Spider mite control in corn aided by moisture

by Kay Ledbetter, Texas A&M AgriLife

The Texas A&M AgriLife Extension Service has a list of do’s and don’ts when it comes to spider mite treatment on corn, according to Dr. Ed Bynum, AgriLife Extension entomologist in Amarillo.

Spider mite damage can reduce corn silage yields about 17 percent and grain yield production by 23 percent or more when not controlled, Bynum said, speaking recently at the High Plains Irrigation Conference in Amarillo.

He said spider mites reduce yields by desiccating leaf tissue, which lowers chlorophyll content and reduces transpiration and photosynthesis. Mite feeding also reduces plant water-use efficiency.

Spider mites are a pest in arid regions that reproduce most rapidly on moderately drought-stressed plants, he said. One female produces 45-100 eggs and they live in colonies. It takes about 10 days for them to go from egg to adult.

“So you can have as many as 11 generations in a summer growing season in the High Plains,” Bynum said.

“The key is to know which species you have and what causes them to increase,” he said. “The Banks grass mite and two-spotted spider mite both feed the same. The Banks grass mite is more common early in the season and stays on the lower portion of the plant, moving up as the population develops. The two-spotted mite is spread out on the plant throughout the growing season.”

Bynum said the list of do’s and don’ts includes:

– Don’t let the corn stress from moisture or heat, especially moisture because it adds to the heat stress.

– Don’t over fertilize.

– Scout fields at least once a week to know if mite and predator populations are increasing, decreasing or staying static.

– Spray before mite populations get out of control.

– Improve the application coverage by getting it down into the canopy.

– Spray early in the morning to keep it from evaporating.

Damage assessment is based on a scale of 1-10, Bynum said.

“We used to advise waiting until a level 5, but with the miticides that we have now, we can’t get the quick control,” he said. “We are advising the damage level of 3, between 21-30 percent plant damage where none of the lower leaves are killed or have excess injury, is the mark for making a miticide application now.”

Bynum said grain corn is only safe after the dent stage is reached, so any infestation prior to that may need to be treated if infestations reach the outlined damage level. Heavily infested plants are prone to stock rot.

“Sometimes people don’t recognize how much loss can occur,” he said. “Some producers may want to control mites at the whorl stage, however at that point you don’t know what the predator populations will do to keep the mite populations under control.

“Most of the control treatments, therefore are made during tassel and grain-filling growth stages. At tassel, the plant chemistry changes and the mites can explode in population.”

In general, Bynum said, spider mites are affected by moisture stress conditions. When corn is kept wet both before and after tassel, mite populations are generally lower. The higher the total season water, the lower density of spider mites. When the crop is kept drier, it supports mites earlier in the season and longer.

Maturity levels of the corn, however, can make a difference. In one study comparing combinations of early maturity, early planted, latematurity, late-planted corn on six levels of water – fully irrigated down to dryland – demonstrated different infestation levels of mites.

Early maturity and early planted with high levels of irrigation had lower mite levels across all water gradients, Bynum said. Early maturity, lateplanted under higher water levels didn’t have as many mites, but mite populations increased as the soil moisture was reduced.

The late-maturity, early planted corn had lower mite infestations at the higher water levels, but increased to high populations during the moderate water levels, and then dropped off to lower populations under drought-stressed conditions.

The late-maturity, late-planted corn generally had low levels of mites at all the different irrigation levels, but Bynum said that might have been because the plants at the drier moisture levels were under greater moisture stress conditions.

“When we allow spider mites to cause excessive damage, we’re really robbing from the water applied to the crop,” he said. “Producers need to be managing those spider mites out there.”

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