shot over a two-and-a-half-day period and was edited down to several minutes to showcase the feeding frenzy.
After the first one-half hour, the bug had tagged the first cotyledon. At six hours, the insect consumed a good portion of the cotyledon. At 12 hours, the cotyledons began to wilt. The first cotyledon was dead at 18 hours. The insect often fed from the bottom of the cotyledon leaf to the top side.
Next, the insect fed on the main terminal - the apical meristem – the plant part that, for example, produces a cauliflower or broccoli head.
“For growers, that’s where the money is made or lost,” Palumbo said.
At 60 hours (two and a half days), the plant was basically destroyed.
“Both cotyledons are essentially desiccated. The terminal is pretty messed up,” Palumbo said. “This is why we see a lot of blind plants with no heads, particularly at the cotyledon stage for seedling plants at stand establishment. This illustrates how active this insect can be under ideal conditions.”
Slow damage rate
In a field setting, plant damage by the Bagrada bug likely occurs at a slower rate.
When the Bagrada bug first appeared three years ago, farmers, PCAs, Palumbo, and others tried to quickly determine which currently registered insecticides could provide the best pest control.
Palumbo has conducted numerous field and greenhouse trials to investigate the effectiveness of various insecticides on Bagrada bugs. One YAC trial last year focused on various soil-applied neonicitinoid insecticides which target the insect’s central nervous system. The ‘neonics’ were banded on the seed line rather than injected 2 inches below the soil surface.
Palumbo’s question was upon plant germination and the sprinkler incorporation of the product could Bagrada bug numbers be greatly reduced.
“This application did not effectively protect the plants from the insect,” Palumbo said. “The cotyledon failed to load up with enough insecticide so damage was still rampant.”
In other studies, Palumbo investigated conventional and alternative foliar insecticides for Bagrada bug control. Much of the research is ongoing.
In summary, Palumbo offered these Bagrada bug management tips for low desert cole crop production.
First, fields near grassy areas (including sudangrass), weedy drains, and residential and desert areas may be at a higher risk for Bagrada bug infestations.
Second, monitoring and scouting for the Bagrada bug is important. The insect spends early morning hours in the soil. Sampling after 9 a.m. is recommended. Look for damage under cotyledons and young leaves. Look for adults on the underside of cotyledons and the soil.
Third, Bagrada bug control should include chemigation at emergence in high-risk areas. While farmers already chemigate for the flea beetle and other insects, Palumbo suggests another chemigation preferably four to five days before the plants are lined out for planting.
“Once the pipe is pulled, you need to protect the plants from the Bagrada bug,” Palumbo said. “Pyrethroids provide the best activity by contact. Other effective products include Lannate and Lorsban, followed by Venom, Scorpion and Belay.”
The Bagrada bug can also damage papaya, potato, maize, sorghum, cotton, capers, and some legumes.