Development of the first generation of adult lygus in Arizona hasn’t peaked in non-cotton hosts — the first 100-degree temperature is when females should begin laying eggs.
Green nymphs begin emerging about a week later. But, the pest that is normally the biggest yield-robbing insect in Arizona’s cotton isn’t likely to exert more than a light to moderate threat to growers this year, says Peter Ellsworth, University of Arizona Extension IPM coordinator and director of the Arizona Pest Management Center.
His early season assessment is based on cropping and weather patterns and sweep netting he’s done this spring in alfalfa fields and experiment trial plots of lesquerella, a lygus host.
“Lygus are out there, reproducing, and happy,” he says. “But their density is fairly unremarkable at this point. Although it’s still very early in the season, so far the situation looks very similar to the past two years.”
For one thing, he says, few fields in the state are planted to safflower and seed alfalfa, which bloom in the spring and early summer. The volatiles in their flowers attract large numbers of adult lygus to these plants, where they reproduce very well, releasing more lygus as those crops senesce. Early in the summer, after alfalfa fields are harvested or safflower begins to mature, lygus start moving into cotton.
With relatively few acres of these crops, any lygus pressure from them this year would be localized, Ellsworth says.
However, alfalfa grown for forage also can become a source for lygus, particularly if the area is all cut at once, forcing adults out of that crop and onto cotton and other nearby hosts. These local factors can be as important in local lygus pressures as any more general factors, like winter rains, Ellsworth says.
Also, little precipitation this spring and past winter should limit growth of the other potential sources of adult lygus — desert plants and weedy species, such as London rocket, black mustard, and globe mallow — which also bloom in the spring and attract the insect. In fact, he says, there have been no significant stands of these plants in the state since the El Niño winter of 2005.
“In general, we’ll continue on this low trajectory for most insect pressures in Arizona this year, with lygus being the principal one.”
Over the last five years the number of insecticide applications made by Arizona farmers to control all cotton insect pests has averaged just 1.5 per season, Ellsworth says. Of those, about half have been to control lygus. Last year, 29 percent of all cotton acres in the state were not sprayed for any insect pest — that’s the highest level of untreated cotton acreage ever.
That achievement actually increases pressures on growers and PCAs to make the right insect control decisions.
“Back when we were spraying frequently, it was relatively simple to spray routinely after a certain interval when a product had lost its activity,” he says. “But, when it’s the first of August and you’ve been out in the field every week without spraying, it’s easy to start doubting yourself and wondering if you might have missed something. That’s when close monitoring of fields and a lot of experience allows you to know when to hold off spraying and when the insect pressure is high enough to pull the trigger.”
In the meantime, Ellsworth advises growers to focus on the fundamentals of controlling lygus:
- Be aware of all sources of lygus that pose a threat to your crop.
- Monitor your fields regularly.
- Keep on top of crop developments in your surrounding area.
He plans to introduce a computer simulation program at his workshops, beginning in June, where growers can see not only how different crop planting patterns affect their own insect pest risks, but those of their neighbors as well — and not just for one season, but for multiple seasons.
“We’re approaching a new era in which farmers will have to manage insect pests in terms of the dynamics in their own fields and in the landscape around them” he says. “The idea is to learn about the distributions of host crops and the ability of lygus to move among different crops, then to exploit these landscape factors for our productive and economic benefit.”
- Be patient.
In his research, Ellsworth has found no insecticide to be terribly effective in killing adult lygus. Also the eggs are laid within plant tissue, where they are protected from insecticides.
It’s hard to stand by and watch adult lygus invading your cotton fields without taking immediate action. But, patience is the best approach, he says.
“The key is not to overreact,” Ellsworth says. “It makes no sense to spray on day 1 of an invasion. It will be a minimum of seven days before the first eggs hatch and, because its mouth parts are too small, the first instar nymph doesn’t damage the plant. Then, it takes another seven days before the nymphs develop into the larger instars that cause damage to the crop.
“So, if you apply an insecticide any sooner than two to three weeks after an invasion starts, you’re likely wasting money.”