Confections See Spots: Lygus Suspected
Tuesday, January 1, 2002
filed under: Insects
Confection Sunflower Seeing Spots
Probable cause is the Lygus—an insect that doesn’t even prefer sunflower as a plant host
Kernel brown spot is becoming a quality problem in confection sunflower, and industry experts suspect that the probable cause is the Lygus bug—an insect that doesn’t even prefer sunflower as a plant host, but moves into it when other broadleaf crops dry down or are harvested.
Confection processors have been noticing an increase in small, brown spots on the meats of confection sunflower, resulting in kernels that are unattractive to buyers, and distasteful to consumers. Oil sunflower hulled and used for confectionery purposes are equally susceptible to brown spot.
The Lygus bug (also called tarnished plant bug) is believed to be the cause of brown spot in confection sunflower kernels. The insect feeds on flowering broadleaf crops such as alfalfa and canola, then moves to late-season crops such as sugarbeets and sunflower. Crop scientists suspect that in the process of feeding on developing sunflower kernels, the adult Lygus (pictured here) injects digestive enzymes, and then extract nutrients with its needle-like mouthparts. The feeding activity is microscopic and can’t be seen with the naked eye, but the damage becomes apparent later as brown spots on hulled sunflower kernels.
The problem does not appear to be specific to any particular hybrid or affected by planting date, though currently there is little research data to confirm these observations. The problem appears to be less frequent in the High Plains, but has become more noticeable over the past two years in the Northern Plains confection growing region (ND, SD, MN, southern Canada). The problem appears to be sporadic. Some confection sunflower haven’t been affected, while other seed samples gathered this year from Northern Plains confection areas have ranged between 1-7% brown spot damage. One confection sunflower sample collected near Baudette, MN had almost 50% brown spot.
Processors are allowed only one half of one percent (0.5%) brown spot damage in their finished product. After seeds are hulled, kernels damaged by brown spot can be removed with some success by color sorting machines. However, for in-shell confection sunflower, the problem is virtually impossible to detect.
“It puts our markets and our reputation of the U.S. as a consistent, quality supplier at risk. Buyers are saying ‘we’ll work with you this year, but get it under control.’ So as an industry, we need to address this,” says Bob Majkrzak, president and CEO of Red River Commodities Inc., Fargo, N.D.
What’s causing it? Some speculate a plant disease. Tom Gulya, USDA-ARS plant pathologist, Fargo, says researchers in Israel have implicated a type of fungus called Alternaria. However, spray trials in ND, conducted by Gulya and USDA entomologist Larry Charlet, appear to rule out the involvement of Alternaria.
More evidence points to insect activity as the cause of brown spot, with the Lygus bug as the prime suspect. USDA spray trials conducted in cooperation with Dahlgren and Agway near the ND cities of Grace City, Harvey, Barlow, and Edmore, had significantly less brown spot on sunflower treated with Warrior insecticide (registered for seed weevil control), compared to the untreated check (see table). Also, in preliminary greenhouse studies at USDA, confection sunflower were grown in pots, with five Lygus bugs bagged onto individual plants. At maturity, the seeds were found to have brown spots similar to those observed on field samples.
Table 1 % Kernel Brown Spot in Four Spray Trials in 1999
This preliminary study conducted by USDA near Grace City, Harvey, Barlow, Edmore, ND, in cooperation with Dahlgren and Agway, indicated less brown spot in confection sunflower applied with insecticide compared to the untreated check.
Hybrid Untreated Foliar Fungicide Warrior
Insecticide Folicur &
Rh 3703 1.56% 1.76 0.12 0.21 0.91 a
Myc 9338 1.41 1.17 0.15 0.16 0.72 a
Bigfoot 2.54 5.26 0.24 0.12 2.0
(Isd=1.09) 1.79 a 2.73 a 0.17 b 0.16 b
This research was partially funded by the NSA with grower checkoff dollars
The lygus bug, also called the tarnished plant bug, can thrive on dozens of cultivated plants (and weeds), and prefers flowering broadleaf plants. A “Lygus Summit” was held late November, 2000, in California, where many of the state’s 250 crops serve as hosts for the Lygus. Of 54 crops covered by the University of California Integrated Pest Management Guidelines, only 10 list a discussion of and control recommendations for Lygus bugs, notes UC entomologist Charles Summers, in proceedings from the summit. Some crops, such as carrots, onions, and lettuce, although a host for Lygus, do not sustain economic injury when grown as a food crop. These same crops, however, are severely injured by Lygus feeding when grown for seed, according to Summers.
Crop scientists surmise that the sharp increase in broadleaf crops the last few years in the Northern Plains may be a key reason why the Lygus population is increasing. The insects seem to prefer softer plant tissue and small-seeded broadleaf plants. Canola, alfalfa, soybeans, sugarbeets, mustard, safflower, buckwheat, mustard, crambe, and sunflower are all susceptible to Lygus. “Although it’s my suspicion that sunflower is probably not a preferred crop, it moves to sunflower and whatever else is still green when other crops are harvested,” says North Dakota State University entomologist Gary Brewer.
Lygus research in Canada is more extensive than it is in the United States, due in part because Canada is a leading producer of canola, a crop that’s highly attractive to the Lygus. There are at least nine Lygus species, and entomologists believe Lygus Lineolaris is the species that is most prevalent in the Northern Plains.
