With the San Joaquin Valley's Pima crop off to a healthy start, could this be a year for growers to consider tolerating some square loss from lygus to save chemicals and dollars that could be used for potential insect problems later in the season?
Conceding he was “sticking his neck out,” Pete Goodell, University of California IPM specialist, offered that thought to attendees at the recent Pima Production Summit, 2004 at Visalia, adding that his impression is lygus will move into cotton from other crops in the mid- to late-season.
“I'm not suggesting you let lygus run away with the crop, but on the other hand, if you lose 10 to 20 percent of the squares and the plants go vegetative and build a larger frame, they could be managed in July and August for somewhat higher yield,” he said.
Although the aphid threat remains to be seen, Goodell suggested growers consider impact on yield potential from aphids that appear in mid-season.
“A lot of growers are worried that if they don't control aphids early they will get out of hand. But we've seen situations where populations in August were not a problem.”
Silverleaf whitefly (SLWF) populations should be monitored closely and insect growth regulators should be the first line of defense in mid-season, Goodell said. Products with different modes of action should be applied to head off resistance.
With the early crop this year, he added, growers need to follow plant development, not the calendar, in defoliating to finish the crop and prevent as many pest problems as possible.
Hinge on plant management
This season appears to be one that will hinge on management of cotton plants, not management of insect pests, he said. “It looks like it will be a case of production limitations rather than limitations from insects.”
On the basis of his annual survey of lygus habitat in the SJV, Goodell said, “generally there's no place for lygus to come from because it all dried up in March and April.”
Exceptions, however, are mustard weeds that support lygus along roadways and another lygus host weed, londonrocket, in some fields. Alfalfa fields around cotton will mitigate some of the pest's movement.
Temperatures, he noted, drive SLWF populations, and this year the pest's development is about 12 days ahead of average. “We had a mild winter and we should be watching the early populations and using insect growth regulators when they reach treatable numbers. This will manage populations in local fields, but we can also expect late season movement from other crops and other cotton fields.”
To simplify identification of insecticides, Goodell said, compounds are being classified by their modes of action in a new system developed by the Insecticide Resistance Action Committee (IRAC), a group of chemical companies, researchers, and regulators.
“They are educating the general agricultural industry about resistance management with a mode of action index, using numbers and letters, such as pyrethroids as 3A or organophosphates as 1B.”
An important effect of the index, he added, will be to make it easier to use certain classes of products early in the season, while conserving others for potential problems later in the season.
Sticky cotton, particularly in Pima, has been a priority in recent years for another summit speaker, Larry Godfrey, UC entomologist, who is probing for new practices to eliminate causes of the soiled cotton that causes severe problems for textile mills.
As with many other IPM practices, management of the stickiness problem has been largely based on experience with Acala, but refinements for Pima are being added, Godfrey said.
Although the jury is still out on susceptibility to other insect pests, researchers have known for years that Pima, with its longer growing season, is more vulnerable to aphids and SLWF that excrete honeydew, the sugary source of the stickiness.
Although a rain can wash off SLWF honeydew from cotton in the field, the aphid deposits persist.
The critical time for SLWF control is the last four to six weeks of the season, and Godfrey said the unregistered materials Oberon, Diamond, and V-10112 showed promise in his 2003 trials. These, he added, hopefully will in time supplement existing compounds Danitol, Orthene, Century, and Warrior.
Godfrey started concentrating on stickiness after PCAs and growers reported that practices used against it in Arizona were not successful in California's more complex cropping systems having hosts such as tomatoes, sugar beets, and corn.
Problem with both
California, Godfrey explained, has both aphid and SLWF in the same fields, while in Arizona the problem is SLWF and in Texas the problem is aphid. Unfortunately, what's done in the SJV to control aphid can explode SLWF and vice versa.
In his research last year, Godfrey sought to improve sampling techniques, but SLWF pressure was light and late. He did learn that SWLF adults first cluster in the top of the plant and later spread, but aphids tend to be evenly distributed across the plant.
Although Pima was once considered more resistant to spider mites than Acala, in recent years of increased Pima acreage and new varieties more of it is being treated for these pests, Godfrey said.