Earlier predictions for moderate to heavy lygus pressure in California cotton fields have luckily been a little off the mark so far this season.
“The early season lygus pressure hasn't been as heavy as we projected,” says Pete Goodell, IPM entomologist at the Kearney Agricultural Center in Parlier, Calif. “There's some scattered locations where early lygus treatments were required, but in general, the pressure is pretty light, and the fruit set is looking very, very good.”
Lygus, also known as the tarnished or western tarnished plant bug, was the second most damaging pest Cotton Belt growers faced in 2001, infesting 55 percent of all U.S. cotton, according to the National Cotton Council's annual insect loss report. While the Mid-South states and Arizona reported the highest losses from the tarnished plant bug, the pest infested 407,400 acres of California cotton in 2001, causing an estimated yield loss of 10,185 bales, the report says.
In his 2002 Lygus Population Projection, Goodell's forecast called for moderate to heavy lygus populations along the east and west rim of the San Joaquin Valley.
The lygus predictions were based on roadside surveys in the San Joaquin Valley during April and May, as well as weather conditions and potential host plant populations.
Rainfall in April and May allowed for the germination and development of short-pod mustard, a reproductive host plant that is very abundant along roadsides and in uncultivated fields, according to Goodell.
“Lygus was found to be very abundant on this host no matter where it was sampled in May, with populations numbering as high as 10 adults and 30 immatures found in 50 sweeps of these plants. These insects probably represent the second generation of lygus,” he says. “The immature population was several weeks from reaching adulthood, but the mustard plants appeared to be capable of supporting the population until adulthood.”
Despite lower than expected lygus numbers, Goodell reminds growers to remain vigilant in their scouting for the pest. “In general, we suggest control not be initiated until a resident population is confirmed to be present, usually by the presence of immature insects or by populations remaining above treatment thresholds for two or more inspection periods.
Cotton growers and pest control advisers should check a field twice within three days before treatment is applied to insure the population is still present.” Goodell says current lygus treatment thresholds are provisional, based on the number of pests found per 50 sweeps and current fruit retention on the cotton plant.
In general, he says, pre-bloom cotton should be treated when you find at least four lygus per 50 sweeps. Once bloom gets going and boll set is coming on strong, the treatment threshold increases to eight lygus per 50 sweeps with the presence of at least one nymph. Later in the season, just prior to open boll, Goodell recommends a threshold of 10 lygus per 50 sweeps. “All of these treatment thresholds may be modified up or down depending on square retention,” he says.
Goodell says, “We're anticipating that as crops bordering cotton are prepared for harvest during July and August, lygus populations may move from field to field. The populations moving from other fields, however, can generally be handled with the existing chemistries we have available.”
For those lygus populations moving into your cotton from a neighboring field, Goodell suggests “knocking down” any infestations with an application of Provado, Centric or Steward. “These products are usually considered suppressive materials, but they seem to knock the lygus down below threshold levels. Dimethoate and Vydate are also being used for single knockdown situations,” he says.
May need pyrethroid
Cotton fields with heavy pressure from lygus infestations may require a pyrethroid treatment for residual contact. However, Goodell recommends treating with a pyrethroid only under “extreme population pressure,” in order to prevent flaring aphid populations.” One unique approach when aphids are present in the field is the use of Leverage, a premixed combination of Baythroid and Provado insecticides.
Growers faced with a slow, continuous infestation coming in over three or four weeks from a neighboring river or creek, may find that a treatment of Temik, applied at the first or second irrigation application, can provide several weeks of residual activity, according to Goodell.