Adult Lygus bugs are about a quarter inch long and an eighth inch wide. Their color can range from dark greenish yellow to brown. Older adults will usually have a distinctive mottled coloration with light wing tips and a pale yellow V-shaped mark near the middle of the back. First-stage nymphs are very small, wingless and bright green in color. Both adults and nymphs are very active, elusive, and usually hide or drop off the plant as soon as the canopy is disturbed. They may look similar to aphids, but are much more mobile and will migrate from crop to crop as harvest occurs.
Ian Wise, entomologist with the Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg, says the Lygus infest crops when the plants are producing buds, flowers, and pods. Adults overwinter in plant residue, and early developing cool season plants such as canola and wild mustard, and perennials such as alfalfa, are common host plants for the first generation nymphs in late May and June. Adults from these nymphs appear in late June and July, depending on the temperatures during nymphal development, and migrate to plants with buds or flowers. The adults feed on the buds and flowers and the females lay eggs in the stems or leaf midribs. The second generation nymphs that emerge from the eggs can reduce yields by feeding on individual seeds, buds, or flowers. Adults from the second generation develop in August to early September, and soon migrate from the crop to search for new food sources.
Adult Lygus may continue to feed on a variety of plants for up to six weeks before overwintering. Crops that typically are late maturing or were seeded late—such as sunflower—often serve as host plants for the second generation adults, according to Wise.
Lygus feed on plants by injecting plant tissues with digestive enzymes, and then extracting nutrients with their needle-like mouthparts. Their feeding either damages or kills the affected area of the plant.
Crop scientists believe adult Lygus feeding activity on immature, developing seed is what’s causing brown spot in confection sunflower. The feeding activity is microscopic and can’t be seen with the naked eye, but the damage becomes apparent later as brown spots on hulled sunflower kernels.
It’s not known if yield is affected, but if the insect does destroy sunflower florets as it feeds, it’s possible the consequence would be fewer seeds per plant, though some experts point out that yield loss may be remote, since individual seeds within heads have the ability to compensate for the loss of adjacent seeds. Lygus activity may result in shriveled seed and thus less test weight, but there is little to no research data to substantiate that.
Nearly all major flowering crops are attacked by Lygus, but the severity of damage to the crops is highly variable, Wise says. Lygus can reduce crop yields directly by damaging individual seeds, or indirectly by killing buds, flowers, and immature pods. The large differences in damage by the plant bugs to crops are caused by the suitability of the crop as a host plant for nymphs, the ability of the crop to compensate for or tolerate feeding injury, and the timing of the crop developmental stages most attractive to adults or sensitive to feeding injury with that of the life history of the plant bugs.
Controlling the problem
NDSU’s Brewer suspects sunflower is susceptible to Lygus damage during flowering, from anthesis through seed hardening. Organophosphate (Lorsban, Methyl Parathion, Parathion) and pyrethroid (Asana XL, Baythroid, Scout X-Tra, Warrior) insecticides have shown success in controlling Lygus. Although none of these products is currently labeled for control of Lygus on sunflower, the good news is that Lygus can be treated at the same time confection sunflower is treated for other insects, such as the seed weevil and banded sunflower moth. “Control the moth and weevil, and you’ll get the Lygus too,” says Brewer.
Entomologists believe two treatments are needed to sufficiently control Lygus: One application at the onset of pollen spread or approximately 10% bloom, then a second treatment 7 days later. This regimen should adequately control insects on confection sunflower throughout flowering.
Confection sunflower growers, like Reg Herman, Brinsmade, ND, say less spraying for insects has occurred over the last several years, because of the low incidence of red seed weevil. But processors have been noticing an increase in insect damage over the last two seasons, much of it due to banded sunflower moth. Since it appears that a two-spray regime will take care of the weevils, the banded moth and the Lygus, “we should look at it as an insect control package, until we learn more about the Lygus,” says Larry Kleingartner, executive director of the National Sunflower Association.
More Research, Awareness Needed
The NSA organized a meeting in Fargo recently to discuss kernel brown spot, and ways to address the problem. The sunflower producers, processors, and crop scientists present agreed that a survey of confection sunflower processors is needed, to gain insight into the current extent of the problem. Such a survey should provide a geographic map of the area affected, which then can be compared to weather patterns of the past few growing seasons. Further, it will provide some data on the level of insect control by those producers who did not see damage.
Crop scientists also plan to compile and disseminate more detailed information about Lygus management this winter, including further recommendations on treatment. Also, formal label inclusion of the Lygus (2EE label status) will be sought for insecticides used on sunflower.
Crop scientists also want to conduct more research on kernel brown spot, the Lygus, and their cause-and-effect relationship, including whether other variables are involved. Key questions that research may address:
? At what growth stage is sunflower most susceptible?
? What impact does the over-wintering generation of Lygus have on sunflower versus in-season generations of the insect?
? What is the impact of cropping systems in the various production regions?
? How much control of Lygus can be expected from spray programs used to control other insects in sunflower?
? What is the economic threshold for confection sunflower?
? How many generations of Lygus occur in the Northern Plains and the High Plains?
? How does the Lygus specifically affect kernel quality, and will it affect the quality of kernels in storage?
? Can seed be X-rayed or some other process used to determine brown spot damage, instead of having to hull seed?
Sunflower leaders also want to build awareness that the Lygus is not just a potential sunflower problem; that it poses a threat to all producers who include broadleaf crops in their cropping systems. Indeed, a key conclusion of the Lygus Summit in California last November is that a community-based, regional management approach involving coordinated strategies across a host of crops will be needed if the primary objective of an overall reduction in the state’s Lygus population is to be realized. – Tracy Sayler