As corn leafhopper continues to spread corn stunt disease and crop losses to California's Central Valley, several evaluated insecticides have only a short-term effect, suggesting that other measures, including a corn-free period, have more potential for success.
Charles G. Summers, University of California entomologist at the Kearney Research and Education Center, Parlier, says a corn-free period, such as from Oct. 31 to April 1, would help in management of the leafhopper.
The most important step for the time being is for growers to destroy volunteer corn plants that provide the insect, capable of overwintering in the valley, a bridge to infest the next crop.
Pesticides, including Thimet, Admire, Capture, and Meta Systox R, provided control in his trials, but only for relatively brief periods ranging from 10 days to 30 days.
Summers said it is not known whether control of four weeks with preplant Thimet or Admire reduced the amount of disease or if the chemicals allowed the plant to develop beyond a susceptible stage for the pathogen to have significant impact on yield.
He cautioned growers, if they decide to apply, to check first to see whether an insecticide is registered for their type of corn. For example, Admire, either foliar or soil applied, is not registered for use on silage corn, while Meta Systox R is registered only for sweet corn and not silage corn.
However, he added, any attempt to use insecticides to control the vector of a disease “is generally not particularly effective. We have data that show in some situations that the use of insecticides can exacerbate the disease more than doing nothing at all.”
Developing plant resistance to the corn stunt pathogen is another potential defense but will take time.
Taking yet another approach, Summers and others found in preliminary - but as yet inconclusive - studies that corn plants grown over mulch of wheat straw for a month after planting had reduced populations of corn leafhoppers.
Mulch trials also showed one-third the incidence of corn stunt disease and 25 percent more yield compared to plants on bare soil. The mulch might be used to repel aphids and whiteflies, as well as leafhoppers, he said.
Summers and a team of several farm advisors and UC specialists surveyed valley counties with sticky traps and D-vac sweeps in 2002 and confirmed the leafhopper can successfully overwinter in the valley counties. They are continuing observations this year.
According to Summers, while leafhopper damage can impair yield and quality, the disease poses the greater threat.
Corn stunt is caused by a bacterial organism, Spiroplasma kunkelii. Beyond stunting of plants, it causes production of multiple ears that fail to fill. In the fall, leaves in the upper portion of infected plants take on a reddish color. It goes to silage and other varieties of corn.
The corn leafhopper is common throughout the southeastern and southwestern U.S. Brownish or tan in color and less than one-eighth of an inch long, it is the only vector of corn stunt disease in California and resides in the whorls of stalks.
Spots below eyes
Seen under magnification, its major distinguishing mark from other leafhoppers is a pair of dark spots between and below its eyes.
Summers said the leafhopper, or something very like it, was first reported in Fresno and Tulare counties in 1942 and a disease fitting the description of S. kunkelii was recorded a few years later.
Today corn stunt is found from Los Angeles County to Yolo County. Outbreaks once lasted only one or two years and were blamed on leafhoppers migrating from Mexico, but since 1996 they have been annual events with increasing severity in the southern San Joaquin Valley.
The disease formerly was seen mostly in corn planted after July but later showed up in fields planted in April or May. “This suggests to me that both the insect and the disease were overwintering in the San Joaquin Valley,” Summers said.
The leafhopper has no diapause period and during winter cold it is inactive, only to resume flight when temperatures reach about 55 degrees. A survey during last winter's comparatively warm temperatures produced heavy catches from October into March on riparian areas, alfalfa, weeds, and triticale.
“We saw leafhoppers at 15 per square foot, so if you multiply that by the number of acres of alfalfa, you get an idea of the huge number of leafhoppers overwintering out there.”
Volunteer corn key
Summers said a “real key” to managing the leafhopper is destroying volunteer corn that provides a bridge between seasons. “After grain corn or silage corn is cut, volunteers always come up, particularly later in the fall. In the winter of 2002-2003, thousands and thousands of leafhoppers survived the entire winter.”
Even though some corn was frozen back, significant numbers found shelter enough in remaining intact whorls. They were discovered in Kings County in February on residue ears that had started to sprout.
Global imaging system surveys analyzed by University of California scientists in 2001-2002 showed heaviest and earliest infestations around the Lemoore area of Kings County, confirming earlier observations on the ground. However, the same surveys from 2002 through July of 2003 identified a shift of hot spots of leafhopper populations to Tulare County as pressure continued in Kings County.
Tulare County farm advisor Carol Frate said corn stunt appears to be scattered throughout the county and she expects that since much of the acreage is late corn, the problem will worsen by the end of the season.
“It's been increasing since 1996-97 and some fields had a 50 percent reduction in yield. We can see the insect in corn but we don't know what the loss will be. Unfortunately, we have a lot of late corn this year, and those fields are most at risk,” she said.
Although they have been found as early as March, infestations of new corn fields typically occur in July when both planting and chopping are in progress, Summers said. “As soon as a field with leafhoppers in it is chopped, they start to bail out and look for something else.”
In August and September, as chopping intensifies, younger fields catch the full impact of the leafhopper movement.