Greenbugs are invading North Texas wheat fields in “the heaviest numbers we've seen in a number of years,” according to Texas A&M Integrated Pest Management specialist Jim Swart.
Swart said numbers are especially high in early-planted fields. “Infestations are so heavy in places damage can be seen from the road,” he said.
He recommended that farmers scout fields carefully and determine if infestations and potential damage, compared with yield potential, justifies control.
“Greenbugs can be controlled with insecticides at a reasonably low cost,” he said.
First symptoms include discolored spots where leaves begin to turn brown. “If these localized areas are not sprayed with an insecticide, the greenbugs can move out to healthy plants.”
Swart said greenbugs cause more concern than other aphids because of their two-pronged attack on wheat plants.
“They inject a toxin into the plant when they feed. Heavy feeding causes localized leaf tissue damage, which can ultimately cause death of the leaf.
“If several leaves are affected on small plants, the whole plant may die. Heavily damaged plants may not recover, and do not produce a normal grain yield.”
Swart said farmers should consider several factors before deciding to spray for greenbugs.
- Plant size
“Large plants can tolerate up to three times as many greenbugs as small plants that have not completed the tillering process. Populations of 25 to 50 greenbugs per foot of row may justify treatment in smaller wheat.”
“The recommended split application of nitrogen appears to have stimulated wheat growth and tillering. In many cases, these plants have put on new leaves and tillers, which is enabling them to outrun the greenbug populations.”
- Planting date
“The later-planted fields (approximately 90 percent of the acreage) exhibit much lighter populations than older wheat. Most of these fields are not in need of spraying now but should be closely monitored.
“In many cases, a greenbug spray decision can be put off until topdressing time, just around the corner for most growers.”
- Time of year
“Late fall and early winter greenbug populations can explode, increasing twenty-fold in one week. This is because greenbugs thrive and reproduce rapidly in cool temperatures.
As temperatures increase in late February and early March, a tiny parasitic wasp that attacks greenbugs becomes active and begins to reduce greenbug populations. When 20 percent of the greenbugs are parasitized, populations crash within a week.
“We are just now beginning to find a few parasitized greenbugs in some fields, but they are still at relatively low levels.”
The key, Swart said, is to scout carefully and keep track of building populations.
Greenbug populations were heavy around San Angelo earlier this winter, and farmers sprayed dimethoate or Lorsban “on a considerable number of acres,” said Chris Sansone, Extension entomologist.
“Infestation was heavy from just after Christmas until about a month ago,” Sansone said, “but producers knew they needed the grazing or had hopes of harvesting grain, so they treated, and numbers are back down.”
He said beneficial insects also are beginning to take a toll on the pests.
“We see mostly lady beetles,” he said. “With warmer weather, beetle numbers are picking up.”
Sansone said the February rain improved wheat outlook considerably. “It's amazing how much better it looks. It was hurting from drought.”
A similar situation exists around Uvalde where Extension entomologist Noel Troxclair says a few farmers sprayed for greenbugs earlier in the season.
“Some of those applications may have been to populations below thresholds,” he said. “We haven't seen anything of economic concern so far.”
He said a long stretch of cool, misty weather encouraged greenbug populations. “But a warm period helped beneficials catch up and they are keeping aphid populations in check.”
Troxclair said greenbugs are still around and “if it stays damp and cool, we could see an explosion. It doesn't take long for aphid populations to blow up.”
Overall, wheat looks “as good as I've seen in several years, although it will be a bit later than usual.”
Ronnie Leps, Extension entomologist in Williamson County (central Texas) said greenbugs rarely pose economic threats to wheat in his area.
“Since 1979, I've seen very few cases I could justify treating greenbugs,” he said. “With warmer weather, wheat grows out of damage and beneficial insects come in to keep populations suppressed.
“At this stage of wheat growth we don't worry much about greenbugs. And we've had so much rain they haven't been able to do much damage.”
Leps said central Texas wheat has done well. “It's growing and is in excellent shape. Our next concern will be disease pressure.”
Other wheat production areas of Texas appear to be largely unaffected by greenbug populations. “We've seen very few greenbugs,” said Roy Parker, Extension entomologist at Corpus Christi.