The corn leafhopper (CLH) continues to be a serious economic pest of corn in the San Joaquin Valley, but cleaning up volunteer corn plants around fields can go far in managing the insect and the costly corn-stunt disease it vectors.
That's advice from Charles G. Summers, University of California (UC) entomologist at Parlier, who described the CLH during a recent corn production meeting at Tulare.
CLH causes damage in the millions for growers and dairymen. As it feeds on corn plants, it deposits sticky honeydew that produces sooty mold, but that's only part of the loss.
“This leafhopper poses a double problem,” Summers said. “It causes damage by its feeding, but the principal concern we have is its transmission of the corkscrew-shaped bacteria Spiroplasma kunkelii, which is responsible for corn-stunt disease,” he told the assembly of growers and PCAs.
Corn-stunt symptoms show as stalks only half the normal height, with close internodes, producing six or seven small ears instead of the normal one or two. The multiple ears, however, do not fill with grain.
CLH, distinguished from other leafhoppers by prominent dark spots between its eyes, is more tolerant of winter cold than other leafhoppers. “The past winter was about as cold as we've seen in the last ten years, and this insect came through it with flying colors.”
Tests at the Kearney Agricultural Center at Parlier showed it can survive temperatures of minus 5 degrees Farenheit, for up to 48 hours.
Although the vector and the bacteria can survive severe winters in the field, the pathogen is killed by heat treatment of planting seed.
CLH, he added, has been collected by researchers throughout the winter months in riparian areas, in alfalfa, in winter forages, and in weeds. When corn is not available, it readily goes to alfalfa, triticale, oats, barley, and wheat. “Practically all the winter cereals can provide enough sustenance for it without corn.” Corn following corn only contributes to an infestation.
The volunteer plants from infected cobs that fall to the ground are a reservoir for the bacteria, which is known to overwinter only on corn.
Germinated by rains as early as February, the seeds sprout to support more stunt disease which is, in turn, taken up and distributed by CLH. Each female lays several hundred virtually invisible eggs over a week's time, so populations build rapidly.
Summers said his “take-home” message to growers in dealing with CLH and corn-stunt disease is to “do the very best job you can in cleaning up volunteer corn plants, because that is the major issue in the problem being overwintered.”
Corn-stunt disease was first discovered in the SJV in 1996 and has been confirmed in Fresno, Kern, Kings, and Tulare counties. CLH is known to be in those counties and others to the north.
In discussing spider mites on field corn, Larry Godfrey, UC Davis entomologist, said their management hinges on the stage of the crop when infested, whether the crop is grown for silage, earlage, or grain, and the use of new effective products so phytotoxicity from Comite is less an issue.
Spider mites, particularly two-spotted spider mites, are important because they have 150 crop hosts, females live for 30 days and deposit more than 100 eggs each, they overwinter in soil and on plants, and they move about with wind currents.
Two-spotted spider mites present a significant challenge because of their ability to develop resistance to chemical controls. Godfrey said more than 250 examples of resistance have been recorded worldwide versus only small fractions of that amount by Pacific and strawberry spider mite species.
In managing resistance, he said there are “no magic answers, just good IMP practices,” which include pest monitoring, use of economic thresholds, use of multiple strategies, correct timing of applications, alternating classes of chemistry, correct rates and application, and protection of beneficial species.
Godfrey pointed to the currently California-registered controls: Comite, Capture 2E, and Oberon. Comite can be phytotoxic, so it must only be used on certain varieties and applied with adjuvants to wet leaves.
Resistance in mites can occur in some crops but can be managed. Capture has a shorter residual and may also provide control of some insect pests. Oberon, registered in 2006, provides very effective control with a new mode of action.
He said extensive sampling studies on mites on field corn in the SJV were done in 2000-2002 and, although some insights were gained, no real answers for growers emerged.
Mite control is worthwhile, Godfrey noted, citing a 1990 study in Stanislaus County showing a 1.2 ton per acre silage loss in untreated versus Comite-treated corn.
He warned that new pests are expected to invade California corn crops from the Midwest. These include western corn rootworm, cucumber beetle, striped cucumber beetle, and European corn borer.
In sharing some tips on corn growth, Carol Frate, Tulare County farm advisor, said development is first by vegetative (V) stages and then, after the tassel stage, by reproductive (R) stages.
She based her talk on a publication from Iowa State University, and noted that the amount of time needed for a corn plant to go through the stages varies with the hybrid and temperatures, and can be affected by stress. If the process is too rapid, yields will be less.
Although traced with degree-days, the process is not as precise as that for cotton. By the V9 stage, all the leaves have formed and the plant demands more nutrients and water.
She said at this point the corn plant does not develop a large taproot but several roots at about 45 degrees to the vertical, so care must be taken with cultivation to avoid the roots.
By the stages V12 to V17, kernel rows and ear size are determined and tassels start appearing. “This is the most critical time to avoid water and nutrient stresses because they can greatly affect yields,” Frate said. This is also the period between the two weeks prior to silking and the two weeks after silking.